Knowledge Base:  
Tech Note 116: Electro-Mechanical Velocity Sensors
Last Updated: 05/10/2017

Electro-Mechanical Velocity SensorsThe base model CMCP535-100V vibration transmitter is designed for use with an IEPE (Integrated Electronics Piezo Electric) type velocity sensor. This type of sensor requires constant current sensor power from the transmitter, and outputs both a vibration velocity signal (typically 100 mV/ips) and a dc bias voltage. The dc bias is monitored by the transmitter and used to determine the “OK” status of the sensor. For this type of sensor the buffered output mirrors the input, i.e.: for a 100 mV/ips input, the buffered output will also be a nominal 100 mV/ips.

A "CMCP535-xxxEV" version is used when the sensor is not the IEPE type, but rather a self-generating (Electro-Mechanical) type velocity sensor. This type of sensor does not require power from the transmitter and does not output a dc bias voltage, so monitoring the “OK” status of the sensor requires different internal input circuitry in the transmitter. A result of this is that there are sensitivity differences at the buffered output. Your customer is using a CEC velocity sensor that is a self-generating (electro-mechanical-velocity) type sensor with a nominal sensitivity of 135 mV/ips. Note that the actual sensitivity of this type of sensor can vary from the nominal value depending on loading across its terminals. When this type of sensor is connected to the CMCP535, the transmitter creates its own dc bias voltage to allow for sensor “OK” status detection; a consequence of which is that the output sensitivity is attenuated. For self-generating sensors with nominal sensitivities from 125mV/ips to 500 mV/ips, this attenuation is normalized so that the sensitivity realized at the buffered output is a nominal 100 mV/ips. An additional factor to consider is that the output impedance at buffered output is 1 kOhm. When the input impedance of any instrument connected to the buffered output is less than 1M Ohm, additional attenuation will result.
If this output is to be connected to a readout device, a nominal value of 100 mV/ips should be used for any calculations.




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