MIL-PRF-24236D
4.6.5 Temperature response.
4.6.5.1 Sensitivity response test (see 3.10) (when specified, see 3.1 and 6.2.2). The switch shall be placed in a
sensitivity test facility. One channel of the facility shall be adjusted to a temperature of 70 degrees F +5 degrees F
below the temperature set point of the switch under test, while the other channel of the facility shall be adjusted to 70
degrees F +5 degrees F above the temperature set point of the switch under test. The air shall be maintained at a
velocity of 20 +2 feet per second (ft/s) in both channels throughout the test. The switch shall be inserted into the
channel operating at the lower temperature, allowed to heat for at least 10 minutes, and then moved, within 1 second
into the channel operating at the higher temperature. The time required for operation of the switch after it has been
moved into the high temperature channel shall not exceed 10 seconds. The switch shall be allowed to soak for at
least 10 minutes, after which it shall be moved back into the channel operating at the lower temperature. The time
required for the switch contacts to open after it has been moved into the lower temperature channel shall not exceed
10 seconds.
4.6.5.1.1 Sensitivity test facility. The sensitivity test facility shall be a gas-flow stove with two similar rectangular
channels that can be operated separately at different temperatures. The channels shall be located side by side so
that the switch can be moved, by a sliding mechanism, from one channel to the other within 1 second. During the
tests, only the temperature sensing portion of the switch need be immersed in the gas flow. To prevent ambient air
from being drawn into the channel where the switch goes through the channel wall, a slight overpressure on the test
section may be maintained. The cross section of the channel test section shall have at least a 1-inch clearance
around the sensing element, except on the mounting side where the mounting flange touches the wall. The velocity
distribution in the test section shall correspond to that of normal turbulent flow in a straight channel. At a distance of
10 percent of the channel width from the channel wall, the flow velocity shall be at least 75 percent of the center
velocity. The test section and the section upstream of the test section shall be insulated so that the wall temperature
will not be more than 10 percent below the center temperature at the test section. The temperature measurement
shall be made with a thermocouple, at least 1 inch of which is parallel to, and in contact with, the wall. Reference
values for the test shall be the temperature and velocity, measured in the center of the channel. The temperature
shall be measured 2-1/2 = 1/2 inch upstream from the switch. The measurement shall be made with a 28 AWG
(0.0126 inch) thermocouple. The thermocouple shall be located at least two channel widths from the outlet end of the
channel. The test section shall be protected from the radiant effect of any flame or surface having a higher
temperature than the gas temperature.
4.6.6 Temperature anticipation (see 3.11) (when specified, see 3.1 and 6.2.2). The switches shall be stabilized at
-65 degrees F for 1 hour. Within 1 minute, the switches shall be inserted in a channel of the sensitivity facility at a
temperature of 70 degrees F +5 degrees F below its actuating temperature with an air velocity of 20 ft/s, and allowed
to remain for 3 minutes. Switches shall then meet the requirements of 3.11.
4.6.7 Seal (as applicable, see 3.12). Switches shall be tested in accordance with 4.6.7.1, or 4.6.7.2, as applicable.
4.6.7.1 Hermetic. Switches shall be tested in accordance with method 112 of MIL-STD-202. The following details
shall apply:
(a) Test condition - C.
(1) Procedure III or IV leakage-rate sensitivity - 1x108 atm cc/sec: for checking gross
leaks, test condition B or D.
(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.
4.6.7.2 Watertight (see 3.12.2). Switches shall be immersed to a depth of 6 +2 inches in a container of water
containing approximately 1/2 of 1 percent aerosol, and shall then be placed in a vacuum chamber. The absolute
pressure shall be 1.3 inches of mercury and this pressure shall be maintained for a period of 1 minute, or until air
bubbles cease to be given off by the water, whichever is longer. The absolute pressure shall then be increased to 2.5
inches of mercury and this pressure maintained for 2 minutes. During the 2-minute period, the switches shall be
observed for evidence of a continuous stream of bubbles. Any bubbles coming from within the switches shall be
considered as leakage. Bubbles, which are the result of entrapped air on the exterior of the switches, shall not be
considered as an indication of leakage.
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