Condition Monitoring Systems


g (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/sec² = 32.1739 ft/sec². / The value of acceleration produced by the force of gravity. / The acceleration produced by the force of gravity, which varies with the latitude and elevation of the point of observation. By international agreement, the value 980.665 cm/sec’ = 386.087 in/sec’ = 32.1739 ft/sec’ has been chosen at the standard acceleration due to gravity.

g units or gravitational units.

A way to express an acceleration, in terms of a ratio. Divide a given acceleration by the appropriate value (9.80665 m/s² or 386.087 in/sec² or 32.1739 ft/sec²).

Gain (magnitude ratio)

For a linear system or element, the ratio of the magnitude (amplitude) of a steady-state sinusoidal output relative to a causal input. In an electrical circuit, it is the amount of amplification used and is sometime expressed in decibels, dB.

Gain accuracy

Measure of deviation of the gain (of an amplifier or other device) from the ideal gain.

Gain, dynamic

For a sinusoidal signal, the magnitude ratio of the steady-state amplitude of the output signal to the amplitude of the input.

Gain, static

The ratio of change of steady state value to a step change in input, provided that the output does not saturate.


The electrical neutral line having the same potential as the surrounding earth; the negative side of a direct current power system; the reference point for an electrical system.


1 gal (seems to mainly be used in Japan) is an acceleration of 1 cm/sec². A more or less typical earthquake measures around 250 gal near the epicenter, so 400 gal is sometimes specified for earthquake testing. 980.6 gal = 1g.

Gate Encoders

Pulse generator for measuring slide gate or valve positions.

Gate Monitors

Device for monitoring slide gate or valve position.

Gear mesh frequency

A potential vibration frequency on any machine employing gears. Multiply the number of teeth on a gear times its RPM, then divide by 60 / A potential vibration frequency on any machine that contains gears; equal to the number of teeth multiplied by the rotational frequency of the gear.

Gear tooth Sensor

Inductive sensor for measuring rotational speed.

Generalised Coordinates

The minimum number of independent coordinates necessary to completely describe a systems position constitutes a set of generalized coordinates. For an N degree of freedom system, N generalized coordinates are required.

Graduation mark

The marks that define the scale intervals on a measuring instrument are known as graduation marks.

Graph Attributes Dialog

In SignalCalc Dynamic Signal Analyzers: A control dialog that determines the appearance of a graph window.

Graph Template

A user-stored graphic format that is used when opening a New Graph; provides a desired graphic appearance with minimum control interaction.

Ground Loop

A current loop created when a signal source and a signal measurement device are grounded at two separate points on a ground bus through which noise currents flow. These currents generate voltage drops between the two ground connections, which cause measurement errors.

Hall effect sensor

A sensor used to detect rotating magnets and send square wave pulses back to a speed switch, rate meter or tachometer.


Highly accelerated life test. See accelerated life test.

Hamming Window

Named after its originator, the Hamming window is a Hanning window sitting on top of a small rectangular pedestal. Its function is similar, but has its first side lobes 42 dB down, whereas the Hanning window’s first side lobes are only 32 dB down. Thus the Hamming has better selectivity for large signals, but it suffers from the disadvantage that the rest of the side lobes are higher, and in fact fall off slowly at 20 dB per octave like those of the rectangular window. The Hamming window had some advantage in the days when FFT Analyzers only had 50 dB or so of dynamic range, but today it is essentially obsolete.

Hand Held Tachometer

A hand held device used to measure rates such as revolutions per minute, inches per mi9nute and feet per minute.

Hanning Window

DSA window function that provides better frequency resolution than the flat top window, but with reduced amplitude accuracy. / The standard Window for general spectrum analysis of continuous signals and all random signals.

Hard failure

A product under test ceases to work correctly. It does not resume correct operation, even when the stressing environment is eased. Differs from soft failure.


A sinusoidal quantity having a frequency that is an integral multiple (x2, x3, etc.) of a fundamental (x1) frequency. / Frequency component at a frequency that is an integer multiple of the fundamental frequency.

Harmonic Distortion

In the output signal of a device, distortion caused by the presence of frequencies not present in the input signal.


Distortion (usually non-harmonic) on a signal. May be viewed on an oscilloscope trace. (slang).


Highly accelerated stress screening. See environmental stress screening (ESS).

Heavy Spot

The angular location of the imbalance vector at a specific lateral location on a shaft. The heavy spot typically does not change with rotational speed.

HELP (button)

Pops up helpful information about the current dialog box or area.

hertz / Hz

Lower-case h, but abbreviated Hz. The unit of frequency. Formerly cps for cycles per second. / Unit of frequency, defined as one cycle per second.

High-Pass Filter

A filter with a transmission band starting at a lower cutoff frequency and extending to (theoretically) infinite frequency.

High Spot

The angular location on the shaft directly under the vibration transducer at the point of closest proximity. The high spot can move with changes in shaft dynamics (e.g., from changes in speed).


The inverse of the system transfer function.


A Test type providing single channel amplitude domain functions including Probability Density (PDH) and Cumulative Density (CDF).


Also called deadband. That portion of a measuring system’s response where a change in input does not produce a change in output. / Non-uniqueness in the relationship between two variables as a parameter increases or decreases. Also called deadband, or that portion of a system’s response where a change in input does not produce a change in output. / The property of an element or sensor, that its output is dependent not only on the value of the input, but on the direction of the current traverse. (The reading of the same value differs as a function of whether the measurement is rising or falling.)

Hysteresis Damping (Hysteric Damping, Structural Damping)

Energy losses within a structure that are caused by internal friction within the structure. These losses are independent of speed or frequency of oscillation but are proportional to the vibration amplitude squared.