MIL-DTL-28827C
4.7.3.1 Circulating liquid bath. The test bath shall contain a liquid capable of safely operating at the
maximum test temperature consistently within ± 0.5°F, and with a maximum viscosity of 20CS at the
actuation point of the switch. The bath shall maintain a minimum velocity of 15 feet per minute
perpendicular to the sensing probe throughout the test. Bath temperature shall be monitored with a
platinum resistance probe and bridge or equivalent devices capable of measuring temperature within ±
0.5°F accuracy.
4.7.3.2 Circulating air oven. Air ovens are recommended for test temperatures above 600°F. The
oven must maintain an air stream velocity perpendicular to the sensing probe of 5400 feet per minute,
and a consistent temperature (±2°F) within the air stream. The air stream temperature shall be monitored
with a platinum resistance probe and bridge or equivalent device capable of measuring temperature
within ± 1°F.
4.7.4 Proof temperature (see 3.8). Switch sensing element shall be exposed to a proof temperature for
one minute or as specified, either liquid or air test devices may be used for this test. Following this test,
the actuation and deactuation points shall be within the tolerance specified.
4.7.5 High temperature exposure (see 3.9). Switches shall be subjected to the highest non-operating
ambient temperature specified (see 3.1) for a minimum of 12 hours. After this period, the temperatures
shall be changed to the highest operating temperature specified and held for a minimum of two hours.
Then, while at this temperature, the switches shall be tested in accordance with calibration (see 4.7.3).
4.7.6 Low temperature exposure (see 3.10). Switches shall be subjected to the lowest non-operating
ambient temperature specified (see 3.1) for a minimum of 12 hours. After this period, the temperature
shall be changed to the lowest operating temperature specified and held for a minimum of two hours.
Then, while at this temperature, the switches shall be tested in accordance with calibration (see 4.7.3).
4.7.7 Time constant (see 3.11). The time constant test facility shall consist of a drum containing a
circulating liquid. The bath temperature shall be such that the actuation point of the thermal switch is 63
percent of the difference between the ambient temperature at the test facility and the bath temperature,
see figure 1. The switch sensing probe shall be immersed in the bath, and the time to actuation recorded.
The switch shall be removed from the bath after actuation and cooled to room temperature before each
immersion. The test shall be repeated for a total of three times. The time constant shall be the average
of the three test values.
4.7.7.1 Time constant (alternate method). For switches where the bath temperature will exceed 600°F
a flowing air stream of specified velocity and density shall be used as a substitute for the oil bath. The
same basic procedures shall apply.
4.7.8 Proof pressure (see 3.12). Switches shall be connected to a device supplying the specified
media or approved substitute at a pressure which can be varied from zero to at least the proof pressure
specified (see 3.1). The pressure shall then be increased to the proof pressure and maintained for one
minute. There shall be no evidence of leakage or deformation.
following details shall apply:
a. Test condition C.
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1. Procedure III or IV; leakage-rate sensitivity 1x10 atm cm /s, for checking gross
leaks, test condition B.
2. For procedure IV Reduced pressure of the chamber and duration of pressurization
Determination made in accordance with the type of equipment used.
b. Measurements after test None.
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