THE DIFFERENCE BETWEEN ELECTROSTATIC VOLTMETERS, FIELDMETERS, STATIC METERS AND STATIC LOCATORS There are several different names associated with instruments used to measure electrostatic charge, voltage and fields. The term electrostatic (or static charge) is commonly used to refer to the voltage measured on the surface of an object. In actuality, charge is the parameter associated with the voltage on the surface of an object and its capacitance where Q (Charge) = C (capacitance) times V (voltage) or (Q=CV) Charge is expressed in Coulombs and can be measured directly using a Coulomb Meter (or Nanocoulombmeter) and Faraday cup. There are two basic types of measurements used to measure the voltage or field potential on the surface of an object. When using a static meter (static locator, fieldmeter), it is the voltage that is actually being measured. ELECTROSTATIC VOLTMETERS measure the actual potential (voltage) at the surface of the object under test. These instruments use non-contacting sensors with a high voltage amplifier to drive the sensing probe to the same potential as that of the surface being measured. By measuring the output of the high voltage amplifier an accurate indication of the unknown potential can be determined. Measurement accuracy is typically 0.1% regardless of probe-to-surface spacing (spacing is usually in the range of 0.1”) with a target resolution 0.1” (2.5mm) or smaller. Electrostatic voltmeters typically can measure over the range from millivolts to several thousand volts. ELECTROSTATIC FIELDMETERS measure the electrostatic field produced by a charged surface located some distance away (voltage per unit distance) from a high input impedance or chopper stabilized sensor that is referenced to ground. (These meters are also called static meters or static locators). The sensor-to-surface distance must be known in order to obtain an accurate measurement. The size of the surface being measured in relation to the field of view of the sensor, the relation of the charged surface to ground and the presence of other objects in proximity to the surface all can have a significant impact on the measurement. These meters are generally calibrated and their accuracy specified when a grounded meter is placed a fixed distance from a large flat plate in free space that is charged to a specific voltage. For example, a 12” x 12” plate in free space is charged to 1kV and the static meter sensor is set exactly 1.0” from the center of the plate. The meter is then calibrated to read 1kV. The Meter is now calibrated to measure field strength of 1kV/inch. The voltage on the surface of an unknown large flat object in free space that is exactly 1.0” from the face of the sensor can be determined with an accuracy of better than 5%. If the surface being measured has voltage gradients (different voltages at different locations) the meter will indicate the average voltage on the surface. Therefore, these meters are not the best choice when making high-resolution measurements on a given surface. For near-field measurements, the relationship between voltage and distance is linear. At a spacing of 2.0” the meter will read 500V etc. Fieldmeters are capable of measuring

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THE DIFFERENCE BETWEEN ELECTROSTATIC VOLTMETERS,FIELDMETERS, STATIC METERS AND STATIC LOCATORS

There are several different names associated with instruments used to measure

electrostatic charge, voltage and fields. The term electrostatic (or static charge) iscommonly used to refer to the voltage measured on the surface of an object. In actuality,charge is the parameter associated with the voltage on the surface of an object and itscapacitance where Q (Charge) = C (capacitance) times V (voltage) or (Q=CV) Charge isexpressed in Coulombs and can be measured directly using a Coulomb Meter (orNanocoulombmeter) and Faraday cup.

There are two basic types of measurements used to measure the voltage or fieldpotential on the surface of an object. When using a static meter (static locator,fieldmeter), it is the voltage that is actually being measured.

ELECTROSTATIC VOLTMETERS measure the actual potential (voltage) at the surface

of the object under test. These instruments use non-contacting sensors with a highvoltage amplifier to drive the sensing probe to the same potential as that of the surfacebeing measured. By measuring the output of the high voltage amplifier an accurateindication of the unknown potential can be determined. Measurement accuracy istypically 0.1% regardless of probe-to-surface spacing (spacing is usually in the range of0.1”) with a target resolution 0.1” (2.5mm) or smaller. Electrostatic voltmeters typicallycan measure over the range from millivolts to several thousand volts.

ELECTROSTATIC FIELDMETERS measure the electrostatic field produced by acharged surface located some distance away (voltage per unit distance) from a highinput impedance or chopper stabilized sensor that is referenced to ground. (Thesemeters are also called static meters or static locators). The sensor-to-surface

distance must be known in order to obtain an accurate measurement. The size of thesurface being measured in relation to the field of view of the sensor, the relation of thecharged surface to ground and the presence of other objects in proximity to the surfaceall can have a significant impact on the measurement.

These meters are generally calibrated and their accuracy specified when a groundedmeter is placed a fixed distance from a large flat plate in free space that is charged to aspecific voltage. For example, a 12” x 12” plate in free space is charged to 1kV and thestatic meter sensor is set exactly 1.0” from the center of the plate. The meter is thencalibrated to read 1kV. The Meter is now calibrated to measure field strength of1kV/inch. The voltage on the surface of an unknown large flat object in free space that isexactly 1.0” from the face of the sensor can be determined with an accuracy of better

than 5%.

If the surface being measured has voltage gradients (different voltages at differentlocations) the meter will indicate the average voltage on the surface. Therefore, thesemeters are not the best choice when making high-resolution measurements on a givensurface.

For near-field measurements, the relationship between voltage and distance is linear. Ata spacing of 2.0” the meter will read 500V etc. Fieldmeters are capable of measuring

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very large voltages at sufficiently large distances. However, if the sensor, and testsurface are calibrated as a total system by placing a known voltage on the surface oronto foil placed on the surface, a fairly accurate measurement can be obtained since allthe variables such as distance, size, interference etc. would be compensated for in thecalibration. With a calibrated test set-up accuracies better than 2% can be obtained.

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