Why we set gap voltage -10 in vibration probe?

Explain why the voltage is set to -10 in a vibration probe

The Gap is usually set at -10V. The reason for setting at -10 volt is, proximitor allows the probe to work from -2 volt to -18 volt. So the voltage difference of 2 to 10 and 10 to 18 is 8 volt. When setting at -10 volt, the probe can equally measure vibration at both sides, That is when the shaft is going away from a probe or towards the probe.