Some Observations about Electrostatic Fieldmeters
If there is one equation that must always be kept in mind during
electrostatic measurements, it is Q = C V, where Q = charge, C = capacitance, and V = voltage.
Variations in voltage measurements in factors of hundreds may readily be seen using electrostatic fieldmeters due to the inverse
proportion of Voltage and Capacitance. The relative capacitance of objects under test must be kept in mind when interpreting any voltage readings obtained.
Today, we commonly hear of the need to keep the voltage levels on printed circuit boards below some set number because of the sensitivity of the
components. Let's say we have a circuit board sitting 1mm above a bench top and we measure 50 volts on that board. If we now lift that board to 10cm without discharging the original charge, the voltage will then rise to 5000volts because the capacitance drop is inversely proportional to the distance of the printed circuit to the bench, and the lower the capacitance, the higher the voltage. Without the awareness of this phenomenon, a false sense of security can be left in quick field surveys.
The practical fieldmeter used today is a ground referenced measuring
device in which readings are proportionally related to the distance from the probe to the surface or object under test. This trait is one of the limiting factors of all fieldmeters and if accurate readings are to be obtained, the distance from the fieldmeter probe to the surface under test must be precisely known.
The fact that most fieldmeters use a ground reference dictates that precautions be taken to be sure the fieldmeter be grounded while in use.
Another common characteristic of fieldmeters is the field of view of the
probe. For accuracy, the target size should be three to four times the distance from the probe to the surface under test. This dictates that the fieldmeter probe should be as close as possible to the test surface, unless the need is to measure over a large area. Fieldmeters can tell little of the voltage on the surface of components and are more accurate measuring large objects, such as packaging and garments.
Also, if the design of the fieldmeter does not allow for continual
grounding or shunting of the probe to ground or reference ground, the meter will be subject to both drift in long term measurements and false readings in ionized environments. Fieldmeters which operate drift free in these two instances are called chopper stabilized, or modulated.
With these concepts in mind, voltage measurements can be obtained in the 2% to 5% range.
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