It provides a constant current to the sensor regardless of the supply voltage or vibration voltage from the sensor. The bias voltage will be measured on the side of the constant current diode connected to the sensor. The bias voltage should be measured periodically to check sensor operation. The best measurement device is a voltmeter. When using the data collector as a voltmeter the DC voltage input is used.
Oscilloscopes can also measure the bias by selecting the DC coupled input.
The bias voltage can be trended with many on-line systems. Trending the bias provides a record of sensor operation. If the sensor is disconnected or slowly develops a fault the bias trend will show when the event occurred. Since the bias changes with temperature the bias trend can show when machines are started, shut down, and crudely detect bearing faults and other heat generating problems. Figure 4 shows a bias voltage trend from an on-line vibration measurement system.