| A |
|
| Absolute
Pressure Sensor |
A sensor which measures pressure in
relation to zero pressure (a vacuum on one
side of the diaphragm). |
| Absolute
Pressure |
Gauge pressure plus the atmospheric
pressure. |
| Absolute
Zero |
The temperature at which thermal energy
is at a minimum. Defined as 0 Kelvin,
calculated to be -273.15 °C or -459.67 °F. |
| ac: |
Alternating current; an electric current
that reverses its direction at regularly
recurring intervals. |
|
Acceleration |
The change in the velocity of a body or
particle with respect to time. The
parameter that an accelerometer measures
(dv/dt). Units expressed in "g". |
| Accuracy |
The closeness of an indication or
reading of a measurement device to the
actual value of the quantity being
measured. Usually expressed as ± percent
of full scale output or reading. |
| Activity
(ai) |
A thermodynamic term for the apparent or
active concentration of a free ion in
solution. It is related to concentration
by the activity coefficient. |
| Activity
Coefficient (fi) |
A ratio of the activity of species i(ai)
to its molality (C). It is a correction
factor which makes the thermodynamic
calculations correct. This factor is
dependent on ionic strength, temperature,
and other parameters. |
| ADC |
Analog-to-Digital Converter: an
electronic device which converts analog
signals to an equivalent digital form, in
either a binary code or a
binary-coded-decimal code. When used for
dynamic waveforms, the sampling rate must
be high to prevent aliasing errors from
occurring. |
| Address |
The
label or number identifying the memory
location where a unit of information is
stored. |
| Aliasing |
If
the sample rate of a function (fs) is less
than two times the highest frequency value
of the function, the frequency is
ambiguously presented. The frequencies
above (fs/2) will be folded back into the
lower frequencies producing erroneous
data. |
| Alloy
11 |
A compensating alloy used in conjunction
with pure copper as the negative leg to
form extension wire for platinum-platinum
rhodium thermocouples Types R and S. |
| Alloy
200/226 |
The combination of compensating alloys
used with tungsten vs. tungsten 26%
rhenium thermocouples as extension cable
for applications under 200°C. |
| Alloy
203/225 |
The combination of compensating alloys
used with tungsten 3% rhenium vs. tungsten
150 rhenium thermocouples as extension
cable for applications under 200°C. |
| Alloy
405/426 |
The combination of compensating alloys
used with tungsten 5% rhenium vs. tungsten
26% rhenium thermocouples as extension
cable for applications under 870°C. |
| ALU |
Arithmetic Logic Unit. The part of a CPU
where binary data is acted upon with
mathematical operations. |
| Alumel |
An aluminum nickel alloy used in the
negative leg of a Type K thermocouple
(Trade name of Hoskins Manufacturing
Company). |
| Ambient
Compensation |
The design of an instrument such that
changes in ambient temperature do not
affect the readings of the instrument. |
| Ambient
Conditions |
The conditions around the transducer
(pressure, temperature, etc.). |
| Ambient
Pressure |
Pressure of the air surrounding a
sensor, transducer or transmitter. |
| Ambient
Temperature |
The average or mean temperature of the
surrounding air which comes in contact
with the equipment and instruments under
test. |
| Ampere
(amp) |
A
unit used to define the rate of flow of
electricity (current) in a circuit; units
are one coulomb (6.28 x 1018 electrons)
per second. |
| Amplifier |
A device which draws power from a source
other than the input signal and which
produces as an output an enlarged
reproduction of the essential features of
its input. |
| Amplitude
Span |
The Y-axis range of a graphic display of
data in either the time or frequency
domain. Usually a log display (dB) but can
also be linear. |
| Amplitude |
A measurement of the distance from the
highest to the lowest excursion of motion,
as in the case of mechanical body in
oscillation or the peak-to-peak swing of
an electrical waveform. |
| Analog
Output |
A voltage or current signal that is a
continuous function of the measured
parameter. |
| Analog-to-Digital
Converter |
A device or circuit that outputs a
binary number corresponding to an analog
signal level at the input. |
| Angstrom |
Ten to the minus tenth meters (10-10) or
one millimicron, a unit used to define the
wave length of light. Designated by the
symbol ‰. |
| Angular
Frequency |
The motion of a body or a point moving
circularly, referred to as the circular
frequency O which is the frequency in
cycles per second (cps) multiplied by the
term (2) and expressed in radians per
second (2pf). |
| Anion |
A negatively charged ion (Cl-, NO3-, S2-
etc.) |
| ANSI |
American National Standards Institute. |
| Anti-reset
Windup |
This is a feature in a three-mode PID
controller which prevents the integral
(auto reset) circuit from functioning when
the temperature is outside the
proportional band. |
| Application
Program |
A computer program that accomplishes
specific tasks, such as word processing. |
| ASCII |
American Standard Code for Information
Interchange. A seven or eight bit code
used to represent alphanumeric characters.
It is the standard code used for
communications between data processing
systems and associated equipment. |
| ASME |
American Society of Mechanical
Engineers. |
|
Assembler |
A
program that translates assembly language
instructions into machine language
instructions. |
| Assembly
Language |
A
machine oriented language in which
mnemonics are used to represent each
machine language instruction. Each CPU has
its own specific assembly language. |
| ASTM |
American Society for Testing and
Materials. |
| Asymmetry
Potential |
The potential developed across the glass
membrane with identical solutions on both
sides. Also a term used when comparing
glass electrode potential in pH 7 buffer. |
|
Asynchronous |
A communication method where data is
sent when it is ready without being
referenced to a timing clock, rather than
waiting until the receiver signals that it
is ready to receive. |
| ATC |
Automatic
temperature compensation. |
|
Auto-Zero |
An
automatic internal correction for offsets
and/or drift at zero voltage input. |
| AWG |
American Wire Gage. |
| Axis of
Rotation |
The axis of rotation (spin axis) is that
straight line about which a body rotates. |
|
Back to
Index |
| B |
|
| Background
Noise |
The total noise floor from all sources
of interference in a measurement system,
independent of the presence of a data
signal. |
| Backup |
A system, device, file or facility that
can be used as an alternative in case of a
malfunction or loss of data. |
| Bandwidth |
A symmetrical region around the set
point in which proportional control
occurs. |
| Basic
Transportation Ref |
The basic transportation section of the
U.S. Government Test Specification
MIL-STD-810D, Method 514.3, Paragraph
I-3.2.1, Page 514.3-5. Basic
transportation defines the test profiles
that have been defined for equipment that
is shipped as secured cargo; by land, by
sea or by air. The test levels are based
upon land transport stress levels because
these are higher than stresses imposed by
air or sea transportation environments. |
| BASIC |
A high-level programming language
designed at Dartmouth College as a
learning tool. Acronym for Beginner's
All-purpose Symbolic Instruction Code. |
| Baud |
A
unit of data transmission speed equal to
the number of bits (or signal events) per
second; 300 baud = 300 bits per second. |
| BCD,
Buffered |
Binary-coded decimal output with output
drivers, to increase line-drive
capability. |
| BCD,
Parallel |
A
digital data output format where every
decimal digit is represented by binary
signals on four lines and all digits are
presented in parallel. The total number of
lines is 4 times the number of decimal
digits. |
| BCD,
Serial |
A digital data output format where every
decimal digit is represented by binary
signals on four lines and up to five
decimal digits are presented sequentially.
The total number of lines is four data
lines plus one strobe line per digit. |
| BCD,
Three-State |
An implementation of parallel BCD, which
has 0, 1 and high-impedance output states.
The high-impedance state is used when the
BCD output is not addressed in parallel
connect applications. |
| Bearing |
A part which supports a journal and in
which a journal revolves. |
| Beat
Frequency |
Beat frequencies are periodic vibrations
that result from the addition and
subtraction of two or more sinusoids. For
example, in the case of two turbine
aircraft engines that are rotating at
nearly the same frequency but not
precisely at the same frequency; Four
frequencies are generated:(f1) the
rotational frequency of turbine one, (f2)
the rotational frequency of turbine two,
(f1 + f2) the sum of turbine rotational
frequencies one and two, and (f1 - f2)
which is the difference or "beat"
frequency of turbines one and two. The
difference of the two frequencies is the
lower frequency and is the one that is
"felt" as a beat or "wow" in this case. |
| Beryllia |
BeO (Beryllium Oxide) A high-temperature
mineral insulation material; toxic when in
powder form. |
| Best
Fit Straight Line (BFSL) |
A line midway between two parallel
straight lines enclosing all output vs.
pressure values. |
| Beta
Ratio |
The ratio of the diameter of a pipeline
constriction to the unconstricted pipe
diameter. |
| BIAS
Current |
A
very low-level DC current generated by the
panel meter and superimposed on the
signal. This current may introduce a
measurable offset across a very high
source impedance. |
| Binary
Coded Decimal (BCD) |
The representation of a decimal number
(base 10, 0 through 9) by means of a 4 bit
binary nibble. |
| Binary |
Refers
to base 2 numbering system, in which the
only allowable digits are 0 and 1.
Pertaining to a condition that has only
two possible values or states. |
| BIOS |
Acronym for basic input/output system.
The commands used to tell a CPU how it
will communicate with the rest of the
computer. |
| Bipolar |
The ability of a panel meter to display
both positive and negative readings. |
| Bit |
Acronym for binary digit. The smallest
unit of computer information, it is either
a binary 0 or 1. |
|
Blackbody |
A theoretical object that radiates the
maximum amount of energy at a given
temperature, and absorbs all the energy
incident upon it. A blackbody is not
necessarily black. (The name blackbody was
chosen because the color black is defined
as the total absorption of light energy.) |
| BNC |
A quick disconnect electrical connector
used to inter-connect and/or terminate
coaxial cables. |
| Boiling
Point |
The temperature at which a substance in
the liquid phase transforms to the gaseous
phase; commonly refers to the boiling
point of water which is 100°C (212°F) at
sea level. |
| BPS |
Bits
per second. |
|
Breakdown Voltage Rating |
The dc or ac voltage which can be
applied across insulation portions of a
transducer without arcing or conduction
above a specific current value. |
| Bridge
Resistance |
Please see Input impedance and Output
impedance. |
| BTU |
British
thermal units. The quantity of thermal
energy required to raise one pound of
water at its maximum density, 1 °F. One
BTU is equivalent to 0.293 watt hours, or
252 calories. One kilowatt hour is
equivalent to 3412 BTU. |
| Buffer
Capacity (B) |
A measure of the ability of the solution
to resist pH change when a strong acid or
base is added. |
| Buffer |
(1) A storage area for data that is used
to compensate for a speed difference, when
transferring data from one device to
another. Usually refers to an area
reserved for I/O operations, into which
data is read, or from which data is
written.(2) Any substance or combination
of substances which, when dissolved in
water, produces a solution which resists a
change in its hydrogen ion concentration
on the addition of an acid or alkali. |
| Bulb
(Thermometer) |
The area at the tip of a liquid-in-glass
thermometer containing the liquid
reservoir. |
| Burn-In |
A long term screening test (either
vibration, temperature or combined test)
that is effective in weeding out infant
mortalities because it simulates actual or
worst case operation of the device,
accelerated through a time, power, and
temperature relationship. |
| Burst
Pressure |
The maximum pressure applied to a
transducer sensing element or case without
causing leakage. |
| Burst
Proportioning |
A fast-cycling output form on a time
proportioning controller (typically
adjustable from 2 to 4 seconds) used in
conjunction with a solid state relay to
prolong the life of heaters by minimizing
thermal stress. |
| Bus |
Parallel lines used to transfer signals
between devices or components. Computers
are often described by their bus structure
(i.e., S-100, IBM PC). |
| Byte |
The representation of a character in
binary. Eight bits. |
|
Back to
Index |
| C |
|
| Calibration |
The process of adjusting an instrument
or compiling a deviation chart so that its
reading can be correlated to the actual
value being measured. |
| Calorie |
The quantity of thermal energy required
to raise one gram of water 1 °C at 15 °C. |
| Cation |
A
positively charged ion (Na+, H+). |
| Cavitation |
The boiling of a liquid caused by a
decrease in pressure rather than an
increase in temperature. |
| Celsius
(centrigrade) |
A temperature scale defined by 0 °C at
the ice point and 100 °C at boiling point
of water at sea level. |
| Center
of Gravity |
(Mass Center) The center of gravity of a
body is that point in the body through
which passes the resultant of weights of
its component particles for all
orientations of the body with respect to a
uniform gravitational field. |
| Centripetal
Force |
A
force exerted on an object moving in a
circular path which is exerted inward
toward the center of rotation. |
| Ceramic
Insulation |
High-temperature compositions of metal
oxides used to insulate a pair of
thermocouple wires The most common are
Alumina (Al2O3), Beryllia (BeO), and
Magnesia (MgO). Their application depends
upon temperature and type of thermocouple.
High-purity alumina is required for
platinum alloy thermocouples. Ceramic
insulators are available as single and
multihole tubes or as beads. |
| Ceramic |
Polycrystalline ferroelectric materials
which are used as the sensing units in
piezoelectric accelerometers. There are
many different grades, all of which can be
made in various configurations to satisfy
different design requirements. |
| CFM |
The volumetric flow rate of a liquid or
gas in cubic feet per minute. |
| Character |
A letter, digit or other symbol that is
used as the representation of data. A
connected sequence of characters is called
a character string. |
| Charge
Sensitivity |
For
accelerometers that are rated in terms of
charge sensitivity, the output voltage
(V)is proportional to the charge (Q)
divided by the shunt capacitance (C). This
type of accelerometer is characterized by
a high output impedance. The sensitivity
is given in terms of charge; picocoulombs
per unit of acceleration (g). |
| Chatter |
The rapid cycling on and off of a relay
in a control process due to insufficient
bandwidth in the controller. |
| Clear |
To restore a device to a prescribed
initial state, usually the zero state. |
| Clipping |
The term applied to the phenomenon which
occurs when an output signal is limited in
some way by the full range of an
amplifier, ADC or other device. When this
occurs, the signal is flattened at the
peak values, the signal approaches the
shape of a square wave, and high frequency
components are introduced. Clipping may be
hard, as is the case when the signal is
strictly limited at some level; or it may
be soft, in which case the clipping signal
continues to follow the input at some
reduced gain. |
| Clock |
The device that generates periodic
signals for synchronization. |
| Closeness
of Control |
Total temperature variation from a
desired set point of system. Expressed as
"closeness of control" is ±2 °C or a
system bandwidth with 4 °C, also referred
to as amplitude of deviation. |
| CMR |
(Common-Mode Rejection) The ability of a
panel meter to eliminate the effect of AC
or DC noise between signal and ground.
Normally expressed in dB at dc to 60 Hz.
One type of CMR is specified between SIG
LO and PWR GND. In differential meters, a
second type of CMR is specified between
SIG LO and ANA GND (METER GND). |
| CMV |
(Common-Mode Voltage) The AC or DC
voltage which is tolerable between signal
and ground. One type of CMV is specified
between SIG LO and PWR GND. In
differential meters, a second type of CMV
is specified between SIG HI or LO and ANA
GND (METER GND). |
| Coherence
Function |
A frequency domain function computed to
show the degree of a linear, noise-free
relationship between a system's input and
output. The value of the coherence
function ranges between zero and one,
where a value of zero indicates there is
no causal relationship between the input
and the output. A value of one indicates
the existence of linear noise-free
frequency response between the input and
the output. |
| Color
Code |
The ANSI established color code for
thermocouple wires in the negative lead is
always red. Color Code for base metal
thermocouples is yellow for Type K, black
for Type J, purple for Type E and blue for
Type T. |
| Common
Mode Rejection Ratio |
The ability of an instrument to reject
interference from a common voltage at its
input terminals with relation to ground.
Usually expressed in db (decibels). |
| Common
Mode |
The output form or type of
control action used by a temperature
controller to control temperature, i.e.
on/off, time proportioning, PID. |
| Communication |
Transmission and reception of data among
data processing equipment and related
peripherals. |
| Compensated
Connector |
A connector made of thermocouple alloys
used to connect thermocouple probes and
wires. |
| Compensating
Alloys |
Alloys used to connect thermocouples to
instrumentation. These alloys are selected
to have similar thermal electric
properties as the thermocouple alloys
(however, only over a very limited
temperature range). |
| Compensating
Loop |
Lead wire resistance compensation for
RTD elements where an extra length of wire
is run from the instrument to the RTD and
back to the instrument, with no connection
to the RTD. |
| Compensation |
An addition of specific materials or
devices to counteract a known error. |
| Compiler |
A program that translates a
high-level language, such as Basic, into
machine language. |
| Conductance |
The measure of the ability
of a solution to carry an electrical
current. (See Equivalent Conductance) |
| Conduction |
The conveying of electrical energy or
heat through or by means of a conductor. |
|
Confidence Level |
The range (with a specified value of
uncertainty, usually expressed in percent)
within which the true value of a measured
quantity exists. |
| Conformity
Error |
For thermocouples and RTDs, the
difference between the actual reading and
the temperature shown in published tables
for a specific voltage input. |
|
Connection Head |
An enclosure attached to the end of a
thermocouple which can be cast iron,
aluminum or plastic within which the
electrical connections are made. |
| Constantan |
A copper-nickel alloy used as the
negative lead in Type E, Type J, and Type
T thermocouples. |
| Continuous
Spectrum |
A frequency spectrum that is
characterized by non-periodic data The
spectrum is continuous in the frequency
domain and is characterized by an infinite
number of frequency components. |
| Control
Character |
A character whose occurrence in a
particular context starts, modifies or
stops an operation that effects the
recording, processing, transmission or
interpretation of data. |
|
Control Mode |
The output form or type of control
action used by a temperature controller to
control temperature, i.e., on/off, time
proportioning, PID. |
| Control
Point |
The
temperature at which a system is to be
maintained. |
|
Convection |
(1) The circulatory motion that occurs
in a fluid at a non-uniform temperature
owing to the variation of its density and
the action of gravity. (2) The transfer of
heat by this automatic circulation of
fluid. |
| Coriolis
Force |
A result of centripetal force on a mass
moving with a velocity radially outward in
a rotating plane. |
| Correction
(Balancing) Plane |
A
plane perpendicular to the shaft axis of a
rotor in which correction for unbalance is
made. |
| Coulomb
Sensitivity |
Charge/unit acceleration, expressed in
Pc/g (charge sensitivity). |
| Coulomb |
A measurement of the quantity of
electrical charge, usually expressed as
pico coulomb (10-12 coulombs). |
| Counter
Weight |
A weight added to a body so as to reduce
a calculated unbalance at a desired place. |
| Counts |
The number of time intervals counted by
the dual-slope A/D converter and displayed
as the reading of the panel meter, before
addition of the decimal point. |
| CPS |
Cycles per second; the rate or number of
periodic events in one second, expressed
in Hertz (Hz). |
| CPU |
Central processing unit. The part of the
computer that contains the circuits that
control and perform the execution of
computer instructions. |
| Critical
Damping |
Critical damping is the smallest amount
of damping at which a given system is able
to respond to a step function without
overshoot. |
| Critical
Speed |
The rotational speed of the rotor or
rotating element at which resonance occurs
in the system. The shaft speed at which at
least one of the "critical" or natural
frequencies of a shaft is excited. |
| Cryogenics |
Measurement of temperature at extremely
low values, i.e., below -200 °C. |
| CSA |
Canadian Standards Administration. |
| Cure
Point |
The temperature at which a normally
magnetic material goes through a magnetic
transformation and becomes non-magnetic. |
| Current
Proportioning |
An output form of a temperature
controller which provides a current
proportional to the amount of control
required. Normally is a 4 to 20 milliamp
current proportioning band. |
| Current |
The rate of flow of electricity. The
unit of the ampere (A) defined as 1 ampere
= 1 coulomb per second. |
| Curve
Fitting |
Curve fitting is the process of
computing the coefficients of a function
to approximate the values of a given data
set within that function. The
approximation is called a "fit". A
mathematical function, such as a least
squares regression, is used to judge the
accuracy of the fit. |
| Cycle
Time |
The time usually expressed in seconds
for a controller to complete one on/off
cycle. |
|
Back to
Index |
| D |
|
| Damping |
The reduction of vibratory movement
through dissipation of energy. Types
include viscous, coulomb, and solid. |
| dB
(Decibel) |
20 times the log to the
base 10 of the ratio of two voltages.
Every 20 dBs correspond to a voltage ratio
of 10, every 10 dBs to a voltage ratio of
3.162. For instance, a CMR of 120 dB
provides voltage noise rejection of
1,000,000/1. An NMR of 70 dB provides
voltage noise rejection of 3,162/1. |
| DC |
Direct
current; an electric current flowing in
one direction only and substantially
constant in value. |
| Dead
Band |
(1). For chart records: the minimum
change of input signal required to cause a
deflection in the pen position. (2) For
temperature controllers: the temperature
band where heat is turned off upon rising
temperature and turned on upon falling
temperature expressed in degrees. The area
where no heating (or cooling) takes place. |
| Dead
Volume |
The volume of the pressure port of a
transducer at room temperature and ambient
barometric pressure. |
| Decimal |
Refers to a base ten number system using
the characters 0 through 9 to represent
values. |
| Default |
The value(s) or option(s) that are
assumed during operation when not
specified. |
| Degree |
An incremental value in the temperature
scale, i.e., there are 100 degrees between
the ice point and the boiling point of
water in the Celsius scale and 180°F
between the same two points in the
Fahrenheit scale. |
| Density |
Mass per unit of volume of a substance.
I.E.: grams/cu.cm. or pounds/cu.ft. |
| Derivative |
The derivative function senses the rate
of rise or fall of the system temperature
and automatically adjusts the cycle time
of the controller to minimize overshoot or
undershoot. |
| Deviation |
The difference between the value of the
controlled variable and the value at which
it is being controlled. |
| Diaphragm |
The sensing element consisting of a
membrane which is deformed by the pressure
differential applied across it. Also known
as a pressure responsive element. |
| Dielectric
Constant |
Related to the force of attraction
between two opposite charges separated by
a distance in a uniform medium. |
| Differential
Input |
A
signal-input circuit where SIG LO and SIG
HI are electrically floating with respect
to ANALOG GND (METER GND, which is
normally tied to DIG GND). This allows the
measurement of the voltage difference
between two signals tied to the same
ground and provides superior common-mode
noise rejection. |
| Differential
Pressure |
The difference in static pressure
between two identical pressure taps at the
same elevation located in two different
locations in a primary device. |
| Differential |
For an on/off controller, it refers to
the temperature difference between the
temperature at which the controller turns
heat off and the temperature at which the
heat is turned back on. It is expressed in
degrees. |
| Digit |
A measure of the display span of a panel
meter. By convention, a full digit can
assume any value from 0 through 9, a
1/2-digit will display a 1 and overload at
2, a 3/4-digit will display digits up to 3
and overload at 4, etc. For example, a
meter with a display span of ±3999 counts
is said to be a 3-3/4 digit meter. |
| Digital
Output |
An output signal which represents the
size of an input in the form of a series
of discrete quantities. |
| Digital-to-Analog
Converter |
A
device or circuit to convert a digital
value to an analog signal level. |
| DIN |
(Deutsche
Industrial Norm) A set of German standards
recognized throughout the world. The 1/8
DIN standard for panel meters specifies an
outer bezel dimension of 96 x 48 mm and a
panel cutout of 92 x 45 mm. |
| DIN
43760 |
The standard that defines the
characteristics of a 100 ohm platinum RTD
having a resistance vs. temperature curve
specified by a = 0.00385 ohms per degree. |
| Discharge
Time Constant |
The time required for the output-voltage
from a sensor or system to discharge 37%
of its original value in response to a
zero rise time step function input. This
parameter determines a low frequency
response. |
| Disk
Operating System |
(DOS) Program used to control the
transfer of information to and from a
disk, such as MS DOS. |
| Displacement |
The measured distance traveled by a
point from its position at rest. Peak to
peak displacement is the total measured
movement of a vibrating point between its
positive and negative extremes.
Measurement units expressed as inches or
millinches. |
| Dissipation
Constant |
The ratio for a thermistor which relates
a change in internal power dissipation to
a resultant change of body temperature. |
| Dissociation
Constant (K) |
A value which quantitatively expresses
the extent to which a substance
dissociates in solution. The smaller the
value of K, the less dissociation of the
species in solution. This value varies
with temperature, ionic strength, and the
nature of the solvent. |
| DMA |
Acronym direct memory access. A high
speed data storage mode of the IBM PC. |
| Double
Precision |
The degree of accuracy that requires two
computer words to represent a number.
Numbers are stored with 17 digits of
accuracy and printed with up to 16 digits. |
| Drift |
A
change of a reading or a set point value
over long periods due to several factors
including change in ambient temperature,
time, and line voltage. |
| Droop |
A common occurrence in time-proportional
controllers. It refers to the difference
in temperature between the set point and
where the system temperature actually
stabilizes due to the time-proportioning
action of the controller. |
| Dual
Element Sensor |
A sensor assembly with two independent
sensing elements. |
| Dual-slope
A/D Converter |
An analog-to-digital converter which
integrates the signal for a specific time,
then counts time intervals for a reference
voltage to bring the integrated signal
back to zero. Such converters provide high
resolution at low cost, excellent
normal-mode noise rejection, and minimal
dependence on circuit elements. |
| Duplex
Wire |
A pair of wires insulated from each
other and with an outer jacket of
insulation around the inner insulated
pair. |
| Duplex |
Pertaining to simultaneous two-way
independent data communication
transmission in both direction. Same as
"full duplex". |
| Duty
Cycle |
The total time to one on/off cycle.
Usually refers to the on/off cycle time of
a temperature controller. |
| Dynamic
Balancing Machine |
A dynamic balancing machine is a
centrifugal balancing machine that
furnishes information for performing
two-plane balancing. |
| Dynamic
Calibration |
Calibration in which the input varies
over a specific length of time and the
output is recorded vs. time. |
| Dynamic
Pressure |
The difference in pressure levels from
static pressure to stagnation pressure
caused by an increase in velocity. Dynamic
pressure increases by the square of the
velocity. |
| Dynamic
Unbalance |
Dynamic unbalance is that condition in
which the central principal axis is not
coincident with the shaft axis. |
|
Back to
Index |
| E |
|
| Echo |
To reflect received data to the sender.
For example, keys depressed on a keyboard
are usually echoed as characters displayed
on the screen. |
| Electrical
Interference |
Electrical noise induced upon the signal
wires that obscures the wanted information
signal. |
| Electrode
Potential (E) |
The difference in potential established
between an electrode and a solution when
the electrode is immersed in the solution. |
| Electrolyte |
Any substance which, when in solution
will conduct an electric current. Acids,
bases, and salts are common electrolytes. |
| Electromotive
Force (emf) |
The potential difference between the two
electrodes in a cell. The cell emf is the
cell voltage measured when no current is
flowing through the cell. It can be
measured by means of a pH meter with high
input impedance. |
| Electronic
Industries Assoc |
(EIA) A standards organization
specializing in the electrical and
functional characteristics of interface
equipment. |
| EMF |
Electromotive Force. A rise in
(electrical) potential energy. The
principal unit is the volt. |
| EMI |
Electromagnetic interference. |
| Emissivity |
The ratio of energy emitted by an object
to the energy emitted by a blackbody at
the same temperature. The emissivity of an
object depends upon its material and
surface texture; a polished metal surface
can have an emissivity around 0.2 and a
piece of wood can have an emissivity
around 0.95. |
| End
Point (Potentiometric) |
The apparent equivalence point of a
titration at which a relatively large
potential change is observed. |
| End
Points |
The end points of a full scale
calibration curve. |
| Endothermic |
Absorbs heat. A process is said to be
endothermic when it absorbs heat. |
| Enthalpy |
The sum of the internal energy of a body
and the product of its volume multiplied
by the pressure. |
| Environmental
Conditions |
All conditions in which a transducer may
be exposed during shipping, storage,
handling, and operation. |
| Eprom |
Erasable Programmable Read-Only Memory.
The PROM can be erased by ultraviolet
light or electricity. |
| Equilibrium
Constant |
The product of the concentrations (or
activities) of the substances produced at
equilibrium in a chemical reaction divided
by the product of concentrations of the
reacting substances, each concentration
raised to that power which is the
coefficient of the substance in the
chemical equation. |
| Equitransference |
Equal diffusion rates of the positively
and negatively charged ions of an
electrolyte across a liquid junction
without charge separation. |
| Equivalent
Conductance (l) |
Equivalent conductance of an electrolyte
is defined as the conductance of a volume
of solution containing one equivalent
weight of dissolved substances when placed
between two parallel electrodes 1 cm
apart, and large enough to contain between
them all of the solution. l is never
determined directly, but is calculated
from the specific conductance (Ls). If C
is the concentration of a solution in gram
equivalents per liter, then the
concentration of a solution in gram
equivalents per liter, then the
concentration per cubic centimeter is
C/1000, and the volume containing one
equivalent of the solute, is, therefore,
1000/C. |
| Error
Band |
The allowable deviations to
output from a specific reference norm.
Usually expressed as a percentage of full
scale. |
| Error |
The difference between the value
indicated by the transducer and the true
value of the measurand being sensed.
Usually expressed in percent of full scale
output. |
| Eutectic
Temperature |
The lowest possible melting point of a
mixture of alloys. |
| Excitation |
The external application of electrical
voltage current applied to a transducer
for normal operation. |
| Exothermic |
Gives off heat. A process is said to be
exothermic when it releases heat. |
| Expansion
Factor |
Correction factor for the change in
density between two pressure measurement
areas in a constricted flow. |
| Explosion-proof
Enclosure |
An enclosure that can withstand an
explosion of gases within it and prevent
the explosion of gases surrounding it due
to sparks, flashes or the explosion of the
container itself, and maintain an external
temperature which will not ignite the
surrounding gases. |
| Exposed
Junction |
A form of construction of a thermocouple
probe where the hot or measuring junction
protrudes beyond the sheath material so as
to be fully exposed to the medium being
measured. This form of construction
usually gives the fastest response time. |
| |
Back to
Index |
| F |
|
| Fahrenheit |
A
temperature scale defined by 32° at the
ice point and 212° at the boiling point of
water at sea level. |
| Ferrule |
A compressible tubular fitting that is
compressed onto a probe inside a
compression fitting to form a gas-tight
seal. |
| Field
Balancing Equipment |
An assembly of measuring instruments for
performing balancing operations on
assembled machinery which is not mounted
in a balancing machine. |
| Field
of View |
A volume in space defined by an angular
cone extending from the focal plane of an
instrument. |
| File |
A set of related records or data treated
as a unit. |
| Filling
Solution |
A solution of defined composition to
make contact between an internal element
and a membrane or sample. The solution
sealed inside a pH glass bulb is called an
internal filling solution. This solution
normally contains a buffered chloride
solution to provide a stable potential and
a designated zero potential point. The
solution which surrounds the reference
electrode internal and periodically
requires replenishing is called the
reference filling solution. It provides
contact between the reference electrode
internal and sample through a junction. |
| Firmware |
Programs stored in PROM’s. |
| Flag |
Any
of various types of indicators used for
identification of a condition or event;
for example, a character that signals the
termination of a transmission. |
| Flow
Rate |
Actual speed or velocity of fluid
movement . |
| Flow |
Travel of liquids or gases in response
to a force (i.e. pressure or gravity). |
| Flowmeter |
A device used for measuring the flow or
quantity of a moving fluid. |
| FM
Approved |
An instrument that meets a specific set
of specifications established by Factory
Mutual Research Corporation. |
| FM |
Factory Mutual Research
Corporation. An organization which sets
industrial safety standards. |
| Forced
Vibration |
Vibration of a system caused by an
imposed force. Steady-state vibration is
an unchanging condition of periodic or
random motion. |
| FORTRAN |
Formula Translation language. A widely
used high-level programming language well
suited to problems that can be expressed
in terms of algebraic formulas. It is
generally used in scientific applications. |
| FPM |
Flow velocity in feet per minute |
| FPS |
Flow velocity in feet per second. |
| Freezing
Point |
The temperature at which the substance
goes from the liquid phase to the solid
phase. |
| Frequency
Mod Output |
A transducer output which is obtained in
the form of a deviation from a center
frequency, where the deviation is
proportional to the applied stimulus. |
| Frequency
of Vibration |
The number of cycles occurring in a
given unit of time. RPM - revolutions per
minute. CPM- cycles per minute. |
| Frequency
Output |
An output in the form of frequency which
varies as a function of the applied input. |
| Frequency,
Natural |
The frequency of free (not
forced) oscillations of the sensing
element of a fully assembled transducer. |
| Frequency |
The number of cycles over a specified
time period over which an event occurs.
The reciprocal is called the period. |
| Full
Bridge |
A
Wheatstone bridge configuration utilizing
four active elements or strain gages. |
| Full
Scale Output |
The algebraic difference between the
minimum output and maximum output. |
| Functions |
Three mode PID controller. A
timeproportioning controller with integral
and derivative functions. The integral
function automatically adjusts the system
temperature to the set point temperature
to eliminate droop due to the time
proportioning function. |
|
Back to
Index |
| G |
|
| g |
The force of acceleration due to gravity
equal to 32.1739 ft/sec2 or 386 in./sec2. |
| Gage
Factor |
A measure of the ratio of the relative
change of resistance to the relative
change in length of a piezoresistive
strain gage. |
| Gage
Length |
The distance between two points where
the measurement of strain occurs. |
| Gauge
Pressure Transducer |
A
transducer which measures pressure in
relation to the ambient pressure. |
| Gauge
pressure |
Absolute pressure minus local
atmospheric pressure. |
| Gain |
The amount of amplification used in an
electrical circuit. |
| Galvanometer |
An instrument that measures small
electrical currents by means of deflecting
magnetic coils. |
| GPH |
Volumetric flow rate in gallons per
hour. |
| GPM |
Volumetric flow rate in gallons per
minute. |
| Ground |
(1) The electrical neutral line having
the same potential as the surrounding
earth. (2) The negative side of DC power
supply. (3) Reference point for an
electrical system. |
| Grounded
Junction |
A
form of construction of a thermocouple
probe where the hot or measuring junction
is in electrical contact with the sheath
material so that the sheath and
thermocouple will have the same electrical
potential. |
| |
Back to
Index |
| H |
|
| Half
Bridge |
Two active elements or strain gages. |
| Half-Duplex |
One way at a time data communication;
both devices can transmit and receive
data, but only one at a time. |
| Handshake |
An interface procedure that is based on
status/data signals that assure orderly
data transfer as opposed to asynchronous
exchange. |
| Hardcopy |
Output in a permanent form (usually a
printout) rather than in temporary form,
as on disk or display terminal. |
| Hardware |
The electrical, mechanical and
electromechanical equipment and parts
associated with a computing system, as
opposed to its firmware or software. |
| Head
Loss |
The loss of pressure in a flow system
measured using a length parameter (i.e.,
inches of water, inches of mercury). |
| Head
Pressure |
Pressure in terms of the height of
fluid, P = yrg, where r = fluid density
and y = the fluid column heights.
Expression of a pressure in terms of the
height of fluid, r = yrg, where r is fluid
density and y = the fluid column height. g
= the acceleration of gravity. |
| Heat
Sink |
(1) Thermodynamic. A body which can
absorb thermal energy. (2) Practical. A
finned piece of metal used to dissipate
the heat of solid state components mounted
on it. |
| Heat
Transfer |
The process of thermal energy flowing
from a body of high energy to a body of
low energy. Means of transfer are:
conduction; the two bodies contact.
Convection; a form of conduction where the
two bodies in contact are of different
phases, i.e. solid and gas. Radiation: all
bodies emit infrared radiation. |
| Heat
Treating |
A
process for treating metals where heating
to a specific temperature and cooling at a
specific rate changes the properties of
the metal. |
| Heat |
Thermal energy. Heat is expressed in
units of calories or BTU's. |
| Hertz
(Hz) |
Units
in which frequency is expressed.
Synonymous with cycles per second. |
| Hexadecimal |
Refers to a base sixteen number system
using the characters 0 through 9 and A
through F to represent the values. Machine
language programs are often written in
hexadecimal notation. |
| Hold |
Meter HOLD is an external input which is
used to stop the A/D process and freeze
the display. BCD HOLD is an external input
used to freeze the BCD output while
allowing the A/D process to continue
operation. |
| Hooke's
Law |
Defines the basis for the measurement of
mechanical stresses via the strain
measurement. The gradient of Hooke's line
is defined by the ratio of which is
equivalent to the Modulus of Elasticity E
(Young's Modulus). |
| Host |
The primary or controlling computer in a
multiple part system. |
| Hydrogen
Ion Activity (aH+) |
Activity of the hydrogen ion in
solution. Related to hydrogen ion
concentration (CH+) by the activity
coefficient for hydrogen (f H+). |
| Hysteresis |
When an electrode system is returned to
a solution, equilibrium is usually not
immediate. This phenomenon is often
observed in electrodes that have been
exposed to the other influences such as
temperature, light, or polarization. |
| Hysteresis |
The difference in output when the
measurand value is first approached with
increasing and then with decreasing
values. Expressed in percent of full scale
during any one calibration cycle. See also
Deadband. |
| |
Back to
Index |
| I |
|
| Icon |
A graphic functional symbol display. A
graphic representation of a function or
functions to be performed by the computer. |
| ICP |
Integrated Circuit Piezoelectric; term
sometimes used to describe an
accelerometer with built-in electronics. |
| Impedance |
The total opposition to electrical flow
(resistive plus reactive). |
| Infrared |
An area in the electromagnetic spectrum
extending beyond red light from 760
nanometers to 1000 microns (106 nm). It is
the form of radiation used for making
non-contact temperature measurements. |
| Initial
Unbalance |
Initial unbalance is that unbalance of
any kind that exists in the rotor before
balancing. |
| Input
Impedance |
The resistance measured across the
excitation terminals of a transducer. |
| Input
Impedance |
The resistance of a panel meter as seen
from the source. In the case of a
voltmeter, this resistance has to be taken
into account when the source impedance is
high; in the case of an ammeter, when the
source impedance is low. |
| Input
Resistance |
(Impedance) The input resistance of a pH
meter is the resistance between the glass
electrode terminal and the reference
electrode terminal. The potential of a
pH-measuring electrode chain is always
subject to a voltage division between the
total electrode resistance and the input
resistance. |
| Insulated
Junction |
See Ungrounded Junction |
| Insulation
Resistance |
The resistance measured between two
insulated points on a transducer when a
specific dc voltage is applied at room
temperature. |
| Integral |
A form of temperature control. See
Automatic Reset, #2 |
| Interchangeability
Error |
A measurement error that can occur if
two or more probes are used to make the
same measurement. It is caused by a slight
variation in characteristics of different
probes. |
| Interface |
The means by which two systems or
devices are connected and interact with
each other. |
| Internal
Ref Electrode |
(Element)
The reference electrode placed internally
in a glass electrode. |
| Interpreter |
A system program that converts and
executes each instruction of a high-level
language program into machine code as it
runs, before going onto the next
instruction. |
| Interrupt |
To stop a process in such a way that it
can be resumed. |
| Intrinsically
Safe |
An
instrument which will not produce any
spark or thermal effects under normal or
abnormal conditions that will ignite a
specified gas mixture. |
| Ionic
Mobility |
Defined
similarly to the mobility of
non-electrolytic particles, viz., as the
speed that the ion obtains in a given
solvent when influenced by unit power. |
| Ionic
Strength |
The weight concentration of ions in
solution, computed by multiplying the
concentration of each ion in solution (C)
by the corresponding square of the charge
on the ion (Z) summing this product for
all ions in solution and dividing by
2:ionic strength - 1/2 _ Z2 C. |
| IPTS-48 |
International
Practical Temperature Scale of 1948. Fixed
points in thermometry as specified by the
Ninth General Conference of Weights and
Measures which was held in 1948. |
| IPTS-68 |
International Practical Temperature
Scale of 1968. Fixed points in thermometry
set by the 1968 General Conference of
Weights and Measures. |
| Isolation |
The reduction of the capacity of a
system to respond to an external force by
use of resilient isolating materials. |
| Isopotential
Point |
A potential which is not affected by
temperature changes. It is the pH value at
which dE/dt for a given electrode pair is
zero. Normally, for a glass electrode and
SCE reference, this potential is obtained
approximately when immersed in pH 7
buffer. |
| Isothermal |
A process or area that is a constant
temperature. |
|
Back to
Index |
| J |
|
| Joule |
The basic unit of thermal energy. |
| Journal |
A journal is that part of a rotor that
is in contact with or supported by a
bearing in which it revolves. |
| Junction |
The point in a thermocouple where the
two dissimilar metals are joined. |
|
Back to
Index |
| K |
When referring to memory capacity, two
to the tenth power (1024 in decimal
notation). |
| Kelvin |
Symbol K. The unit of absolute or
thermodynamic temperature scale based upon
the Celsius scale with 100 units between
the ice point and boiling point of water.
0°C = 273.15K (there is no degree (°)
symbol used with the Kelvin scale). |
| Kilowatt
(kw) |
Equivalent to 1000 watts. |
| Kilowatt
Hour (kwh) |
1000 watthours. Kilovolt amperes (kva):
1000 volt amps. |
| Kinetic
Energy |
Energy associated with mass in motion,
i.e., 1/2 rV2 where r is the density of
the moving mass and V is its velocity. |
| KVA |
Kilovolt amperes (1000-volt amps). |
|
Back to
Index |
| L |
|
| Lag |
(1) A time delay between the output of a
signal and the response of the instrument
to which the signal is sent. (2) A time
relationship between two waveforms where a
fixed reference point on one wave occurs
after the same point of the reference
wave. |
| Laminar
Flow |
Streamlined flow of a fluid where
viscous forces are more significant than
inertial forces, generally below a
Reynolds number of 2000. |
| Large
Scale Integration (LSI) |
The combining of about 1,000 to 10,000
circuits on a single chip. Typical
examples of LSI circuits are memory chips
and microprocessor. |
| Latent
Heat |
Expressed in BTU per pound. The amount
of heat needed (absorbed) to convert a
pound of boiling water to a pound of
steam. |
| Leakage
Rate |
The maximum rate at which a fluid is
permitted or determined to leak through a
seal. The type of fluid, the differential
Limits of Error: A tolerance band for the
thermal electric response of thermocouple
wire expressed in degrees or percentage
defined by ANSI specification MC-96.1
(1975). |
| Least-squares
Line |
The straight line for which the sum of
the squares of the residuals (deviations)
is minimized. |
| Life
Cycle |
The minimum number of pressure cycles
the transducer can endure and still remain
within a specified tolerance. |
| Limits
of Error |
A
tolerance band for the thermal electric
response of thermocouple wire expressed in
degrees or percentage defined by ANSI
specification MC-96.1 (1975). |
| Linearity |
The closeness of a calibration curve to
a specified straight line. Linearity is
expressed as the maximum deviation of any
calibration point on a specified straight
line during any one calibration cycle. |
| Liquid
Junction Potential |
The potential difference existing
between a liquid-liquid boundary. The sign
and size of this potential depends on the
composition of the liquids and the type of
junction used. |
| Load
Impedance |
The impedance presented to the output
terminals of a transducer by the
associated external circuitry. |
| Load |
The electrical demand of a process
expressed as power (watts), current (amps)
or resistance (ohms). |
| Logarithmic
Scale |
A method of displaying data (in powers
of ten) to yield maximum range while
keeping resolution at the low end of the
scale. |
| Loop
Resistance |
The total resistance of a thermocouple
circuit caused by the resistance of the
thermocouple wire. Usually used in
reference to analog pyrometers which have
typical loop resistance requirements of 10
ohms. |
| LS-TTL
Compatible |
For digital input circuits, a logic 1 is
obtained for inputs of 2.0 to 5.5 V which
can source 20 µA, and a logic 0 is
obtained for inputs of 0 to 0.8 V which
can sink 400 µA. For digital output
signals, a logic 1 is represented by 2.4
to 5.5 V with a current source capability
of at least 400 µA; and a logic 0 is
represented by 0 to 0.6 V with a current
sink capability of at least 16 MA. "LS"
stands for low-power Schottky. |
| LS-TTL
Unit Load |
A load with LS-TTL voltage levels, which
will draw 20 µA for a logic 1 and -400 µA
for a logic 0. |
| LSD
(Least-Significant Digit) |
The rightmost active (non-dummy) digit
of the display. |
|
Back to
Index |
| M |
Mega; one million. When referring to
memory capacity, two to the twentieth
power (1,048,576 in decimal notation). |
| Machine
Language |
Instructions that are written in binary
form that a computer can execute directly.
Also called object code and object
language. |
| Mandrel
(Balancing Arbor) |
An accurately machined shaft on which
work is mounted for balancing. |
| Manual
Reset (Adjustment) |
The adjustment on a proportioning
controller which shifts the proportioning
band in relationship to the set point to
eliminate droop or offset errors. |
| Manual
Reset (Switch) |
The switch in a limit controller that
manually resets the controller after the
limit has been exceeded. |
| Mass
Flow Rate |
Volumetric flowrate times density, i.e.
pounds per hour or kilograms per minute. |
| Mass
Storage |
A device like a disk or magtape that can
store large amounts of data readily
accessible to the central processing unit. |
| Maximum
Elongation |
The strain value where a deviation of
more than ±5% occurs with respect to the
mean characteristic (diagram of resistance
change vs strain). |
| Maximum
Excitation |
The maximum value of excitation voltage
or current that can be applied to the
transducer at room conditions without
causing damage or performance degradation
beyond specified tolerances. |
| Maximum
Operating Temp |
The maximum temperature at which an
instrument or sensor can be safely
operated. |
| Maximum
Power Rating |
The maximum power in watts that a device
can safely handle. |
| Mean
Temperature |
The average of the maximum and minimum
temperature of a process equilibrium. |
| Measurand |
A physical quantity, property, or
condition which is measured. |
| Measuring
Junction |
The thermocouple junction referred to as
the hot junction that is used to measure
an unknown temperature. |
| Mechanical
Hysteresis |
The difference of the indication with
increasing and decreasing strain loading,
at identical strain values of the
specimen. |
| Medium
Effect |
For solvents other than water the medium
effect is the activity coefficient related
to the standard state in water at zero
concentration. It reflects differences in
the electrostatic and chemical
interactions of the ions with the
molecules of various solvents. Solvation
is the most significant interaction. |
| Melting
Point |
The temperature at which a substance
transforms from a solid phase to a liquid
phase. |
| Membrane |
The pH-sensitive glass bulb is the
membrane across which the potential
difference due to the formation of double
layers with ion-exchange properties on the
two swollen glass surfaces is developed.
The membrane makes contact with and
separates the internal element and filling
solution from the sample solution. |
| Method
of Correction |
A procedure whereby the mass
distribution of a rotor is adjusted to
reduce unbalance, or vibration due to
unbalance, to an acceptable value.
Corrections are usually made by adding
material to, or removing it from, the
rotor. |
| Mica |
A transparent mineral used as window
material in high-temperature ovens. |
| Microamp |
One millionth of an ampere, 10-6 amps,
µA. |
| Microcomputer |
A computer which is physically small. It
can fit on top of or under a desk; based
on LSI circuitry, computers of this type
are now available with much of the power
currently associated with minicomputer
systems. |
| Micron |
One millionth of a meter, 10-6 meters. |
| Microvolt |
One millionth of a volt, 10-6 volts. |
| Mil |
One thousandth of an inch (0.001"). |
| Milliamp |
One thousandth of an amp, 10-3 amps,
symbol mA. |
| Millimeter |
One thousandth of a meter, symbol mm. |
| Millivolt |
Unit of electromotive force. It is the
difference in potential required to make a
current of 1 millampere flow through a
resistance of 1 ohm; one thousandth of a
volt, symbol mV. |
| Insulated
Thermocouple |
A type of thermocouple cable which has
an outer metal sheath and mineral
(magnesium oxide) insulation inside
separating a pair of thermocouple wires
from themselves and from the outer sheath.
This cable is usually drawn down to
compact the mineral insulation and is
available in diameters from 0.375 to 0.010
inches. It is ideally suited for
high-temperature and severe-duty
applications. |
| Minor
Scale Division |
On an analog scale, the smallest
indicated division of units on the scale. |
| Modem |
Modulator/Demodulator. A device that
transforms digital signals into audio
tones for transmission over telephone
lines, and does the reverse for reception. |
| Molality |
A measure of concentration expressed in
mols per kilogram of solvent. |
| Monovalent
Ion |
An ion with a single positive or
negative charge (H+, C1-). |
| Motherboard |
The pc board of a computer that contains
the bus lines and edge connectors to
accommodate other boards in the system. In
a microcomputer, the motherboard contains
the microprocessor and connectors for
expansion boards. |
| Mounting
Error |
The error resultant from installing the
transducer, both electrical and
mechanical. |
| MSD |
(Most-Significant Digit) The leftmost
digit of the display. |
| Mueller
Bridge |
A
high-accuracy bridge configuration used to
measure three-wire RTD thermometers. |
| Multiplex |
A technique which allows different input
(or output) signals to use the same lines
at different times, controlled by an
external signal. Multiplexing is used to
save on wiring and I/O ports. |
|
Back to
Index |
| N |
|
| N/C (No
Connection) |
A connector point for which there is no
internal connection. |
| NEC |
National Electric Code. |
| Negative
Temp Coefficient |
A decrease in resistance with an
increase in temperature. |
| NEMA-4 |
A standard from the National Electrical
Manufacturers Association, which defines
enclosures intended for indoor or outdoor
use primarily to provide a degree of
protection against windblown dust and
rain, splashing water, and hose-directed
water. |
| NEMA-7 |
A standard from the National Electrical
Manufacturers Association, which defines
explosion-proof enclosures for use in
locations classified as Class I, Groups A,
B, C or D, as specified in the National
Electrical Code. |
| NEMA-12 |
A standard from the National Electrical
Manufacturers Association, which defines
enclosures with protection against dirt,
dust, splashes by non-corrosive liquids,
and salt spray. |
| NEMA-Size
Case |
An older US case standard for panel
meters, which requires a panel cutout of
3.93 x 1.69 inches. |
| Nernst
Equation |
A mathematical description of electrode
behavior: E is the total potential, in
millivolts, developed between the sensing
and reference electrodes; Ex varies with
the choice of electrodes, temperature, and
pressure: 2.3RT/nF is the Nernst factor (R
and F are constants, n is the charge on
the ion, including sign, T is the
temperature in degrees Kelvin), and ai is
the activity of the ion to which the
electrode is responding. |
| Nernst
Factor |
(S, Slope) The term 2.3RT/nF is the
Nernst equation, which is equal (at T = 25
°C) to 59.16 mV when n = 1 and 29.58 mV
when n - 2, and which includes the sign of
the charge on the ion in the term n. The
Nerst factor varies with temperature. |
| Network |
A group of computers that are connected
to each other by communications lines to
share information and resources. |
| Nibble |
One half of a byte. |
| Nicrosil/Nisil |
A nickel chrome/nickel silicone thermal
alloy used to measure high temperatures.
Inconsistencies in thermoelectric voltages
exist in these alloys with respect to the
wire gage. |
| NMR |
(Normal-Mode
Rejection) The ability of a panel meter to
filter out noise superimposed on the
signal and applied across the SIG HI to
SIG LO input terminals. Normally expressed
in dB at 50/60 Hz. |
| Noise |
An unwanted electrical interference on
the signal wires. |
| Normal
(axial) Stress |
The force per unit area on a given plane
within a body a = F/A |
| Normal
Hydrogen Electrode |
A reversible hydrogen electrode (Pt) in
contact with hydrogen gas at 1 atmosphere
partial pressure and immersed in a
solution containing hydrogen ions at unit
activity. |
| Normal-mode
Reject Ratio |
The ability of an instrument to reject
interference usually of line frequency
(50-60 Hz) across its input terminals. |
| NPT |
National Pipe Thread. |
| Null |
A condition, such as balance, which
results in a minimum absolute value of
output. |
|
Back to
Index |
| O |
|
| O.D |
Outside
diameter. |
| Octal |
Pertaining to a base 8 number system. |
| Offset |
The difference in temperature between
the set point and the actual process
temperature. Also, referred to as droop. |
| ofhc |
Oxygen-free high-conductivity copper.
The industrial designation of the pure
copper used in a Type T thermocouple. |
| Ohmeter |
An instrument used to measure electrical
resistance. |
| On/off
Controller |
A controller whose action is fully on or
fully off. |
| Open
Circuit |
The lack of electrical contact in any
part of the measuring circuit. An open
circuit is usually characterized by rapid
large jumps in displayed potential,
followed by an off-scale reading. |
| Operating
System |
A collection of programs that controls
the overall operation of a computer and
performs such tasks as assigning places in
memory to programs and data, processing
interrupts, scheduling jobs and
controlling the overall input/output of
the system. |
| Operational
pH |
The determination of sample pH by
relating to pH measurements in a primary
standard solution. This relationship
assumes that electrode errors such as
sensitivity and changes in asymmetry
potential can be disregarded or
compensated for, provided the liquid
junction potential remains constant
between standard and sample. |
| Optical
Isolation |
Two networks which are connected only
through an LED transmitter and
photoelectric receiver with no electrical
continuity between the two networks. |
| Outboard
Rotor |
A two-journal rotor which has its center
of gravity between the journals. |
| Output
Impedance |
The resistance as measured on the output
terminals of a pressure transducer. |
| Output
Noise |
The RMS, peak-to-peak (as specified) ac
component of a transducer's dc output in
the absence of a measurand variation. |
| Output |
The electrical signal which is produced
by an applied input to the transducer. |
| Overshoot |
The number of degrees that a process
exceeds the set point temperature when
coming up to the set point temperature. |
|
Back to
Index |
| P |
|
| Parallax |
An optical illusion which occurs in
analog meters and causes reading errors.
It occurs when the viewing eye is not in
the same plane, perpendicular to the meter
face, as the indicating needle. |
| Parallel
Transmission |
Sending all data bits simultaneously.
Commonly used for communications between
computers and printer devices. |
| Parity |
A technique for testing transmitting
data. Typically, a binary digit is added
to the data to make the sum of all the
digits of the binary data either always
even (even parity) or always odd (odd
parity). |
| Peltier
Effect |
When a current flows through a
thermocouple junction, heat will either be
absorbed or evolved depending on the
direction of current flow. This effect is
independent of joule I2 R heating. |
| Perfectly
Balanced Rotor |
A rotor is perfectly balanced when its
mass distribution is such that it
transmits no vibratory force or motion to
its bearings as a result of centrifugal
forces. |
| Peripheral |
A device that is external to the CPU and
main memory, i.e., printer, modem or
terminal, but is connected by the
appropriate electrical connections. |
| pH
Junctions |
The Junction of a reference electrode or
combination electrode is a permeable
membrane through which the fill solution
escapes (called the liquid junction). |
| pH(S)
(Standard pH Scale) |
The conventional standard pH scale
established on the basis that an
individual ionic activity coefficient can
be calculated from the Debye-H¸ckel law
for primary buffers. |
| Phase
Difference |
The time expressed in degrees between
the same reference point on two periodic
waveforms. |
| Phase
Proportioning |
A form of temperature control where the
power supplied to the process is
controlled by limiting the phase angle of
the line voltage. |
| Phase |
A time based relationship between a
periodic function and a reference. In
electricity, it is expressed in angular
degrees to describe the voltage or current
relationship of two alternating waveforms. |
| PID |
Proportional, integral, derivative. A
three mode control action where the
controller has time proportioning,
integral (auto reset) and derivative rate
action. |
| Piezoelectric
Accelerometer |
A
transducer that produces an electrical
charge in direct proportion to the
vibratory acceleration. |
| Piezoresistance |
Resistance that changes with stress. |
| Pixel |
Picture element. Definable locations on
a display screen that are used to form
images on the screen. For graphic
displays, screens with more pixels provide
higher resolution. |
| Plane
Separation |
Of a balancing machine, is the operation
of reducing the correction plane
interference ratio for a particular rotor. |
| Platinel |
A non-standard, high temperature
platinum thermocouple alloy whose
thermoelectric voltage nearly matches a
Type K thermocouple (Trademark of
Englehard Industries). |
| Platinum
6% Rhodium |
The platinum-rhodium alloy used as the
negative wire in conjunction with
platinum-30% rhodium to form a Type B
thermocouple. |
| Platinum
10% Rhodium |
The platinum-rhodium alloy used as the
positive wire in conjunction with pure
platinum to form a Type S thermocouple. |
| Platinum
13% Rhodium |
The platinum-rhodium alloy used as the
positive wire in conjunction with pure
platinum to form a Type R thermocouple. |
| Platinum
30% Rhodium |
The platinum-rhodium alloy used as the
positive wire in conjunction with platinum
6% rhodium to form a Type B thermocouple. |
| Platinum
67 |
To develop thermal emf tables for
thermocouples, the National Bureau of
Standards paired each thermocouple alloy
against a pure platinum wire (designated
Platinum 2 prior to 1973, and currently
Platinum 67). The thermal emf's of any
alloy combination can be determined by
summing the "vs. Pt-67" emf's of the
alloys, i.e., the emf table for a Type K
thermocouple is derived from the Chromel
vs. Pt-67 and the Alumel vs. Pt-67 values. |
| Platinum |
A noble metal which in its pure form is
the negative wire of Type R and Type S
thermocouples. |
| Poisson
Ratio |
The ratio between the strain of
expansion in the direction of force and
the strain of contraction perpendicular to
that force v = -Et/E1. |
| Polarity |
In
electricity, the quality of having two
oppositely charged poles, one positive one
negative. |
| Polarization |
The inability of an electrode to
reproduce a reading after a small
electrical current has been passed through
the membrane. Glass pH electrodes are
especially prone to polarization errors
caused by small currents flowing from the
pH meter input circuit and from static
electrical charges built up as the
electrodes are removed from the sample
solution, or when the electrodes are
wiped. |
| Port |
A signal input (access) or output point
on a computer. |
| Positive
Temp Coefficient |
An increase in resistance due to an
increase in temperature. |
| Potential
Energy |
Energy
related to the position or height above a
place to which fluid could possibly flow. |
| Potentiometer |
(1) A variable resistor often used to
control a circuit. (2) A balancing bridge
used to measure voltage. |
| Power
Supply |
A separate unit or part of a circuit
that supplies power to the rest of the
circuit or to a system. |
| PPM |
Abbreviation for "parts per million,"
sometimes used to express temperature
coefficients. For instance, 100 ppm is
identical to 0.01%. |
| Primary
Device |
Part of a flowmeter which is mounted
internally or externally to the fluid
conduit and produces a signal
corresponding to the flowrate and from
which the flow may be determined. |
| Primary
Standards |
Aqueous pH buffer solutions established
by the National Bureau of Standards within
the 2.5 to 11.5 pH range of ionic strength
less than 0.1 and which provide stable
liquid junction potential and uniformity
of electrode sensitivity. |
| Principal
Axes |
The axes of maximum and minimum normal
stress. |
| Probe |
A generic term that is used to describe
many types of temperature sensors. |
| Process
Meter |
A panel meter with sizeable zero and
span adjustment capabilities, which can be
scaled for readout in engineering units
for signals such as 4-20 mA, 10-50 mA and
1-5 V. |
| Program |
A list of instructions that a computer
follows to perform a task. |
| Prom |
Programmable read-only memory. A
semiconductor memory whose contents cannot
be changed by the computer after it has
been programmed. |
| Proof
Pressure |
The specified pressure which may be
applied to the sensing element of a
transducer without causing a permanent
change in the output characteristics. |
| Proportioning
Band |
A temperature band expressed in degrees
within which a temperature controller's
time proportioning function is active. |
| Proportioning
Control Mode |
A time proportioning controller where
the amount of time that the relay is
energized is dependent upon the system's
temperature. |
| Proportioning
Control plus Derivative Function |
A time proportioning controller with
derivative function. The derivative
function senses the rate at which a
system's temperature is either increasing
or decreasing and adjusts the cycle time
of the controller to minimize overshoot or
undershoot. |
| Proportioning
Control plus Integral |
A two-mode controller with time
proportioning and integral (auto reset)
action. The integral function
automatically adjusts the temperature at
which a system has stabilized back to the
setpoint temperature, thereby eliminating
droop in the system. |
| Proportioning
Control with Integral and Derivative
Functions |
Three mode PID controller. A time
proportioning controller with integral and
derivative functions. The integral
function automatically adjusts the system
temperature to the set point temperature
to eliminate droop due to the time
proportioning function. The derivative
function senses the rate of rise or fall
of the system temperature and
automatically adjusts the cycle time of
the controller to minimize overshoot or
undershoot. |
| Protection
Head |
An enclosure usually made out of metal
at the end of a heater or probe where
connections are made. |
| Protection
Tube |
A metal or ceramic tube, closed at one
end into which a temperature sensor is
inserted. The tube protects the sensor
from the medium into which it is inserted. |
| Protocol |
A formal definition that describes how
data is to be exchanged. |
| PSIA |
Pounds per square inch absolute.
Pressure referenced to a vacuum. |
| PSID |
Pounds per square inch differential.
Pressure difference between two points. |
| PSIG |
Pound per square inch gauge. Pressure
referenced to ambient air pressure. |
| PSIS |
Pounds per square inch standard.
Pressure referenced to a standard
atmosphere. |
| Pulse
Width Modulation |
An output in the form of duty cycle
which varies as a function of the applied
measurand. |
|
Back to
Index |
| R |
|
| Radiation |
See Infrared |
| RAM |
(Random Access Memory) Memory that can
be both read and changed during computer
operation. Unlike other semi-conductor
memories, RAM is volatile-if power to the
RAM is disrupted or lost, all the data
stored is lost. |
| Range |
Those values over which a transducer is
intended to measure, specified by its
upper and lower limits. |
| Rangeability |
The ratio of the maximum flowrate to the
minimum flowrate of a meter. |
| Rankine
(°R) |
An absolute temperature scale based upon
the Fahrenheit scale with 180° between the
ice point and boiling point of water.
459.67°R = 0°F. |
| Rate
Action |
The derivative function of a temperature
controller. |
| Rate
time |
The time interval over which the system
temperature is sampled for the derivative
function. |
| Ratiometric
Measurement |
A measurement technique where an
external signal is used to provide the
voltage reference for the dual-slope A/D
converter. The external signal can be
derived from the voltage excitation
applied to a bridge circuit or pick-off
supply, thereby eliminating errors due to
power supply fluctuations. |
|
ROM |
(Read Only Memory) Memory that contains
fixed data. The computer can read the
data, but cannot change it in any way. |
| Real
Time |
The time interval over which the system
temperature is sampled for the derivative
function. |
| Record |
A collection of unrelated information
that is treated as a single unit. |
| Recovery
Time |
The length of time which it takes a
transducer to return to normal after
applying a proof pressure. |
| Redox
Potential |
The potential developed by a metallic
electrode when placed in a solution
containing a species in two different
oxidation states. |
| Reference
Junction |
The cold junction in a thermocouple
circuit which is held at a stable known
temperature. The standard reference
temperature is 0°C (32°F). However, other
temperatures can be used. |
| Reference
Mark |
Any diagnostic point or mark which can
be used to relate a position during
rotation of a part to its location when
stopped. |
| Reference
Plane |
Any plane perpendicular to the shaft
axis to which an amount of unbalance is
referred. |
| Refractory
Metal Thermocouple |
A class of thermocouples with melting
points above 3600°F. The most common are
made from tungsten and tungsten/rhenium
alloys Types G and C. They can be used for
measuring high temperatures up to 4000°F
(2200°C) in non-oxidizing, inert, or
vacuum environments. |
| Register |
A storage device with a specific
capacity, such as a bit, byte or word. |
| Relay
(Mechanical) |
An electromechanical device that
completes or interrupts a circuit by
physically moving electrical contacts into
contact with each other. |
| Relay
(Solid State) |
A solid state switching device which
completes or interrupts a circuit
electrically with no moving parts. |
| Remote |
Not hard-wired; communicating via
switched lines, such as telephone lines.
Usually refers to peripheral devices that
are located a site away from the CPU. |
| Repeatability |
The ability of a transducer to reproduce
output readings when the same measurand
value is applied to it consecutively,
under the same conditions, and in the same
direction. Repeatability is expressed as
the maximum difference between output
readings. |
| Reserved
Word |
A word that has a defined function in
the language, and cannot be used as a
variable name. |
| Residual
(Final) Unbalance |
Residual unbalance is that unbalance of
any kind that remains after balancing. |
| Resistance
Ratio Characteristic |
For thermistors, the ratio of the
resistance of the thermistor at 25 °C to
the resistance at 125 °C. |
| Resistance
Temperature Characteristic |
A relationship between a thermistor's
resistance and the temperature. |
| Resistance |
The resistance to the flow of electric
current measured in ohms (1/2) for a
conductor. Resistance is function of
diameter, resistivity (an intrinsic
property of the material) and length. |
| Resolution |
The smallest detectable increment of
measurement. Resolution is usually limited
by the number of bits used to quantize the
input signal. For example, a 12-bit A/D
can resolve to one part in 4096 (2 to the
12 power equals 4096). |
| Resonant
Frequency |
The measurand frequency at which a
transducer responds with maximum
amplitude. |
| Response
Time |
(time
constant) The time required by a sensor to
reach 63.2% of a step change in
temperature under a specified set of
conditions. Five time constants are
required for the sensor to stabilize at
600 of the step change value. |
| Response
Time |
The length of time required for the
output of a transducer to rise to a
specified percentage of its final value as
a result of a step change of input. |
| Reynolds
Number |
The ratio of inertial and viscous forces
in a fluid defined by the formula Re =
rVD/µ, where: r = Density of fluid, µ =
Viscosity in centipoise (CP), V =
Velocity, and D = Inside diameter of pipe. |
| RFI |
Radio frequency interference. |
| Rheostat |
A variable resistor. |
| Rigid
Rotor |
A rotor is considered rigid when it can
be corrected in any two (arbitrarily
selected) planes [see "Correction
(Balancing)Plane"] and after that
correction, its unbalance does not
significantly exceed the balancing
tolerances (relative to the shaft axis) at
any speed up to maximum operating speed
and when running under conditions which
approximate closely to those of the final
supporting system. |
| Rise
Time |
The time required for a sensor or system
to respond to an instantaneous step
function, measured from the 10% to 90%
points on the response waveforms. |
| Room
Conditions |
Ambient environmental conditions under
which transducers must commonly operate. |
| Root
Mean Square (RMS) |
Square root of the mean of the square of
the signal taken during one full cycle. |
| Rotor |
A rotor is a rotating body whose
journals are supported by bearings. |
| RTD |
Resistance temperature detector. |
|
Back to
Index |
| S |
|
| Salt
Bridge |
The salt bridge of a reference electrode
is that part of the electrode which
contains the filling solution to establish
the electrolytic connection between
reference internal cell and the test
solution. Auxiliary Salt Bridge: A glass
tube open at oneend to receive
intermediate electrolyte filling solution,
and the reference electrode tip and a
junction at the other end to make contact
with the sample. |
| Salt
Effect (fx) |
The effect on the activity coefficient
due to salts in the solution. |
| SAMA |
Scientific Apparatus Makers Association.
An association that has issued standards
covering platinum, nickel, and copper
resistance elements (RTDs). |
| SCE |
Saturated calomel electrode. |
| SCR |
Silicone controlled rectifier. |
| Scroll |
To move all or part of the screen
material up to down, left or right, to
allow new information to appear. |
| Secondary
Device |
A part of the flowmeter which receives a
signal proportional to the flowrate, from
the primary device, and displays, records
and/or transmits the signal. |
| Secondary
Standard |
pH buffer solutions which do not meet
the requirements of primary standard
solutions but provide coverage of the pH
range not covered by primary standards.
Used when the pH value of the primary
standard is not close to the sample pH
value. |
| Seebeck
Coefficient |
The derivative (rate of change) of
thermal EMF with respect to temperature
normally expressed as millivolts per
degree. |
| Seebeck
Effect |
When a circuit is formed by a junction
of two dissimilar metals and the junctions
are held at different temperatures, a
current will flow in the circuit caused by
the difference in temperature between the
two junctions. |
| Seebeck
EMF |
The open circuit voltage caused by the
difference in temperature between the hot
and cold junctions of a circuit made from
two dissimilar metals. |
| Self
Heating |
Internal
heating of a transducer as a result of
power dissipation. |
| Sensing
Element |
That part of the transducer which reacts
directly in response to the input. |
| Sensitivity
Shift |
A change in slope of the calibration
curve due to a change in sensitivity. |
| Sensitivity |
The minimum change in input signal to
which an instrument can respond. |
| Sequential
Access |
An access mode in which records are
retrieved in the same order in which they
were written. Each successive access to
the file refers to the next record in the
file. |
| Serial
Transmission |
Sending one bit at a time on a single
transmission line. Compare with parallel
transmission. |
| Set
Point |
The temperature at which a controller is
set to control a system. |
| Settling
Time |
The time taken for the display to settle
within one digit final value when a step
is applied to the meter input. |
| Shear
Modulus |
The ratio of the shear stress and the
angular shear distortion. |
| Shear
Stress |
Where normal stress is perpendicular to
the designated plane, shear stress is
parallel to the plane. |
| Shearing
Strain |
A measure of angular distortion also
directly measurable, but not as easily as
axial strain. |
| Sheath
Thermocouple |
A thermocouple made out of
mineral-insulated thermocouple cable which
has an outer metal sheath. |
| SI |
System Internationale. The name given to
the standard metric system of units. |
| Signal
Conditioner |
A circuit module which offsets,
attenuates, amplifies, linearizes and/or
filters the signal for input to the A/D
converter. The typical output signal
conditioner is +2 V dc. |
| Signal
Conditioning |
To process the form or mode of a signal
so as to make it intelligible to, or
compatible with, a given device, including
such manipulation as pulse shaping, pulse
clipping, compensating, digitizing, and
linearizing. |
| Signal |
An electrical transmittance (either
input or output) that conveys information. |
| Single
Precision |
The degree of numeric accuracy that
requires the use of one computer word. In
single precision, seven digits are stored,
and up to seven digits are printed.
Contrast with double precision. |
| Single-ended
Input |
A signal-input circuit where SIG LO (or
sometimes SIG HI) is tied to METER GND.
Ground loops are normally not a problem in
AC-powered meters, since METER GND is
transformer-isolated from AC GND. |
| Single-Plane
(Static) Balancing Machine |
A single plane balancing machine is a
gravitational or centrifugal balancing
machine that provides information for
accomplishing single plane balancing. |
| Slope |
(Electrode Sensitivity, Span) See Nernst
factor. |
| Smallest
Bending Radius |
The smallest radius that a strain gage
can withstand in one direction, without
special treatment, without suffering
visible damage. |
| Software |
Generally, programs loaded into a
computer from external mass storage but
also extended to include operating systems
and documentation. |
| Solvation |
Ions
in solution are normally combined with at
least one molecule of solvent. This
phenomenon is termed solvation. |
| Source
Code |
A non-executable program written in a
high-level language. A compiler or
assembler must translate the source code
into object code (machine language) that
the computer can understand and process. |
| Span
Adjustment |
The ability to adjust the gain of a
process or strain meter so that a
specified display span in engineering
units corresponds to a specified signal
span. For instance, a display span of
200°F may correspond to the 16 mA span of
a 4-20 mA transmitter signal. |
| Span |
The difference between the upper and
lower limits of a range expressed in the
same units as the range. |
| Spare |
A connector point reserved for options,
specials, or other configurations. The
point is identified by an (E#) for
location on the electrical schematic. |
| Specific
Gravity |
The ratio of mass of any material to the
mass of the same volume of pure water at
4°C. |
| Specific
Heat |
The ratio of thermal energy required to
raise the temperature of a body 1° to the
thermal energy required to raise an equal
mass of water 1°. |
| Spectral
Filter |
A filter which allows only a specific
band width of the electromagnetic spectrum
to pass, i.e., 4 to 8 micron infrared
radiation. |
| Spectrum
Analysis |
Utilizing frequency components of a
vibration signal to determine the source
and cause of vibration. |
| Spectrum |
The resolving of overall vibration into
amplitude components as a function of
frequency. |
| Spot
Size |
The diameter of the circle formed by the
cross section of the field of view of an
optical instrument at a given distance. |
| Spurious
Error |
Random or erratic malfunction. |
| SSR |
Solid state relay (see relay, solid
state). |
| Stability |
The quality of an instrument or sensor
to maintain a consistent output when a
constant input is applied. |
| Stagnation
Pressure |
The sum of the static and dynamic
pressure. |
| Standard
Electrode Potential |
(E0) The standard potential E0 of an
electrode is the reversible emf between
the normal hydrogen electrode and the
electrode with all components at unit
activity. |
| Standardization |
A process of equalizing electrode
potentials in one standardizing solution
(buffer) so that potentials developed in
unknown solutions can be converted to pH
values. |
| Static
Calibration |
A calibration recording pressure versus
output at fixed points at room
temperature. |
| Static
Error Band |
The error band applicable at room
temperature. |
| Static
Pressure |
Pressure of a fluid whether in motion or
at rest. It can be sensed in a small hole
drilled perpendicular to and flush with
the flow boundaries so as not to disturb
the fluid in any way. |
| Static
Unbalance |
Static unbalance is that condition of
unbalance for which the central principal
axis is displayed only parallel to the
shaft axis |
| Steady
Flow |
A flow rate in the measuring section of
a flow line that does not vary
significantly with time. |
| Steady
State Vibration |
That condition of vibration induced by
an unchanging continuing periodic force. |
| Stiffness |
The ratio of the force required to
create a certain deflection or movement of
a part expressed as (Force/deflection)
lbs/in or grams/cm. |
| Stop
Bit |
A signal following a character or block
that prepares the receiving device to
receive the next character or block. |
| Strain
Gage |
A measuring element for converting
force, pressure, tension, etc., into an
electrical signal. |
| Strain |
The ratio of the change in length to the
initial unstressed reference length. |
| String |
A sequence of characters. |
| Strouhal
Number |
A nondimensional parameter important in
vortex meter design defined as: s = Fh/V
where f = frequency, V = velocity, and h =
a reference length |
| Super
Cooling |
The cooling of a liquid below its
freezing temperature without the formation
of the solid phase. |
| Super
Heating |
(1) The heating of a liquid above its
boiling temperature without the formation
of the gaseous phase. (2) The heating of
the gaseous phase considerably above the
boiling-point temperature to improve the
thermodynamic efficiency of a system. |
| Surge
Current |
A current of short duration that occurs
when power is first applied to capacitive
loads or temperature dependent resistive
loads such as tungsten or molybdenum
heaters-usually lasting no more than
several cycles. |
| Suspension
Effect |
The source of error due to varied
reference liquid junction potential
depending upon whether the electrodes are
immersed in the supernatant fluid or
deeper in the sediment. Normally
encountered with solutions containing
resins or charged colloids. |
| Syntax |
The rules governing the structure of a
language. |
|
Back to
Index |
| T |
|
| Teflon |
A fluorocarbon polymer used for
insulation of electrical wires (trademark
of DuPont Company). |
| TEMPCO |
Abbreviation for "temperature
coefficient": the error introduced by a
change in temperature. Normally expressed
in %/°C or ppm/°C. |
| Temperature
Error |
The maximum change in output, at any
measurand value within the specified
range, when the transducer temperature is
changed from room temperature to specified
temperature extremes. |
| Temperature
Range, Compensated |
The range of ambient temperatures within
which all tolerances specified for Thermal
Zero Shift and Thermal Sensitivity Shift
are applicable (temperature error). |
| Temperature
Range, Operable |
The range of ambient temperatures, given
by their extremes, within which the
transducer may be operated. Exceeding
compensated range may require
recalibration. |
| Terminal |
An input/output device used to enter
data into a computer and record the
output. |
| Thermal
Coefficient of Resistance |
The change in resistance of a
semiconductor per unit change in
temperature over a specific range of
temperature. |
| Thermal
Conductivity |
The property of a material to conduct
heat in the form of thermal energy. |
| Thermal
emf |
See Seebeck emf |
| Thermal
Expansion |
An increase in size due to an increase
in temperature expressed in units of an
increase in length or increase in size per
degree, i.e. inches/inch/degree C. |
| Thermal
Gradient |
The distribution of a differential
temperature through a body or across a
surface. |
| Thermal
Sensitivity Shift |
The sensitivity shift due to changes of
the ambient temperature from room
temperature to the specified limits of the
compensated temperature range. |
| Thermal
Zero Shift |
An error due to changes in ambient
temperature in which the zero pressure
output shifts. Thus, the entire
calibration curve moves in a parallel
displacement. |
| Thermistor |
A temperature-sensing element composed
of sintered semiconductor material which
exhibits a large change in resistance
proportional to a small change in
temperature. Thermistors usually have
negative temperature coefficients. |
| Thermocouple |
The junction of two dissimilar metals
which has a voltage output proportional to
the difference in temperature between the
hot junction and the lead wires (cold
junction) (refer to Seebeck emf). |
| Thermopile |
An arrangement of thermocouples in
series such that alternate junctions are
at the measuring temperature and the
reference temperature. This arrangement
amplifies the thermoelectric voltage.
Thermopiles are usually used as infrared
detectors in radiation pyrometry. |
| Thermowell |
A closed-end tube designed to protect
temperature sensors from harsh
environments, high pressure, and flows.
They can be installed into a system by
pipe thread or welded flange and are
usually made of corrosion-resistant metal
or ceramic material depending upon the
application. |
| Thomson
Effect |
When current flows through a conductor
within a thermal gradient, a reversible
absorption or evolution of heat will occur
in the conductor at the gradient
boundaries. |
| Transducer
Vibration |
Generally, any device which converts
movement, either shock or steady state
vibration, into an electrical signal
proportional to the movement; a sensor. |
| Transducer |
A device (or medium) that converts
energy from one form to another. The term
is generally applied to devices that take
physical phenomenon (pressure,
temperature, humidity, flow, etc.) and
convert it to an electrical signal. |
| Transient
Vibration |
A temporary vibration or movement of a
mechanical system. |
| Transitional
Flow |
Flow between laminar and turbulent flow,
usually between a pipe Reynolds number of
2000 and 4000. |
| Transmitter
(Two-Wire) |
(1) A device which is used to transmit
data from a sensor via a two-wire current
loop. The loop has an external power
supply and the transmitter acts as a
variable resistor with respect to its
input signal. (2) A device which
translates the low level output of a
sensor or transducer to a higher level
signal suitable for transmission to a site
where it can be further processed. |
| Triac |
A solid state switching device used to
switch alternating current wave forms. |
| Triboelectric
Noise |
The generation of electrical charges
caused by layers of cable insulation. This
is especially troublesome in high
impedance accelerometers. |
| Triple
Point (Water) |
The thermodynamic state where all three
phases, solid, liquid, and gas may all be
present in equilibrium. The triple point
of water is .01°C. |
| Triple
Point |
The temperature and pressure at which
solid, liquid, and gas phases of a given
substance are all present simultaneously
in varying amounts. |
| True
RMS |
The true root-mean-square value of an AC
or AC-plus-DC signal, often used to
determine power of a signal. For a perfect
sine wave, the RMS value is 1.11072 times
the rectified average value, which is
utilized for low-cost metering. For
significantly non-sinusoidal signals, a
true RMS converter is required. |
| TTL
Unit Load |
A load with TTL voltage levels, which
will draw 40 µA for a logic 1 and -1.6 mA
for a logic 0. |
| TTL-Compatible |
For digital input circuits, a logic 1 is
obtained for inputs of 2.0 to 5.5 V which
can source 40 µA, and a logic 0 is
obtained for inputs of 0 to 0.8 V which
can sink 1.6 mA. For digital output
signals, a logic 1 is represented by 2.4
to 5.5 V with a current source capability
of at least 400 µA; and a logic 0 is
represented by 0 to 0.6 V with a current
sink capability of at least 16 mA. |
| TTL |
Transistor-to-transistor logic. A form
of solid state logic which uses only
transistors to form the logic gates. |
| Turbulent
Flow |
When forces due to inertia are more
significant than forces due to viscosity.
This typically occurs with a Reynolds
number in excess of 4000. |
| Typical |
Error is within plus or minus one
standard deviation (±1%) of the nominal
specified value, as computed from the
total population. |
|
Back to
Index |
| U |
|
| UL |
Underwriters Laboratories, Inc. An
independent laboratory that establishes
standards for commercial and industrial
products. |
| Ultraviolet |
That portion of the electromagnetic
spectrum below blue light (380
nanometers). |
| Unbalance |
That condition which exists in a rotor
when vibratory force or motion is imparted
to its bearings as a result of centrifugal
forces. |
| Unbalance
Tolerance |
The unbalance tolerance with respect to
a radial plane (measuring plane or
correction plane) is that amount of
unbalance which is specified as the
maximum below which the state of unbalance
is considered acceptable. |
| Undershoot |
The difference in temperature between
the temperature a process goes to, below
the set point, after the cooling cycle is
turned off and the set point temperature. |
| Ungrounded
Junction |
A form of construction of a thermocouple
probe where the hot or measuring junction
is fully enclosed by and insulated from
the sheath material. |
| Union |
A form of pipe fitting where two
extension pipes are joined at a separable
coupling. |
|
Back to Index |
| V |
|
| Vacuum |
Any pressure less than atmospheric
pressure. |
| Velocity |
The time rate of change of displacement;
dx/dt. |
| Vibration
Error Band |
The error recorded in output of a
transducer when subjected to a given set
of amplitudes and frequencies. |
| Vibration
Error |
The maximum change in output of a
transducer when a specific amplitude and
range of frequencies are applied to a
specific axis at room temperature. |
| Viscosity |
The inherent resistance of a substance
to flow. |
| Volt |
The (electrical) potential difference
between two points in a circuit. The
fundamental unit is derived as work per
unit charge-(V = W/Q). One volt is the
potential difference required to move one
coulomb of charge between two points in a
circuit while using one joule of energy. |
| Voltage |
An electrical potential which can be
measured in volts. |
| Voltmeter |
An instrument used to measure voltage. |
| Volume
Flow Rate |
Calculated using the area of the full
closed conduit and the average fluid
velocity in the form, Q = V x A, to arrive
at the total volume quantity of flow. Q =
volumetric flowrate, V = average fluid
velocity, and A = cross sectional area of
the pipe. |
|
Back to Index |
| W |
|
| Watt
Density |
The watts emanating from each square
inch of heated surface area of a heater.
Expressed in units of watts per square
inch. |
| Wheatstone
Bridge |
A network of four resistances, an emf
source, and a galvanometer connected such
that when the four resistances are
matched, the galvanometer will show a zero
deflection or "null" reading. |
| Word |
Number of bits treated as a single unit
by the CPU. In an 8-bit machine, the word
length is 8 bits; in a sixteen bit
machine, it is 16 bits. |
| Working
Standard |
A standard of unit measurement
calibrated from either a primary or
secondary standard which is used to
calibrate other devices or make comparison
measurements. |
|
Back to
Index |
| Y |
|
| Young's
Modulus |
Young's Modulus (the Modulus of
Elasticity) is equivalent to the ratio of
normal stress to strain. |
|
Back to
Index |
| Z |
|
| Zero
Adjustment |
The ability to adjust the display of a
process or strain meter so that zero on
the display corresponds to a non-zero
signal, such as 4 mA, 10 mA, or 1 V dc.
The adjustment range is normally expressed
in counts. |
| Zero
Offset |
(1) The difference expressed in degrees
between true zero and an indication given
by a measuring instrument. (2) See Zero
Suppression |
| Zero
Point |
The electrical zero point where zero
millivolts would be displayed. Used in
conjunction with the slope control to
provide a narrower range calibration. |
|
|
| Zero
Power Resistance |
The resistance of a thermistor or RTD
element with no power being dissipated. |
|
|
| Zero
Suppression |
The span of an indicator or chart
recorder may be offset from zero (zero
suppressed) such that neither limit of the
span will be zero. For example, a
temperature recorder which records a 100°
span from 400° to 500° is said to have
400° zero suppression. |
|
|
| Zero
Voltage Switching |
The making or breaking of circuit timed
such that the transition occurs when the
voltage wave form crosses zero voltage;
typically only found in solid state
switching devices. |
