MIL-PRF-87260B(USAF)
4.5.22 Electrical resistivity test.
All volume resistivity measurements of the conductive material shall be conducted in
accordance with the basic requirements of ASTM D257. A standard set of electrodes shall be
used for all measurements and shall be constructed from stainless steel in accordance with the
guidelines of FIGURE 4. The measurements shall be made on qualification and production
specimens per 4.5.4, and on full size buns through the bun thickness. For qualification, the
resistivity and resistivity uniformity shall be established by evaluating a standard bun of the
finished material at several locations (four minimum) by using production specimens, per 4.5.4,
from various positions in the bun (top, middle, bottom of the bun height). For production tests,
the resistivity specimens shall be taken from the top, middle, and bottom 3 inches of the test
section (per 4.5.4). Qualification and production specimens shall be tested at standard
laboratory conditions. The full size bun measurements may be conducted in the manufacturing
area after final trimming has been accomplished, with temperature and humidity recorded.
These measurements shall be conducted without cutting the bun, but by measuring the
resistance through the bun thickness. The electrical resistivity values shall then be extrapolated
to standard conditions (72 ±5°F, 55 ±5 percent relative humidity). The extrapolation shall be
accomplished with a calibration curve(s) for the product as a function of relative humidity (see
4.5.22.3 for the required correlation data).
4.5.22.1 Test equipment and preconditioning environment.
A suggested electrical resistivity test set-up employs a Beckman Megohmmeter Model L-8 (or
equivalent) with a variable resistance (106 to 1013 ohms) and a variable supply voltage (1-1000
volts), in combination with the stainless steel electrodes on FIGURE 4. The production test
specimen size shall be the same as that for air pressure drop (see 4.5.4) and shall be taken
from the same locations (top, middle, bottom). One set may be used for both air pressure drop
and electrical resistivity. If the resistance of the 1-inch test specimen is below the lower limit
(106 ohms) of the megohmmeter being used, it may be necessary to use more than one
specimen. By stacking two or three 1-inch thick specimens together a value of resistance may
be obtained that can be measured on the meter. Production tests shall be conducted in a
laboratory environment following a minimum of 1 hour of preconditioning in the test
environment. For full size bun testing, the bun should also be preconditioned for at least 1 hour
prior to testing, and the temperature and humidity in the production test location recorded along
with the measured resistance and calculated resistivity.
4.5.22.2 Test procedure and calculation.
test sample (specimen or bun) placed between the electrodes and the electrodes centered
directly over each other to produce a vertical "field" between the plates. The megohmmeter
voltage shall be set to 500 volts and the variable resistance adjusted until a steady-state
resistance reading is obtained on the meter. Allow the meter to stabilize for 1 minute and
record the resistance for the specimen along with the temperature, relative humidity, and the
thickness of the specimen. Calculate the sample volume resistivity as follows:
m easu r ed r esist an ce ( oh m s) X 1 5 5 . 7 ( cm 2 )
ev =
sam p le t h ick n ess ( in ch es) X 2 . 5 4 ( cm / in )
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