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Description / Abstract:
Purpose: This method is suitable for determining the volume
permittivity, (dielectric constant) and loss tangent (dissipation
factor) of insulating materials at 1 MHz. It is not dependent on
either direct or indirect measurement of specimen thickness and
therefore is very useful for thin films and laminates but may also
be used on specimens up to approximately 6.35 mm [0.25"] thick.
It is useful for all ranges of permittivity and for loss tangent
as low as 0.0005 providing the range and accuracy of the bridge
used are adequate.
Description of Method: The two fluid method utilizes air as one
fluid and a suitable liquid, normally Dow 200 1.0CS silicon fluid,
as the second. Using an established value for the permittivity of
air, the values for the permittivity of the fluid and the sample
may easily be calculated. The cell spacing is fixed during all
readings but does not need to be known accurately for the series of
readings required. Since specimens do not require any electrodes to
be applied and since many specimens can be measured at one time
without changing any spacings or machine settings, the method is
not only very accurate but very rapid.
The method has been used for measurement of PTFE and epoxy glass
laminates and flexible films, e.g. polyimide.
Reproducibility lab to lab is excellent for permittivity
provided minimal precautions are observed and bridge accuracy is
appropriate. On most materials, the effects of small changes in
moisture or temperature are larger than any error due to the
method. Lab to lab correlation on stable material such as PTFE have
shown results to be consistently within 0.005 or (0.2%).