To enhance repeatability of successive measurements allow the meaterial to rest between mea-
surements. One general guideline is recording the electrification period required for a steady state
measurement and then add one minute to the measurement time to calculate the minimum rest
period between measurements. For example, if 1 minute is used to obtain a steady state measure-
ment – plus 1 minute – the rest time between measurements of the same sample is 2 minutes. On
the other hand, if a steady state measurement is obtained in 15 seconds, the total minimum time
between measurements is 1 minute 15 seconds. Typically, materials will be preconditioned at de-
fined temperature and relative humidity for 48 hours or more, and measured in the same environ-
ment using defined procedures to reduce the effect of ambient variables.
E. Calculating Volume Resistivity (ohm-cm)
Resistance is a measurement, whereas Volume Resisivity is a calculation based on (1) a resistance
measurement, (2) fixture dimensions and, (3) material thickness. To convert resistance measure-
ments in ohms obtained with the PRF-930 to volume resistivity (
ρ
) in ohm-cm, we use the follow-
ing equation to convert the resistance measurement to resistivity.
Where,
R = Resistance obtained from the material measurement in ohms
A
= Area of the fixture’s center plate in cm
2
d
= Distance in cm between the measurement and power plates.
The relationship of
area
A
divided by
distance
d
is often re-
ferred to as a
correction factor
in cm (
CF
), which is multiplied
by the measured resistance to obtain resistivity. (See
Sample
Calculations
, below)
Exact fixture dimensions can be obtained during fixture clean-
ing using the measurement procedures described below. When
the fixture is disassembled, and after it is cleaned, measure the
concentric center plate with a micrometer as shown in Figure 8,
at right.
The center plate area
A
is calculated using the equation:
Where
p
is approximately 3.1416 and
r
is the radius of the cen-
ter plate, which is one-half the plate diameter. The distance
d
between plates is measured with a micrometer (Figure 9) after
the fixture is reassembled. The measurement is made at the
center point of the measurement plates, midway between the
acrylic frame vertical supports.
Figure 8: Measuring Diameter of Measure-
ment Center Plate
Figure 9: Measuring the Distance
d
be
-
tween Plates
10
Rev. A / January 2010
PRF-930 Advanced Powder Fixture
