Downloaded from https://www.everyspec.com MIL-STD-1311D METHOD 1226 ENVELOPE TEMPERATURE When a measurement of envelope temperature is required, the hot spot shall be located as specified in 1 below, and the temperature measured as specified in either 2 or 3 below. The method to be used shall be specified on the TSS. When the use of thermocouples is not practical because of high voltage or stray rf power, a temperature-sensitive paint or lacquer may be used. 1. Hot-spot location.
For each tube type the hottest point on the tube envelope shall be determined using a temperature lacquer (Tempilaq, or equivalent) with melting point not more than 5 ° C less than the maximum permissible bulb temperature for the type. The tube shall be placed in an oven, and with the specified test conditions applied, the oven temperature shall be raised until the lacquer just begins to melt locally. The central point of the melted area shall be taken as the location of the hottest point. 2. Temperature by loop-thermocouple measurement.
A length of wire, type 304, or 302, spring temper, corrosion-resistant steel, .004 inch diameter approximately equal to the perimeter of the tube envelope at its hottest point shall be formed into a loop by spot welding the ends together. At approximately 25 percent the length of the loop from the weld, a V-shaped kink shall be made at right angles to the plane of the loop to insure spring-tightness of the loop on the envelope at approximately 50 percent the length of the loop from the kink, the junction of a No. 40 B and S or equivalent, gauge thermocouple of chromel and alumel shall be spot-welded to the inside of the loop.
The junction of the thermocouple shall be calibrated to within ± 0.5 ° C. The loop thermocouple shall be placed on the tube envelope so that the junction is in direct contact with the hottest point. (See 1 above.) The tube with the thermocouple in position shall be placed in the oven. The cold junctions of the oven thermocouple and the loop thermocouple shall be immersed in a melting ice bath, or a cold junction compensated potentiometer may be used provided the necessary accuracy is obtained. All voltages shall be read with a potentiometer with a sensitivity of at least 0.02 millivolt (or 0.5 ° C, if direct reading). 3. Temperature by conduction-band measurements.
Envelope temperature measurements shall be made by use of thermocouple wires welded to opposite sides of a split phosphor bronze ring in good thermal contact with the envelope. The dimensions of the ring shall be as shown on figure 1226-1, or such that equivalent envelope contact is made. Unless otherwise specified, the ring shall be located at the hottest spot on the envelope (see 1 above). METHOD 1226 1226-1 19 April 1968