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4.0 Testing Hints and Other Tests
4.1
General
The measurement of electrostatic charge is dependent on several factors,
mainly sample characteristics, humidity, and to a much lesser extent,
temperature. It is very common for a 10% change in one condition, such as
humidity, to cause a significant change in decay rate. It is very important to try to
duplicate
measurements. Many times this is not possible, such as when an antistatic
additive blossoms to the surface of a material over a period of time. The
following are some hints and additional test procedures that will help to
understand the many factors affecting electrostatic measurements.
4.2
Free Air Measurement
Measure the electrostatic field with 5kV applied to just the electrodes (no sample
in place). This is the free air electrostatic field caused by the charge on the
electrodes. It is approximately 1,500-1800 Volts with magnetic electrodes and
2000-2200 Volts with clamp electrodes. Other electrode configurations will result
in different voltage levels.
If a sample is inserted the maximum charge measured when 5,000 volts is
applied is the free air value. This indicates that the sample is a very good
insulator and will not accept any conducted charge.
The charging voltage is applied to the ends of the sample. If the sample has very
high insulating properties the electrons will not flow across the sample. However,
since the electrons will not flow, the material may have developed a deficiency or
an excess of electrons during manufacture or by handling. This condition results
in the material having an initial charge associated with it that will not change by
conductive charging or by grounding. When the material is rubbed or separated,
voltages may be generated by triboelectric charging. The only way to remove
this initial charge is to place it in front of an ionizer for several seconds. This will
must be taken into account when testing finished product that
may have a large area or multiple sides such as a box or bin.
To determine a Pass/Fail criteria first perform a standard static
test of the material the product is made from. Then perform the
nondestructive test on the product. The ratio of the two decay
times will determine the Pass/Fail criteria for the nondestructive
test.
To test liquids and powders place the product in a glass or other
insulated container that is at least 8x6" (203x153mm) and
approximately 1" (25mm) deep. Place the Fixture on the material
and perform the test in the normal manner. The electrodes should
not penetrate the material by more than approximately ¼" (6mm).
The electrodes are stainless steel and the conductive rubber is
nickel impregnated silicone rubber.
a
set
of
conditions
exactly
when
24
attempting
to
compare
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