The vibration monitoring industry has specialized jargon that is commonly used when talking about vibration sensors, accelerometers and associated applications. It's important that you understand common terminology to communicate effectively with vibration monitoring, reliability, condition monitoring and process automation professionals. We hope you find our glossary useful to find specialized terms and definitions used across vibration, condition monitoring and predictive maintenance.

**ABSOLUTE PRESSURE**

The pressure measured relative to a perfect vacuum.

**ABSOLUTE PRESSURE TRANSDUCER**

A transducer that has an internal reference chamber sealed at or close to 0 psia (full vacuum).

**ACCELERATION**

A vector that specifies the time rate of change (derivative) of velocity.

**ACCELERATION OF GRAVITY**

Standard, by international agreement, g = 9.80665 m/s2 = 386.088 in/s2 = 32.174 ft/s2. Actual acceleration of gravity varies depending on altitude and latitude of location of measurement. Variation is less than +/- 0.5% in industrialized areas of the world, and less than 0.3% in the 48 contiguous states of the USA.

**ACCELERATION LIMIT (Environmental)**

The maximum acceleration level specified for an instrument to which it can be subjected without physical damage. The maximum vibration or shock acceleration to which an instrument can be subjected without permanent damage.

**ACCELEROMETER**

A transducer which converts input accelerations into outputs (usually electrical) which are proportional to the input acceleration values.

**ACCURACY**

The ratio of the error to the full-scale Output or the ratio of the Error to the Output, as specified, expressed in percent. More correctly: “Uncertainty”. Note: Accuracy may be expressed in terms of units of Measurand, or as with +/- percent of Full Scale Output.

**ATMOSPHERIC PRESSURE**

The pressure caused by the weight of the earth’s atmosphere; varies with geographic location, altitude, and weather.

**ACOUSTIC SENSITIVITY**

The output of a transducer, not due to rigid body motions, in response to a specified acoustical environment. Usually expressed as equivalent RMS measurand induced by a specified sound pressure level spectrum having an overall value of 140 dB referred to 0.0002 μbar (20 μPa) RMS.

**AMBIENT CONDITIONS**

The conditions (pressure, temperature, etc.) of the medium surrounding the case of the instrument.

**AMPLITUDE LINEARITY**

The closeness of the calibration curve of sensitivity to a straight line over a stated range of measurand amplitudes, at a stated fixed frequency.

**ANGULAR FREQUENCY**

Angular frequency (also known as circular frequency (omega)) is the torsional vibration frequency in radians per second. Multiply by 2π and express in cycles per second (cps) or hertz (Hz).

**ANTIRESONANCE**

A condition that exists in a system when with a constant applied excitation where any change in frequency causes an increase in system response.

**BAND-PASS FILTER**

A Filter having a single transmission band extending from a lower cutoff frequency greater than zero to a finite upper cutoff frequency.

**BAROMETRIC PRESSURE**

See ATMOSPHERIC PRESSURE.

**BASE STRAIN SENSITIVITY**

The sensitivity to strains applied to the base by bending, in the absence of any rigid body motion of the transducer. Usually expressed in equivalent measurand units at 250 μstrain applied to the mounting structure.

**BEL**

The ratio of two amounts of power expressed as a logarithm to the base 10. A decibel (dB) is one-tenth of a bel.

**BEST STRAIGHT LINE**

A line midway between the two parallel straight lines closest together and enclosing all output vs. measurand values on a calibration curve.

**BRIDGE**

A Wheatstone bridge configuration utilizing four resistive elements.

**BRIDGE RESISTANCE**

See INPUT IMPEDANCE and OUTPUT IMPEDANCE.

**BROADBAND**

Vibration signals that are unfiltered. Signals at all frequencies contribute to the measured value.

**CALIBRATION**

The comparison of transducer voltage outputs against the outputs of a reference standard.

**CALIBRATION (As applied to vibration sensors)**

An orderly procedure for determining sensitivity as a function of frequency, temperature, amplitude, etc. Yields deviations from correct values used for inferring true magnitudes from indicated magnitudes. Calibration may also refer to adjusting an instrument, lessening deviations from a standard sensitivity.

**CASE ISOLATION**

The minimum resistance, measured at a specified dc voltage, between the output common of an instrument and the entire metallic enclosure of the instrument.

**CENTER OF SEISMIC MASS**

The point within an accelerometer where acceleration forces are considered to be summed.

**CHARGE AMPLIFIER**

An amplifier that presents an output voltage that is proportional to the total electrical charge presented to the input.

**COMMON MODE PRESSURE**

See LINE PRESSURE.

**COMPENSATION**

Provision of a supplemental device, circuit, or special materials to counteract known sources of error.

**COMPLIANCE**

The ratio of change in translational (or rotational) displacement of an elastic element to the corresponding change of force (or torque). Compliance is the reciprocal of stiffness.

**COMPLIANCE VOLTAGE**

The maximum voltage that a constant current source will go to maintain the selected current.

**COMPRESSIONAL WAVE**

A wave in an elastic medium causing an element of the medium to change volume, a result of compressive or tensile stresses. It is normally a longitudinal wave.

**CONSTANT CURRENT**

Electric current independent of either voltage or resistances and fixed at a specific value. A constant current power supply varies its output voltage, up to its maximum compliance voltage, to maintain the fixed current into the load.

**CREST FACTOR**

For an oscillating quantity, the ratio of the peak value to the rms value.

**CRITICAL DAMPING**

Critical damping (Cc) is the minimum viscous damping that will allow a displaced system to return to its initial position without oscillation or overshoot. Cc=2 √mk

**CRITICAL FREQUENCY**

A particular resonant frequency (see Resonance) at which damage (or degradation in performance) is likely.

**CRITICAL SPEED**

A characteristic speed of a rotating system, such that the predominant excitation occurs at resonances of the system.

**CROSS TALK**

The signal observed in one channel due to a signal in another channel. “Cross talk” is erroneously applied to transverse sensitivity of transducers including shakers.

**CUT-OFF FREQUENCY**

The frequency above or below the frequency of maximum response of a filter at which the response to a sinusoidal signal is 3 dB below the maximum response.

**CYCLE**

The complete sequence of values of a periodic quantity that occurs during any one period.

**DAMPING**

Dissipation of oscillatory or vibratory energy with motion or with time. Critical damping (q.v.) is that value that provides the most rapid response to a step function without overshoot. Damping ratio is then C/Cc.

**DAMPING RATIO**

For a system with linear viscous damping, the ratio of the actual damping coefficient to the critical damping coefficient.

**DAMPING RATIO CHANGE**

The change in the damping over a specified temperature range. Usually specified in %/°C. This specification usually found in damped accelerometers such as piezoresistive and variable capacitance accelerometers.

**DEAD VOLUME**

The volume inside the pressure port of a transducer at room temperature and barometric pressure.

**DECIBEL**

Ratios of identical quantities are expressed in decibel or deciBel or dB units. Magnitude thus refers to some standard value, in terms of the base 10 logarithm of that ratio. In measuring acoustic or vibration power (as in PSD or ASD of random vibration), the number of dB = 10 log10 P/P0. P0, the reference level, equals 0 dB. In measuring the more common voltage-like quantities such as acceleration, the number of dB = 20 log10 E/E0. E0, the reference level, equals 0 dB.

**DEFLECTION**

The change in length along the primary axis or the distance a diaphragm moves at the center between no-load and rated-load conditions.

**DEGREES OF FREEDOM**

In mechanics, the total number of directions of motion (of all the points being considered) on a structure being modeled or otherwise evaluated. In statistics, the number of independent variables used in constructing a mathematical model representing some collection of random variables.

**DETERMINISTIC VIBRATION**

A vibration whose instantaneous value at any future time can be predicted by an exact mathematical expression. Sinusoidal vibration is the classic example. Complex vibration is less simple (two or more sinusoids). See also Periodic Vibration.

**DIAPHRAGM**

The sensing membrane which is deformed when pressure is applied.

**DIFFERENTIAL PRESSURE**

The difference in pressure between two measurement points.

**DIFFERENTIAL INPUT (Amplifier)**

A symmetrical input circuit configured such that both input lines have equal impedance and transfer characteristics with respect to the transducer grounding structure. The amplifier then amplifies only the difference between the two inputs, rejecting any common signal (Common Mode).

**DISCHARGE TIME CONSTANT**

A term sometimes found on IEPE datasheets. It is the time constant created by the output characteristics of the internal electronics. The time constant determines the low frequency point of the accelerometer e.g., the -5% low frequency point is 0.5/time constant. It is the time required for the sensor’s electronics to discharge to 37%of its original value following a step function input.

**DISPLACEMENT**

The change in position of a body or point with respect to a reference point. A vector quantity specifying the change of position of a body or particle, usually measured from the mean or rest position.

**DISTORTION**

In mechanics, any unwanted motion. If sinusoidal motion were desired at a fundamental frequency, any motion at harmonics or subharmonics of that frequency, or any mechanical "hash" (perhaps due to parts colliding). In electronic measurements, any unwanted signal; e.g. amplifiers may generate unwanted signals by clipping, nonlinearity or harmonic distortion.

**DOUBLE AMPLITUDE**

The total excursion of a simple harmonic quantity; the peak-to-peak value. DURATION: Of a shock pulse, how long it lasts. For "classical" pulses, time is usually measured between instants when the amplitude is greater than 10% of peak value.

**DYNAMIC MASS (Apparent Mass, Effective Mass)**

The complex ratio of force to acceleration at a point in a mechanical system during simple harmonic motion. Usually expressed as: Za=F/a

**ENDEVCO**

An acronym for ENgineering DEVelopment COmpany. A leader in developing instrumentation for the sensing of physical phenomena.

**END POINTS**

The outputs at the specified upper and lower limits of the Range.

**EXCITATION, ELECTRICAL**

The voltage or current applied to the input terminals of the transducer.

**EQUALIZATION**

The process whereby the response of a vibration system is adjusted to a flat or shaped spectrum.

**ERROR**

The algebraic difference between the indicated value and the true value of the measurand. In a transducer, usually expressed in percent of the full-scale output reading of the transducer.

**ERROR BAND**

The band of maximum deviations of output values from a specified reference line or curve as a result of causes due to the instrument.

**EXCITATION**

The external electrical voltage and/or current applied to a transducer for its proper operation. Mechanical excitation is a mechanical input driving force into a structure such as from a hammer, shaker, etc.

**FAST FOURIER TRANSFORM (FFT)**

A shortcut method implemented in computers and digital measurement instrumentation to quickly create a frequency domain spectrum, often in real-time.

**FIGURE OF MERIT (ACCELEROMETER)**

The measure of the efficiency of an accelerometer’s design. F of M = Charge Sensitivity X Fn 2.

**FILTER**

A device to pass certain frequencies (pass band) but block other frequencies (stop band). Classified as low-pass (high-stop), high-pass(low-stop), band-pass or band-stop. Filters may be mechanical, electrical, or optical.

**FLUSH DIAPHRAGM**

Sensing element is located on the very tip of the transducer (No pressure port).

**FORCED VIBRATION**

The vibratory motion of a system caused by some mechanical excitation. If excitation is periodic and continuous, motion eventually becomes steady-state.

**FOURIER SERIES**

A series which expresses the values of a periodic function in terms of discrete frequency components that are harmonically related to each other

**FREE FIELD (Sound)**

The field in a homogeneous, isotropic medium free from acoustically reflecting boundaries. Usually a field in which the effects of the boundaries are negligible over the region of interest.

**FREE VIBRATION**

Free vibration occurs without force, as after a reed is plucked.

**FREQUENCY**

The reciprocal of the period in seconds (of a periodic function) (1/T). Usually given in hertz (Hz), meaning cycles per second (cps).

**FREQUENCY, NATURAL**

The frequency of free oscillations of the sensing element of a fully assembled transducer. The frequency of a sinusoidally applied measurand at which the transducer output lags the measurand by 90 degrees.

**FREQUENCY RESPONSE**

The change, with frequency, of the sensitivity with respect to the reference sensitivity of a transducer, for sinusoidally varying applied measurand within a stated range of frequencies. The output signal expressed as a function of the frequency of the input signal.

**FREQUENCY SPECTRUM**

A description of the resolution into frequency components, giving the amplitude (sometimes also phase) of each component.

**FULL-SCALE OUTPUT**

The algebraic difference between the end points of the range. Usually expressed as plus or minus one-half of the algebraic difference.

**GAGE PRESSURE**

The pressure above (or below) atmospheric. Represents positive difference between measured pressure and existing atmospheric pressure. Can be converted to absolute by adding actual atmospheric pressure value.

**GAIN**

Transfer characteristic expressing the ratio of output voltage to the input signal of an amplifier. For a voltage amplifier: expressed as volts/volt, as mV/mV, or as dimensionless number. For a charge amplifier: expressed as millivolts per Pico Coulomb (mV/pC).

**GAIN RANGE**

The minimum and maximum values of gain that are available in an amplifier without causing any degradation in performance parameters beyond the limits of the specification. For a charge amplifier: expressed as from ____mV/pC to ____mV/pC.

**GROUND LOOP (Earth Loop)**

The closed electrical circuit formed by the connection of a ground wire to several ground terminals at different locations.

**FUNDAMENTAL FREQUENCY**

The number of cycles per second of the lowest-frequency component of a complex, cyclic motion. (See also Harmonic and Subharmonic). g: The acceleration produced by Earth's gravity. By international agreement, the value for 1 gravitational unit is 9.80665 m/s2 = 386.087 in/sec 2 = 32.1739 ft/sec2. g UNITS or GRAVITATIONAL UNITS: A way to express acceleration in terms of the gravitational constant, is equal to in/sec 22/ 386.087 in/sec2 or to m/s2 / 9.80665 m/s2 . GAGE FACTOR: The ratio of the relative change of resistance to the relative change in length of a Resistive Strain Transducer. GF = ΔR/R ΔL/L

**HYSTERESIS**

The maximum difference between output readings for the same measurand point, one point obtained while increasing from zero and the other while decreasing from full scale. The points are taken on the same continuous cycle. The deviation is expressed as a percent of full scale.

**IMPACT PRESSURE**

The pressure in a moving fluid which is exerted parallel to the direction of flow, caused by the inertial effects of the mass of the fluid. Also called DYNAMIC PRESSURE or VELOCITY PRESSURE.

**INDEPENDENT LINEARITY**

Maximum deviation from the linear regression line (least squares fit) for all measured points, expressed as percent of full scale output.

**INPUT IMPEDANCE**

The resistance measured across the excitation terminals of a transducer at room temperature.

**INSULATION (ISOLATION) RESISTANCE**

The DC resistance, expressed in ohms, measured between any electrical connector pin or lead wire and the transducer body or case. Normally measured at 50 Vdc.

**LINEARITY**

The maximum deviation of the calibration curve from a specified straight line expressed as a percent of full scale output and only measured on increasing measurand.

**LINE PRESSURE**

The maximum pressure in the pressure vessel or pipe for differential pressure measurement. Also called COMMON MODE PRESSURE.

**MEASURAND**

The physical quantity, property, or condition which is measured. (e.g.: pressure, load, weight, acceleration).

**MEDIUM (MEDIA)**

The fluid(s) in contact with the diaphragm, the pressure of which is being measured.

**NEAR FIELD**

Describes locations within a few inches to a few feet from the explosive event. The area is subjected to a very fast rise time, high amplitude, high frequency energy.

**NONLINEARITY**

Used interchangeably with “linearity.”

**NONREPEATABILITY**

Used interchangeably with “repeatability.”

**OUTPUT**

The electrical signal measured at the output terminals which is produced by an applied input to a transducer.

**OUTPUT IMPEDANCE**

The resistance as measured on the output terminals of a transducer at standard temperature, with no measurand applied, and with the excitation terminals open-circuited.

**OVERRANGE**

The maximum pressure or load which may be applied to the transducer without causing a permanent change in the performance specifications.

**PARTIAL PRESSURE**

The pressure which would be exerted by one constituent of a mixture of gases, if it alone were to occupy the same volume as the mixture. See also TOTAL PRESSURE.

**PASCAL**

Pressure of one Newton (force) per square meter.

**PHASE SHIFT**

The phase angle between the output and the applied signal.

**PRESSURE HEAD**

The height of a liquid column at the base of which a given pressure would be developed due to gravity acting on the fluid mass.

**PSI**

Pounds per square inch.

**PSIA**

Pounds per square inch absolute.

**PSID**

Pounds per square inch differential.

**PSIG**

Pounds per square inch gage.

**RANGE**

The measurand values, over which a transducer is intended to measure, specified by their upper and lower limits.

**REFLECTED OVERPRESSURE**

The total pressure that results at the interface when a shock wave traveling in a medium encounters a discontinuity such as a rigid surface or another shock wave.

**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 as a percent of full scale.

**RESOLUTION**

A measure of ability to delineate, detail, or distinguish between nearly equal values of quantity. Also referred to as “threshold” – lowest level of valid measurement.

**RESPONSE TIME**

The time required for the output of a transducer to increase from zero to some specified percentage of its final value when excited by a step change in measurand.

**RISE TIME**

The time required for the output of a transducer to rise from 10% to 90% of its final value as a result of a step change of measurand.

**SEALED (or SEALED GAGE) PRESSURE**

Pressure measured with reference to the pressure in a sealed container; the container is usually within the sensor.

**SENSING ELEMENT**

The part of the transducer which reacts directly in response to the measurand.

**SENSITIVITY**

The ratio of change in transducer output to a change in the value of the measurand. Specified sensitivity is usually averaged over the full scale range of the measurand.

**SHUNT CALIBRATION**

The change in electrical output caused by placing a fixed resistor between the appropriate transducer terminals. Used “in the field” for quick calibration.

**S.I. SYSTEM**

The international (metric) system of units.

**SPAN**

The algebraic difference between the limits of the range from zero to full scale.

**SPECIFICATIONS**

The group of error limits within which each device will operate.

**STAGNATION PRESSURE**

The sum of the static pressure and the impact pressure. It can be measured at a point where the velocity of the fluid is zero.

**STATIC PRESSURE**

The pressure of a fluid, exerted normal to the direction along which the fluid flows.

**STRAIN GAGE**

A measuring element for converting mechanical strain into an electrical signal.

**SUPPLY VOLTAGE**

See EXCITATION.

**TEMPERATURE COMPENSATED**

The range of temperature over which a transducer can operate up to full scale and still meet all specifications. Meggitt’s Endevco pressure transducers incorporate temperature compensation.

**TEMPERATURE COMPENSATION**

The utilization of supplementary devices, materials, or components with the transducer to minimize sources of error caused by changing temperature.

**TERMPERATURE, OPERATING**

The range of temperature over which a transducer may be safely operated up to full scale without causing failure; but specifications may not be met.

**THERMAL SENSITIVITY SHIFT**

The change in sensitivity due to a change in ambient temperature. Usually expressed as the maximum percentage change in sensitivity.

**THERMAL ZERO SHIFT**

The change in zero balance due to a change in ambient temperature. Usually expressed as the maximum percentage change of FSO over the compensated temperature range.

**TOTAL PRESSURE**

The sum of the pressures (partial pressures) which each gas (in a mixture of gases) would exert were it to occupy the containing vessel alone.

**TRANSDUCER**

A device (or medium) that converts energy from one form to another. The term is generally applied to devices that take a physical phenomenon (pressure, temperature, humidity, flow, etc.) and convert it to an electrical signal.

**VACUUM**

Pressure measured below atmospheric pressure and with reference to atmospheric pressure (Negative gage pressure).

**VELOCITY PRESSURE**

See IMPACT PRESSURE.

**ZERO ADJUSTMENTS**

Used when “setting up” a transducer to adjust the output signal to zero when zero load/pressure is applied.

**ZERO BALANCE**

The output signal of the transducer with rated excitation and with no-load applied, usually expressed in millivolts. Also called ZMO and zero pressure output.

**ZERO RETURN**

The difference in zero balance measured immediately before rated load application of specified duration and measured after removal of the load, and when the output has stabilized.

**GROUNDED OR INSULATED (Ungrounded) TRANSDUCER**

Refers to the presence or absence of an electrical connection between the "low" side (signal return) of the transducer element and the portion of the transducer intended to be in contact with the test structure.