Monroe Electronics Electrostatic Voltmeter - Isoprobe® - model 279L - Manual (Page 24)

279L-0401-RCA Page 21
APPENDIX IV
ACHIEVING SPACING INDEPENDENCE
Eliminating Zero Shift Over a Large Probe-to-Surface Spacing Range
This instrument’s basic operating principle is to null the electric field between a surface under measurement and the
sensitive electrode in the probe. When the potential of the surface under measurement is zero, the instrument
drives the sensitive electrode and the probe housing (a reference surface) to zero potential, thereby nulling the
electric field between the probe and the surface under measurement.
Ideally, only the surface under measurement contributes to this process. However, if other voltage sources exist in
the vicinity of the sensitive electrode, which are independent of the unknown to be measured, offsets are produced.
Such voltage sources include contact potential differences among internal probe parts, small specs of charged dust,
etc. The ZERO1 control is provided to counter such offsets by applying a voltage directly to the sensitive electrode.
The influence of these voltage sources is usually not the same at all probe-to-surface separations. Their influence
will vary in proportion to the probe-to-surface separation and will cause the instrument zero offset voltage to vary
with probe-to-surface spacing changes. This effect can be minimized to afford a high degree of zero stability over a
range of spacing changes. This is known as spacing independence.
The following procedure is used to optimize the spacing independence of the instrument:
1. Position the probe at the minimum spacing to the surface under test. Apply zero volts to the surface under
test.
2. Switch the instrument to the OPERATE mode and adjust the GAIN for a critically damped or somewhat
over-damped condition.
3. Adjust the ZERO2 control for a zero output voltage from the instrument.
4. Re-position the probe to the maximum spacing from the surface under test.
5. Adjust the ZERO1 control for a zero output voltage from the instrument.
6. Return the probe to the minimum spacing to the surface under test.
7. Repeat steps 3 through 7 until the zero shift is minimized to the desired level.