MIL-PRF-27208F
g. Measurement during test: While resistors are still in the oven, resistance shall be measured between the end
terminals at the end of the 0.5 hour "off" period after 50 hours ±4 hours, 100 hours ±8 hours, 250 hours ±12
hours, 500 hours ±12 hours, 750 hours ±12 hours, and 1,000 hours ±12 hours have elapsed and compared to
the similar readings taken (see 4.7.16c).
h. Measurements after test: After resistors have been removed from the oven and returned to room
temperature, setting stability and total resistance shall be measured as specified in 4.7.9.1 and 4.7.2.1,
respectively, and compared to the similar readings taken in 4.7.16c. Dielectric withstanding voltage (at
atmospheric pressure), and torque shall be measured as specified in 4.7.6.1 and 4.7.8.1, respectively.
i. Examination after test: Resistor shall be examined for evidence of mechanical damage.
4.7.17 Low temperature operation (see 3.22).
4.7.17.1 Mounting. Resistors shall be mounted in such a manner as to allow electrical connections to be made to
the terminals.
4.7.17.2 Procedure. Total resistance shall be measured as specified in 4.7.2.1. The resistors shall be placed in a
chamber at room temperature. The temperature shall be gradually decreased to -55oC +0oC, -5oC within a period of
not less than 1.5 hours. For conformance inspection only, and at the option of the manufacturer, the resistors may be
placed in the chamber when the chamber is already at the extreme low temperature. After one hour of stabilization at
this temperature, setting stability shall be measured as specified in 4.7.9.1. Full rated continuous working voltage
(see 3.1 and 3.6) shall be applied for 45 minutes. The resistors may be loaded individually or in parallel. Fifteen
minutes +5 minutes, -0 minutes after removal of voltage, setting stability shall be measured as specified in 4.7.9.1.
The temperature in chamber shall be gradually increased to room temperature within a period of not more than 8
o
o
hours. The resistors shall be removed from the chamber and maintained at a temperature of 25 C ±5 C for a period
of approximately 24 hours. Total resistance shall be measured as specified in 4.7.2.1. Resistors shall then be
examined for evidence of mechanical damage.
4.7.18 High temperature exposure (see 3.23).
4.7.18.1 Mounting. Resistors shall be mounted in such a manner as to allow electrical connections to be made to
the terminals.
4.7.18.2 Procedure. Total resistance and setting stability shall then be measured as specified in 4.7.2.1 and
o
o
o
4.7.9.1, respectively. The resistors shall then be exposed to an ambient temperature of 150 C +5 C, -0 C for a
period of 250 hours ±8 hours. Not less than 2 hours after the end of the exposure period, setting stability and total
resistance shall be measured as specified in 4.7.9.1 and 4.7.2.1, respectively. Torque shall be measured as
specified in 4.7.8.1, except that it shall be determined during the movement of the contact arm from the position for
setting stability to the position for total resistance. Dielectric withstanding voltage (at atmospheric pressure), and
insulation resistance shall be measured as specified in 4.7.6.1 and 4.7.7, respectively. Resistors shall then be
examined for evidence of mechanical damage.
4.7.19 Rotational life (see 3.24).
4.7.19.1 Mounting. Resistors shall be mounted by their normal mounting means on a 0.0625 inch thick, glass
base, epoxy laminate. The resistors shall be ganged in pairs, and each pair shall be connected in series, as shown in
figure 4, so that a nominally constant current flows through the resistors, irrespective of the contact arm position
during the turning of the operating shaft.
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