Figure 3-8. Altimeter calibration-operating Setup.

TM 9-6685-202-14

MI (C) 6685-202-14-3-8

Figure 3-8. Altimeter calibration-operating Setup.

with the vernier adjusting knob. Align the altitude index

valve completely and open the relief valve slightly to

on the vernier slide with the desired altitude mark on the

allow the pressure to rise to the proper point.

scale. The altitude is marked at the following intervals:

WARNING

-1,000 to 10,000 feet, marked each 100 feet,

The operator should not break the

10,000 to 0,000 feet, marked each 500 feet,

seal between the valve subassembly

50,000 to .0,000 feet, marked etch 1,000 feet.

and the barometer tube while

If unmarked altitudes are desired, position the vernier

vacuum Is applied.

slide to the pressure equivalent of the desired altitude.

(5) After the pressure has been adjusted to

(4) Open the vacuum valve slightly. The

align the mercury meniscus with the sighting ring, the

mercury will fall and the altimeters will indicate an

pressure in the test chamber will be the same as that for

increase in altitude. As the, pressure nears the desired

the altitude which the vernier slide was set. Read and

point, the meniscus will be inside the sighting ring but

record the altimeter readings.

will be visible through the open side of the ring.

b. Type A-1 Modified. The modified barometer

Continue to decrease the pressure until the highest point

does not have an altitude scale for direct comparison of

of the meniscus appears to barely touch the lower edge

altimeters. However, the modified barometer can be

of the sighting ring as in figure 3-5. If the pressure has

used for

been reduced too much as indicated by light being

visible above the entire meniscus, close the vacuum

3-15