MIL-DTL-17896E
nor more than four inches. It shall have a diameter (as measured across the free aperture of the
crystal) of 4.75 inches ± 0.375 inch (120.65 ± 9.52 mm). A circular mounting flange shall be a part of
the case. The flange shall be secured to the case by brazing, silver soldering, or mechanical assembly.
The flange shall be sturdily made and shall not exceed seven inches in diameter. Three mounting
holes with rubber bushing inserts having a 0.188 inch (4.77 mm) inside diameter shall be spaced at
uniform distances around the flange. They shall be located so that the barometer can be readily
mounted on a wall or similar support without the use of special tools. The top hole shall be on the
vertical centerline in line with the 30.0-inch mark. A zero adjusting screw shall be provided for
setting the index hand. The adjusting screw shall be accessible through an opening in the case; the
opening shall be labeled "Regulator". The opening shall provide for the passage of air to the
movement and shall be protected by a fine-mesh screen insert, which shall be readily removable for
access to the zero adjusting screw.
3.4.1 Exterior finish. The exterior for the brass or bronze case shall be brush finished
and protected by a transparent lacquer or plastic coating. Aluminum die cast cases shall be
anodized or alodined and painted. The finish paint shall be in accordance with MIL-DTL-15090.
3.4.2 Bezel ring. The bezel ring, if used, shall be the same material as the case and shall
have a polished or brushed finish and a protective coating of the same finish as the case. It shall
be sufficiently thick to be rigid. The bezel ring shall be attached to the case by threads or other
means adequate to ensure a tight fit. The bezel ring shall hold the window snugly in the case.
The bezel ring shall allow removal of the window and aneroid mechanism.
3.4.3 Window. The window shall be either glass in accordance with ANSI Z26.1 or
plastic in accordance with ASTM D 788, class 3. The window shall show no defects that will
interfere with the assembly or operation of the barometer. A hole shall be provided in the center
of the window for the reception of the set hand knob assembly.
3.5 Dial. The dial shall be metal and have a dull white or silver white matte finish.
Lettering, numerals, and graduations shall be engraved, deep etched, or stamped into the surface
and shall be filled with a durable black enamel. Means shall be provided to prevent the dial from
shifting its position with respect to the index hand or case.
3.5.1 Type I barometer graduations. For type I barometers, two sets of graduations shall
be provided on two concentric scales: an outer scale graduated in inches of mercury and an
inner scale graduated in millibars. The graduations of the inch scale shall extend 360 degrees on
the dial ranging from 26.5 to 31.5 inches. The graduations shall be divided in two hundredths
with tenths and inches accentuated and identified. The millibar scale shall range from 900 to
1060 millibars and shall be graduated in millibars with fives and tens accentuated and tens
identified (990, 1000, 1010). The indications of the millibar scale shall be equivalent to the
corresponding values of the inch scale throughout the range of the millibar scale.
3.5.2 Type II barometer graduations. For the type II barometer, the graduations shall be
in inches of mercury extending 360 degrees on the dial, ranging on a scale from 28 to 38 inches.
The graduations shall be divided into tenths with tenths and inches accentuated and identified.
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