Gap Voltage for Axial Displacement and Zero Position

Dear all

During our outage, found gap voltage for Bently Nevada axial displacement probe was around 6.4 Vdc. I&C team adjusted this one to 10.2 Vdc since 10 Vdc is the recommended setting. After unit back to service, the axial displacement reading increased too much while the turbine was running at rated speed (from +20s mils to around +33 mils). The alarm is 25 and danger is 30 mils.

GE said once you correct the gap voltage on the field, you have to also correct the zero position on the software. The as found zero position was 42 mils. They advised to change it to 55 mils. Now the readings back to normal again.

I hope someone can explain the relation between the gap voltage in field and zero position ? How can we predict the required adjustment for the zero position once gap voltage has been adjusted?

Note: As per GE specs the normal axial float for 7FA e+ is 13-17 mils.
25 mils is the alarm setting
30 mils is danger