GLOSSARY OF IMPORTANT TERMS
Absolute Pressure Transducer:
A transducer which measures pressure in relation to zero pressure (a vacuum on
one side of the diaphragm).
pressure plus atmospheric pressure.
at which thermal energy is at a minimum. Defined as 0 Kelvin, calculated to be
-273.15°C or -459.67°F.
Acceleration: A 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".
Accelerometer: A device which converts the
effects of mechanical motion into an electrical signal that is proportional to
the acceleration value of the motion. A sensor. A transducer.
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.
Acoustics: The degree of sound. The nature,
cause, and phenomena of the vibrations of elastic bodies; which vibrations
create compressional waves or wave fronts which are transmitted through various
media, such as air, water, wood, steel, etc.
Alumel: An aluminum nickel alloy used in
the negative leg of a Type K thermocouple (Trade name of Hoskins Manufacturing
design of an instrument such that changes in ambient temperature do not affect
the readings of the instrument.
conditions around the transducer (pressure, temperature, etc.).
Pressure of the air surrounding a transducer.
average or mean temperature of the surrounding air which comes in contact with
the equipment and instruments under test.
Bearing: A part which supports a journal and in which a journal
BTU: British thermal units. The quantity of thermal energy required to
raise one pound of water at its maximum density, 1 degree F. One BTU is
equivalent to .293 watt hours, or 252 calories. One kilowatt hour is equivalent
to 3412 BTU.
Calorie: The quantity of thermal energy
required to raise one gram of water 1°C at 15°C.
Cation: A positively charged ion (Na+,
Cavitation: The boiling of a liquid caused by
a decrease in pressure rather than an increase in temperature.
temperature scale defined by 0°C at the ice point and 100°C at boiling point of
water at sea level.
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.
Force: A force
exerted on an object moving in a circular path which is exerted inward toward
the center of rotation.
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.
Conduction: The conveying of electrical
energy or heat through or by means of a conductor.
Level: The range
(with a specified value of uncertainty, usually expressed in percent) within
which the true value of a measured quantity exists.
thermocouples and RTDs, the difference between the actual reading and the temperature
shown in published tables for a specific voltage input.
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.
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.
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
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.
to the force of attraction between two opposite charges separated by a distance
in a uniform medium.
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.
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
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 milli inches.
Calibration in which the input varies over a specific length of time and the
output is recorded vs. time.
difference in pressure levels from static pressure to stagnation pressure
caused by an increase in velocity. Dynamic pressure increases by the square of
Dynamic unbalance is that condition in which the central principal axis is not
coincident with the shaft axis.
Echo: To reflect received data to the
sender. For example, keys depressed on a keyboard are usually echoed as
characters displayed on the screen.
Electrode: See Isopotential point.
Electrolyte: Any substance which, when in
solution will conduct an electric current. Acids, bases, and salts are common
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.
Point (Potentiometric): The apparent equivalence point of a titration at which a relatively
large potential change is observed.
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.
conditions in which a transducer may be exposed during shipping, storage,
handling, and operation.
Exothermic: Gives off heat. A process is said
to be exothermic when it releases heat
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
An assembly of measuring instruments for performing balancing operations on
assembled machinery which is not mounted in a balancing machine.
speed or velocity of fluid movement .
Flow: Travel of liquids or gases in
response to a force (i.e. pressure or gravity).
meter: A device
used for measuring the flow or quantity of a moving fluid.
FPM: Flow velocity in feet per minute.
FPS: Flow velocity in feet per second.
temperature at which the substance goes from the liquid phase to the solid
of Vibration: The
number of cycles occurring in a given unit of time. RPM - revolutions per
minute. CPM- cycles per minute.
Output: An output
in the form of frequency which varies as a function of the applied input.
frequency of free (not forced) oscillations of the sensing element of a fully
Frequency: The number of cycles over a
specified time period over which an event occurs. The reciprocal is called the
Factor: A measure
of the ratio of the relative change of resistance to the relative change in
length of a piezoresistive strain gage.
distance between two points where the measurement of strain occurs.
A transducer which measures pressure in relation to the ambient pressure.
Absolute pressure minus local atmospheric pressure
GPH: Volumetric flow rate in gallons
GPM: Volumetric flow rate in gallons
Loss: The loss of
pressure in a flow system measured using a length parameter (i.e., inches of
water, inches of mercury).
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.
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
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.
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.
(Hz): Units in which
frequency is expressed. Synonymous with cycles per second.
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).
Isothermal: A process or area that is a
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.
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).
to 1000 watts.
Hour (kwh): 1000
watt hours. Kilovolt amperes (kva): 1000 volt amps.
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
flow of a fluid where viscous forces are more significant than inertial forces,
generally below a Reynolds number of 2000.
Volumetric flow rate times density, i.e. pounds per hour or kilograms per
strain value where a deviation of more than ±5% occurs with respect to the mean
characteristic (diagram of resistance change vs strain).
Operating Temperature: The maximum temperature at which an instrument or sensor can be
Power Rating: The
maximum power in watts that a device can safely handle.
average of the maximum and minimum temperature of a processequilibrium.
Measurand: A physical quantity, property, or
condition which is measured.
thermocouple junction referred to as the hot junction that is used to measure
an unknown temperature.
difference of the indication with increasing and decreasing strain loading, at
identical strain values of the specimen.
temperature at which a substance transforms from a solid phase to a liquid
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.
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 .375 to .010
inches. It is ideally suited for high-temperature and severe-duty applications
Noise: An unwanted electrical
interference on the signal wires.
The force per unit area on a given plane within a body a = F/A
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.
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.
O.D.: Outside diameter.
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
PID: Proportional, integral,
derivative. A three mode control action where the controller has time
proportioning, integral (auto reset) and derivative rate action.
transducer that produces an electrical charge in direct proportion to the
Piezoresistance: Resistance that changes with
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.
Temperature Coefficient: An increase in resistance due to an increase in temperature.
related to the position or height above a place to which fluid could possibly
Potentiometer: 1. A variable resistor often used
to control a circuit. 2. A balancing bridge used to measure voltage.
Axes: The axes of
maximum and minimum normal stress.
specified pressure which may be applied to the sensing element of a transducer
without causing a permanent change in the output characteristics.
temperature band expressed in degrees within which a temperature controller's
time proportioning function is active.
Control Mode: A
time proportioning controller where the amount of time that the relay is
energized is dependent upon the system's temperature.
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.
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.
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.
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 gage.
Pressure referenced to ambient air pressure.
PSIS: Pounds per square inch standard.
Pressure referenced to a standard atmosphere.
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.
(°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.
derivative function of a temperature controller.
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
RTD: Resistance temperature detector.
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: Ssilicone controlled rectifier.
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.
derivative (rate of change) of thermal EMF with respect to temperature normally
expressed as millivolts per degree.
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.
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.
heating of a transducer as a result of power dissipation
ratio of the shear stress and the angular shear distortion.
Stress: Where normal
stress is perpendicular to the designated plane, shear stress is parallel to
Strain: A measure
of angular distortion also directly measurable, but not as easily as axial
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.
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.
ratio of mass of any material to the mass of the same volume of pure water at
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°.
Filter: A filter
which allows only a specific band width of the electromagnetic spectrum to
pass, i.e., 4 to 8 micron infrared radiation.
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.
diameter of the circle formed by the cross section of the field of view of an
optical instrument at a given distance.
Error: Random or
SSR: Solid state relay (see relay,
Stability: The quality of an instrument or
sensor to maintain a consistent output when a constant input is applied.
Pressure: The sum
of the static and dynamic pressure.
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
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.
calibration recording pressure versus output at fixed points at room
Error Band: The
error band applicable at room temperature.
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.
unbalance is that condition of unbalance for which the central principal axis
is displayed only parallel to the shaft axis
Flow: A flow rate
in the measuring section of a flow line that does not vary significantly with
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.
Gage: A measuring
element for converting force, pressure, tension, etc., into an electrical
Strain: The ratio of the change in length
to the initial unstressed reference length.
Coefficient of Resistance: The change in resistance of a semiconductor per unit change in
temperature over a specific range of temperature.
property of a material to conduct heat in the form of thermal energy.
emf: See Seebeck
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.
distribution of a differential temperature through a body or across a surface.
The sensitivity shift due to changes of the ambient temperature from room
temperature to the specified limits of the compensated temperature range.
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.
R Platinum/Platinum 13% Rhodium
S Platinum/Platinum 10% Rhodium
B Platinum 6% Rhodium/Platinum30% Rhodium
G* Tungsten/Tungsten 26% Rhenium
C* Tungsten 5% Rhenium/Tungsten 26% Rhenium
D* Tungsten 3% Rhenium/Tungsten 150 Rhenium
*Not ANSI symbols.
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
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
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
current flows through a conductor within a thermal gradient, a reversible
absorption or evolution of heat will occur in the conductor at the gradient
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.
temporary vibration or movement of a mechanical system.
between laminar and turbulent flow, usually between a pipe Reynolds number of
2000 and 4000.
(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.
generation of electrical charges caused by layers of cable insulation. This is
especially troublesome in high impedance accelerometers.
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.
temperature and pressure at which solid, liquid, and gas phases of a given
substance are all present simultaneously in varying amounts.
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.
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
Vacuum: Any pressure less than
Velocity: The time rate of change of
Viscosity: The inherent resistance of a
substance to flow.
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.
watts emanating from each square inch of heated surface area of a heater.
Expressed in units of watts per square inch.
Modulus (the Modulus of Elasticity) is equivalent to the ratio of normal stress
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.