Glossary

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.

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.

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

HALF-SINE SHOCK PULSE

An ideal shock pulse for which the acceleration-time relation has the shape of the positive (or negative) section of one cycle of a sine wave.

HARMONIC

A sinusoidal quantity having a frequency that is an integral multiple (X2, X3, etc.) of a fundamental (X1) frequency. HARMONIC DISTORTION: The maximum harmonic content of a periodic output signal, for any given frequency or amplitude within the specified limits, expressed as a percentage of the rms value of the signal.

HAVERSINE (Versine) SHOCK PULSE

An ideal shock pulse for which the acceleration-time curve has the shape of one full cycle of a versine curve beginning at zero.

HIGH-PASS FILTER

A wave filter having a single transmission band extending from some critical or cutoff frequency, other than zero, up to an infinite frequency.

HYSTERESIS

The phenomenon exhibited by a system in which the response of the system to changes is dependent upon its past response to change. The maximum difference in output of a device, at any measurand value within its specified range, when the value is approached first with increasing and then with decreasing measurand.

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.

INERTANCE (or accelerance)

The ratio of acceleration to force. 1/M = F/A

INPUT IMPEDANCE

The equivalent value of resistance and inductance or capacitance, in series or parallel with the input terminals.

INPUT RESISTANCE

The resistance measured on the excitation voltage leads on a piezoresistive device. By convention, these are the black and red leads.

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.

ISOLATION FROM POWER SUPPLY

Isolation that is the absence of a direct electrical connection between the lines of the power supply and the amplifier common.

JERK

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

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.

LINEAR SYSTEM

A system is linear if its response is directly proportional to excitation over a stated range of amplitudes at a stated fixed frequency. A plot of input versus output amplitudes of a linear system will be a straight line.

LINEARITY

The closeness of a calibration curve to a specified straight line. (See: Amplitude Linearity.)

LOAD IMPEDANCE

The impedance presented to the output terminals of an instrument by the associated external circuitry.

LOW NOISE CABLE

Cable treated with low-noise material to reduce the effects of triboelectric noise. To be used on charge-mode piezoelectric accelerometers.

LOW-PASS FILTER

A wave filter having a single transmission band extending from zero frequency to some finite critical or cutoff frequency.

MASS

A physical property, dynamically computed as acceleration/force. Statically computed as W (which can be measured on a scale)/g. Ordinary structures are not pure masses as they contain reactive elements, i.e., springs and damping.

MEASURAND

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

MECHANICAL IMPEDANCE

At a point in a mechanical system, the complex ratio of force to velocity during simple harmonic motion. Usually expressed as: F/v where all terms are phasors, having a magnitude and direction. Mechanical impedance is the reciprocal of Mobility.

MEDIUM (MEDIA)

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

MOUNTED RESONANCE FREQUENCY

The resonance frequency of an accelerometer rigidly mounted directly to the vibration generator with a solid stud, or adhesive mounted as specified by the manufacturer.

NATURAL FREQUENCY (Damped)

The frequency of free vibration of a damped linear system. fd=½π x √(k/m-c/4m2)

NATURAL FREQUENCY (Undamped)

The frequency of free vibration resulting from only elastic and inertial forces of the system. fn=½π x √k/m. Regardless of damping present, the frequency of sinusoidal excitation of a single-degree-of-freedom accelerometer at which the output signal lags behind the motion of the case (housing) by a phase angle of 90°.

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.

NODE

A point, line, or surface in a standing wave where some characteristic of the wave field has essentially zero amplitude.

NOISE

Any disagreeable or undesired acoustical or electrical oscillations.

NONLINEARITY

Used interchangeably with “linearity.”

NONREPEATABILITY

Used interchangeably with “repeatability.”

OCTAVE

The interval between two frequencies differing by exactly 2:1.

OUTPUT

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

OUTPUT IMPEDANCE

The open-circuit equivalent generator impedance, expressed in equivalent value of resistance, and inductance or capacitance, in series or parallel with the output generator. The effective internal impedance in series with the output terminals of an amplifier or circuit.

OUTPUT RESISTANCE

Resistance measured on the signal output leads of a piezoresistive device. By convention, these are the green and white wires.

OVERLOAD RECOVERY

The maximum time for an amplifier to return to a minimum defined level of linear operation following an input overload of defined amplitude, duration, and pulse shape.

OVERRANGE

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

OVERSHOOT

The amount of output measured beyond the final steady-state output value, in response to a step change in the measurand.

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.

PEAK-TO-PEAK VALUE

For an oscillating quantity, the algebraic difference between the extreme values of the quantity. (Same as double amplitude.)

PHASE SHIFT

The amount of time by which the output of an instrument lags a sinusoidally varying input. Usually expressed as a fraction of a cycle of the frequency, in degrees.

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.

PERIODIC VIBRATION (See also Deterministic Vibration)

An oscillation whose waveform regularly repeats. Compare with Probabilistic Vibration.

PIEZOELECTRICITY

That property of asymmetric crystalline materials that develop electric polarization proportional to strain.

PIEZORESISTIVE TRANSDUCER

A transducer that relies on deformation of its sensitive element to change the resistance of the element. Some semi-conductors give greater resistance change for a given deformation than metals.

POLARITY

The relationship between the transducer output and direction of applied measurand; taken as "standard" or "positive" when a positive charge or voltage appears on the "high" side of the transducer output for an acceleration directed from the mounting surface into the body of the accelerometer.

POWER SPECTRAL DENSITY, G(f), PSD

The mean square measurand at a specified frequency divided by the bandwidth used to measure it, as the bandwidth approaches zero. In vibration measurement it is usually expressed as g2/Hz.

PRECISION

The relative closeness of the distribution of measurements of a quantity about their mean value. A high precision implies that the measurement has small random errors such as: a) Errors of judgment b) Fluctuating conditions (temperature, pressure, line voltage) c) Small disturbances (vibration, spurious electrical pickup, etc.)

PROBABILISTIC VIBRATION

(As compared to Deterministic Vibration), one whose magnitude at any future time can only be predicted statistically. See also Random Vibration.

PSI

Pounds per square inch.

PSIA

Pounds per square inch absolute.

PSID

Pounds per square inch differential.

PSIG

Pounds per square inch gage.

PULSE DURATION

The time during which the faired magnitude of the pulse is above 10% of its maximum value.

PYROELECTRIC OUTPUT

Electrical signal (charge) which appears on the surfaces of a piezoelectric crystal when temperature gradients occur across the crystal.

Q (Quality Factor) (Amplification Factor)

The quantity Q is a measure of the sharpness of resonance or frequency selectivity of a resonant vibratory system having a single degree of freedom, either mechanical or electrical. It is often used for the ratio of the sensitivity of a transducer, at its resonance frequency, to its reference sensitivity. It is also used to express transmissibility at resonance).

QUASI-DIFFERENTIAL CIRCUIT

A circuit that is configured to provide electrical characteristics similar to those of a true differential circuit over a narrow range of operating conditions.

RANGE

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

RANDOM VIBRATION

(See Probabilistic Vibration). One whose in°stantaneous magnitudes cannot be predicted. Adjective "Gaussian" applies if they follow the Gaussian distribution. May be broad-band, covering a wide, continuous frequency range, or narrow-band, covering a relatively narrow frequency range. Random vibration contains no periodic or deterministic components.

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.

REFERENCE SENSITIVITY

The ratio of the change in charge or voltage generated by a transducer to the change in value of measurand that is measured under a set of defined conditions. For accelerometers, it is typically measured at 100 Hz (in the USA) and standard room conditions (q.v.).

REPEATABILITY

The maximum deviation from the mean of corresponding data points taken under identical conditions. (2) The maximum difference in output for identically-repeated stimuli (no change in other test conditions). The maximum difference in outputs, at the same input, with the input changing in the same direction. Do not confuse with Accuracy.

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.

RESIDUAL NOISE

The RMS noise measured at the output of an accelerometer over a specified bandwidth. The bandwidth can be broadband with noise expressed in ng or μg RMS or a series of narrow band specifications expressed in ng or μg/√Hz

RESONANCE

Resonance of a system in forced oscillation exists when any change, however small, in the frequency of excitation causes a decrease in the response of the system.

RESPONSE SIGNAL

The signal from a "response sensor" measuring the mechanical response of a mechanical system to an input vibration or shock.

RELATIVE HUMIDITY. R.H.

The ratio, expressed as a percentage, of the amount of water in a given volume of air at a given temperature to the amount required to saturate the air at that temperature. RELIABILITY: The characteristic of an item expressed by the probability that it will perform a required function under given conditions for a specific period of time.

RESOLUTION

The resolution is the smallest change of measurand, i.e., displacement, velocity, or acceleration, for which a change in output is discernible.

RESPONSE

The vibratory motion or force that results from some mechanical input. In general, the output of a system that results from some input.

RESPONSE SIGNAL

The signal from a "response sensor" measuring the mechanical response of a mechanical system to an input vibration or shock. RISE TIME: The length of time required for the output of an instrument to rise from a small percentage of its final value to a large percentage of its final value as a result of a step change of input measurand. Usually assumed to be between 10% and 90% of the final value.

RISE TIME

The length of time required for the output of an instrument to rise from a small percentage of its final value to a large percentage of its final value as a result of a step change of input measurand. Usually assumed to be between 10% and 90% of the final value.

RMS or ROOT-MEAN-SQUARE value

The square root of the time-averaged squares of a series of measurements. Refer to a textbook on electrical engineering. In the exclusive case of a sine wave, the RMS value is 0.707 X the peak value.

RSS or ROOT-SUM-Square

The square root of the sum of the squares of a series of measurements. Most often used to statistically sum independent errors.

ROOM CONDITIONS (ISA S37.1(1969)

Ambient environmental conditions, which have been established as follows: a) Temperature: 25 ±10° C (77 ±18° F) b) Relative Humidity: 90% or less c) Barometric Pressure: 26 to 32 inches Hg (880 to 1083 mbar).

SAWTOOTH SHOCK PULSE (Terminal Peak)

An ideal shock pulse for which the acceleration-time curve has the shape of a triangular wave wherein the acceleration increases linearly to a maximum value and then drops instantaneously to zero.

SEALED (or SEALED GAGE) PRESSURE

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

SELF-HEATING

Internal heating resulting from electrical energy dissipated within a current-carrying element.

SENSING ELEMENT

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

SENSITIVITY

The ratio of the change in transducer output to a change in the value of the measurand (e.g., mV/g).

SHEAR WAVE

A wave in an elastic medium that causes an element to change its shape without a change in volume, a result of shear stresses. Normally a transverse wave.

SHOCK (Mechanical Shock)

The application of a sudden change in force, position, velocity, or acceleration that excites transient disturbances in a system. Change is normally considered sudden if it takes place in a time that is short compared with natural periods of concern.

SHOCK RESPONSE SPECTRUM

A plot (amplitude vs. frequency) of the response of a number of single-degree-of-freedom systems to an applied shock.

SHOCK SPECTRUM, PRIMARY (INITIAL)

A shock spectrum derived from response measurement within the duration of a simple shock pulse.

SHOCK SPECTRUM, RESIDUAL

A shock spectrum derived from response measurements after a shock has ended.

SHOCK PULSE

A form of shock excitation characterized by a rise and decay of acceleration in a relatively short period of time.

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.

SHUNT CALIBRATION RESISTOR

A shunt resistor which, when placed across a specified element of the electrical circuit of a transducer, will electrically simulate a specified percentage of the transducer full-scale output at room conditions.

SHUNTING RESISTANCE (Transducer Resistance) (Leakage Resistance)

The electrical resistance observed between the two terminals of a piezoelectric transducer or its integral cable.

SIMPLE HARMONIC MOTION

Periodic vibration that is a sinusoidal function of time.

SINGLE-DEGREE-OF-FREEDOM SYSTEM

A system requiring but one coordinate to completely define its configuration at any instant.

SINGLE-ENDED INPUT

An input circuit configured such that one of the input lines is electrically and directly connected to signal ground.

S.I. SYSTEM

The international (metric) system of units.

SKIRT (Rolloff)

The terminal slope of attenuation of the frequency response of a filter, usually expressed at 6 n dB/octave, where n is the order of the filter.

SLEW RATE

The maximum rate at which the output voltage of an instrument can be made to change understated load conditions.

SOURCE CAPACITANCE

The total external capacitance shunting the input terminals of an amplifier. Includes transducer and cable capacitance.

SOURCE RESISTANCE

The total of all dc resistive leakage paths shunting the input terminals of an amplifier.

SPAN

The algebraic difference between the limits of the Range.

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.

STATIONARY PROCESS

An ensemble of time-histories such that their statistical properties are invariant with respect to translations in time.

STANDARD DEVIATION (sigma)

A statistical term: the square root of the variance, i.e., the square root of the mean of the squares of the deviations from the mean value. If the mean is zero, standard deviation is equal to RMS.

STATIC PRESSURE

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

STEADY STATE VIBRATION

Periodic vibration for which the statistical measurement properties (such as the peak, average, RMS and mean values) are constant.

STIFFNESS

The ratio of change of force (or torque) to the corresponding change in translational (or rotational) displacement of an elastic element. Usually expressed as: k = F/d = W/δ.

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.

TEMPERATURE RANGE, OPERATING

The range of ambient temperatures, given by their extremes, within which an instrument is intended to operate.

THERMAL SENSITIVITY SHIFT

(Temperature Sensitivity Error): The change in sensitivity due to changes in ambient temperature from a reference temperature to the specified limits of the Operating Temperature Range.

THERMAL TRANSIENT RESPONSE

The output (response) of an accelerometer when subjected to a temperature change. (See Transient Temperature Error).

THERMAL ZERO SHIFT

The change in zero measurand output due to changes in ambient temperature from a reference temperature to the specified limits of the operating temperature range.

TIME CONSTANT

The length of time required for the output of an instrument to rise to 63% of its final value as a result of a step change of the input measurand.

THRESHOLD (RESOLUTION)

Output noise level of a transducer expressed in g. Threshold resolution = Residual Noise / Sensitivity.

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.

TRACKING FILTER

A band-pass filter (usually narrow-band) whose center frequency can be made to follow an external synchronizing signal.

TRANSDUCER

A device designed to receive energy from one system and to supply energy, of either the same or of a different kind, to another in such a manner that the desired characteristics of the input energy appear at the output.

TRANSIENT TEMPERATURE ERROR

The output of a transducer when subjected to a specified transient temperature change within the specified operating temperature range. (See Thermal Transient Response.)

TRANSIENT TEMPERATURE SENSITIVITY

Usually expressed as equivalent measurand per degree temperature change measured in accordance with ANSI S2.11-1969, Section 4.9.

TRANSFER CHARACTERISTIC

The ratio between output and input of an instrument usually expressed in terms of gain and phase shift as a function of frequency.

TRANSMISSIBILITY

The non-dimensional ratio of the response amplitude of a system in steady-state forced vibration to the excitation amplitude.

TRANSVERSE SENSITIVITY

The maximum sensitivity of a transducer to an acceleration or other measurand perpendicular to the sensitive axis of the transducer (Intended measuring direction). Transverse sensitivity is usually expressed as a percentage of the sensitivity in the sensitive axis.

TRIBOELECTRIC NOISE

Unwanted noise caused by the by electrical charges generated by the rubbing the insulation layers in a cable.

TURN-ON TIME

See Warm-Up Time

UNDERSHOOT (Amplifier)

The trailing edge of the output of an amplifier that cannot respond to dc components of an input signal, that appears in opposite polarity to a unipolar primary pulse.

VELOCITY

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

VACUUM

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

VELOCITY PRESSURE

See IMPACT PRESSURE.

WARM-UP TIME (Period)

The period of time, starting with the application of excitation or power to an instrument, required to assure that the instrument will perform within all specified tolerances.

ZERO ADJUSTMENTS

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

WHITE-NOISE (White Random Vibration)

A random noise or vibration having equal energy per unit bandwidth (uniform power spectral density) over the spectrum of interest.

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.

ZERO MEASURAND OUTPUT (Zero Balance, ZMO)

A measure of the difference between the real output and the desired output of zero measurands from an instrument or system which is properly energized.