Normalization See “Standardization”. Mass Loading The change in actual sensitivity of a back-to-back accelerometer when loaded with masses signifi- cantly different from the transfer standard with which it was originally calibrated. Correction curves of actual sensitivity vs. frequency are referred to as “mass loading correction curves”. Phase Shift The difference in phase angle between input meas- ured and output electrical signal, measured in degrees. The response may lag or lead the input. Picocoulomb A measure of electrostatic charge output frompiezoelectric sensors. 1 Picocoulomb= 1 x 10 Linearity Actually the non-linearity or deviation from a straight line in a plot of output amplitude vs. input amplitude of a sensor or system. At Dytran, we use the popular zero based best straight line method of determining linearity. Full scale is determined, a zero based best straight line plot of output vs. input is made, an error band is created using a specified percent of full scale. All points on the curve must fall within this error band. Triboelectric Noise Zero Shift Transient Thermal Response The unwanted generation of electrical charge by the chafing together of insulation layers inside industrial electrical cables during flexing caused by vibration and shock induced motion. The change in baseline level of the output voltageof an accelerometer immediately after a mechani-cal shock. The time history of the change in output (voltage or charge) from a sensor resulting from a sudden change in ambient temperature. Transverse Sensitivity Thermal Coefficient of Sensitivity The output of an accelerometer in response to motion in directions orthogonal to its sensitive axis, expressed as a ratio of measured output to cross axis input, in percent. The measured change in the sensitivity (of scale factor) of a sensor, from its room temperature (ref- erence) value to the value at a higher or lower sta- bilized ambient temperature. This parameter is measured in percent of change in sensitivity per degree of temperature deviation. Standardization As applied to systems, refers to the application of gain or attenuation as needed to correct the sensi- tivity of a sensor to the exact nominal (or design) sensitivity. As applied to sensors, refers to the process by which the sensor sensitivity is brought close to the nominal value. In specification sheets, it is the highest acceptable deviation of the meas- ured sensitivity from the nominal value, expressed in percent. Sensor Bias When IEPE sensors are supplied with constant cur- rent, a DC voltage bias exists at the center conduc- tor of the sensor connector. The output signal is superimposed on this DC bias. The power unit extracts the signal from this bias by various means, effectively “blocking” it. The normal range for this voltage is +9 to +12 Volts.

glossary of TERMS

Acceleration Compensation A design incorporating components within the sen-sor to cancel the effect of motion (vibration) on the sensor output signal. Acceleration Sensitivity In a industrial pressure transducer or impulse hammer, this refers to the unwanted output signal from the sen- sor in response to vibration in the sensitive axis of the sensor. This parameter is specified in terms of psi/g for pressure sensors and Lbs/g for impulse hammers. Certain Dytran sensors such as the Series 2200 and 2300 pressure sensors and the Dynapulse LIVM Frequency Response Low Impedance Voltage Mode (LIVM=IEPE), Dytran’s Trademark describing its line of piezoelec- tric sensors with built-in impedance converting elec- tronics. The highest and lowest frequencies at which meas- ured deviations from a reference sensitivity (usual- ly 100 Hz) lie within a specified error. The devia- tions are usually specified as -5%, but in some cases -3dB is specified. Fault Monitor Meter Discharge Time Constant (TC) A Dc voltmeter incorporated into IEPE power units to read the DC voltage at the output of the current source. If a sensor is connected, this meter reads the sensor bias voltage and can be used to indicate open and shorted cables and sensors or to verify normal system operation.
™ series of impulse hammers are acceler-ation compensated to minimize this effect. The time required for the output voltage from a sensor or system to discharge to 37% of its original value in response to a zero rise time step function input. This parameter determines low frequency response. Charge Mode Current Source Power Unit Sensors which contain piezoelectric industrial crystals but no built-in electronics. These sensors may use quartz or piezoceramics to generate electrostatic charge signals in response to input measured. A power supply expressly designed for use with Low Impedance Voltage Mode (LIVM=IEPE) sensors consisting of an energy source (batteries of Industrial dc power supplies) and a constant current element. These units are characterized by stable constant current output and high dynamic impedance. They also serve as signal decoupling devices.
GLOSSARY OF TERMS Rise Time Sensitivity The time required for a sensor or system to respond to an instantaneous input step function, measured from the 10% to 90% points on the response wave- forms. The scale factor of a sensor or system, measured in terms of change in output signal per change ininput measured. Accelerometer sensitivity is expressed in terms of mV or pC/g, pressure is expressed in terms of mV or pC/psi and force sen- sor sensitivity is expressed in terms of mV or pC/LbF.
-12 Coulomb. Resonant Frequency Also called “natural frequency”. The lowest fre- quency of a second order system (spring-mass sys- tem) which satisfies the solution to the differential equation of motion. At this frequency, the ampli- tude increases by factors of as much as 100. The usable upper frequency of a sensor is determined by this parameter. All sensors are spring-mass sys- tems with intrinsic natural frequencies. RMS Root Mean Square. In AC theory, this is the “heat- ing value” of an AC waveform and it is equal in amplitude to .707 times the peak value of the waveform. Most AC meters are calibrated to read in RMS. To obtain the peak value of the waveform, multiply the RMS value by 1.414.
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