Hatch – I think your terminology might be a little confusing.
Depending on the brand, the proximeter is usually a 3 terminal device – the terminals are labelled something like –Vt, COM and signal.
The proximiter needs to be powered up; usually with -24Volts (applied between –Vt and COM) to get a linear response range of up to about 120mils. You can vary the -24volts power supply from around -18 to -30 volts (I think that’s about the correct range) and get a larger or smaller linear range accordingly.
The vibration signal output (between Signal and COM) is a voltage proportional to the gap between the prox probe and the shaft surface. With everything at rest, the output from the proximity probe system is a dc ‘gap voltage’ – you can adjust this voltage by moving the probe towards or away from the shaft surface. If you have a prox probe system of 200mV/mil by adjusting probe to between -8 and -12 volts dc during setup you can be sure that it is sitting in the middle of its linear range (to accommodate motion towards and away from the probe tip).
When the shaft begins to turn, you get a varying dc voltage as the shaft moves towards and away from the probe tip. You can treat this as a dc voltage with an ac voltage superimposed on it, where the dc portion represents the average gap between the probe tip and shaft (good for shaft centreline plots) and the ac portion represents vibration. For a typical 200mV/mil system you would expect a dc gap voltage to be in the range -7 to -13 volts, and for a typical piece of rotating equipment, you wouldn’t want to see more than 10 mils pk-pk corresponding to 2 volts ac pk-pk.
The dc gap voltage is fixed by the position of the shaft within its journal (either at rest or on its oil film) and the ac voltage is fixed by the amount of vibration that is present.
By reducing –Vt you do not change anything other than the linear range of the system: you could keep reducing –Vt until your analyser appears to work, but by then you have lost the signal.
If fitted, then using the AC input effectively ‘blocks’ the dc component of the signal (that’s the capacitor working). If you have a dc component of -10 volts and an ac component of 2 volts pk to pk, then your input signal will vary from -9 to -11 volts. If you put this through a 50% potential divider, reducing the variation seen by the analyser input from -4.5 volts to -5.5 volts and then tell the analyser that you are looking at a 100mv/mil signal, you can use the analyser with a +/-10 volts input range.
