Monroe Electronics Electrostatic Voltmeter - Isoprobe® - model 244A - Manual (Page 14)

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c. While closing the switch, observe the instrument output and adjust
GAIN for fastest speed-of-response with minimum overshoot in
step response.
Once properly adjusted, the GAIN control should require no
further attention unless the probe-to-surface spacing is altered.
The static follow-up error (difference in DC potential between the
unknown and the probe) of a carefully zeroed and adjusted
instrument is typically less than 0.05% throughout its range.
d. With the probe aperture viewing a zero volt (or grounded) surface,
set ZERO control at mid-range. Adjust BAL (balance) control for
near zero reading.
B. SPACING INSENSITIVITY OVER LARGE PROBE-TO-SURFACE SPACING
RANGE:
This adjustment procedure will minimize offset errors due to variations in probe-
to-surface spacing. This adjustment is preferably done with a mechanical
positioning device and a grounded plate.
1. Position probe for minimum probe-to-surface spacing.
2. Adjust for a meter reading of zero, as described in Sect. A3d above.
3. Re-position to maximum probe-to-surface spacing and adjust BAL
control for a meter reading of zero.
4. Return to the minimum spacing and re-adjust ZERO. Repeat this
procedure until the degree of spacing insensitivity desired is reached.
The stability of this adjustment is directly related to zero drift and the
probe-to-surface spacing.