MTI AccuMeasure 9000, User Manual

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7
The Accumeasure
TM
System measures the electrical impedance of the capacitance between a sensing
electrode in the probe and a ground-referenced target. The magnitude of the impedance is proportional to
the reciprocal of the capacitance value as defined by the equation below:
C
Z
c

1

where:

is proportional to the frequency at which the capacitance measurement is performed.
Substituting the equation for capacitance into the impedance equation shows that the impedance is directly
proportional to the gap value D, as shown in the following equation:
A
D
Z
C


The Accumeasure
TM
probe amplifier produces a DC voltage that is linearly proportional to the average value
of the probe gap impedance, and an AC voltage variation from the DC voltage that is directly proportional to
the amplitude of the target vibration. The amplifier electronic circuitry eliminates the effects of both the probe
cable capacitance and the stray capacitance at the edge of the probe sensing area that could cause non-
linearity of the gap and vibration measurements.
The standard probe cable length is 2.5m (8.2ft) and standard full scale gap probes from 25um to 5mm
(0.001 to 0.200 inches) are available. Custom cable lengths from 300mm to 15m (1 to 50ft) and custom
probes from 12.5um to 12.7mm (0.0005 to 0.5 inches) can be used with the Accumeasure
TM
probe
amplifier. Contact the factory for details on custom cables and/or probe lengths.