MIL-DTL-24251F
APPENDIX A
INSTRUMENTED TUBES
A.1 Scope. This appendix covers the instrumented tubes for use in the heat dissipation tests specified in 4.5.7.
The tube type recommended to represent each bulb type was chosen for its high power dissipation. The tube types
are listed in table IV. This appendix is a mandatory part of this specification. The information contained herein is
intended for compliance.
A.2 Tube thermocouple instrumentation.
A.2.1 Locating hot spots. A temperature sensitive paint shall be applied to the tube envelope. The appropriate
value paint shall be determined by trial and error to provide an accurate indication of the hot spot location when the
tube is operated at or near its maximum power dissipation rating and the hot spot is located by visual observation.
A.2.2 Preparing the bulb. The tube envelope diameter shall be measured with a micrometer and recorded. The
tube shall be prepared for grooving by masking the entire bulb with masking tape, leaving only a .062 inch (1.57 mm)
± .016 inch (0.41 mm) strip uncovered, running lengthwise along the bulb, and through the hot spot. The upper end
of the bare strip shall stop .125 inch (3.18 mm) to .188 inch (4.78 mm) from the beginning of the curved shoulder at
the top of the tube. The bare strip shall be allowed to run out at the bottom of the tube.
A.2.3 Etching the groove. A sand blaster with a .250 inch (6.35 mm) to .500 inch (12.70 mm) diameter nozzle may
be used. The etching material may be No. 120 or 220 grit aluminum oxide powder. Air pressure may be 45 psi. The
nozzle may be positioned 4-inches (101.6 mm) from the tube surface, so that the etching powder will impinge at right
angles to the surface to be etched.
A.2.3.1 Etching technique. Approximately 25 to 50 passes may be required with the nozzle over the exposed
glass. Each pass shall be extended beyond both ends of the tube envelope to prevent over etching at the ends of the
groove. On the 6336A tube, it may be necessary to roughen (by sandblasting) a small surface area, delta in shape,
at the lower end of the groove on the outside of the glass, to serve as a better anchor for the thermocouple and
protective tube.
A.2.3.2 Depth of the groove. The depth of the groove shall be .010 inch (0.25 mm) + .003-inch (0.08 mm) - .001
inch (0.03 mm). Periodic measurements of groove depth shall be made, as the speed of etching will vary with the
cleanliness, age, and moisture content of the etching material.
A.2.4 Thermocouple installation. The thermocouple used shall be a welded junction of 36 gauge iron and
constantan wire. The thermocouple shall be laid into the groove with its junction on the hot spot of the tube, held
down with a spring or other means and the leads running out of the lower end of the groove. The groove shall be
filled with Sauereisen, Insalute #1 cement, or equivalent, and the cement allowed to dry at 100°C for 24 hours or 36
to 48 hours at room temperature. The excess shall then be scraped off to restore the original contour of the tube
envelope.
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