Figure 3-4. Sighting the mercury level.

TM 9-6685-202-14

(2) Lock vernier clamp nut and make the final

NOTE

adjustment with the vernier adjusting knob (fig. 1-2).

Rapping the table before each

(3) When sighting the meniscus, use the

reading will aid in adjusting the

barometers light to provide an illuminated background.

meniscus shape in the tube and

cistern.

The error incurred by

(4) Move the eye up and down to be sure it is

improper meniscus shape may be as

aligned with both the front and rear edges of the sighting

ring. Correct alignment is achieved at the point where

large as 0.15 mm.

the greatest amount of light is visible behind the

meniscus. At the correct position of sighting ring and

(1) Unlock the vernier clamp nut and adjust

eye, the two edges of the sighting ring and meniscus will

the vernier slide assembly so that the lower edge of the

appear to just touch as in figure 3-4.

sighting ring is approximately in line with the top of the

mercury column.

Figure 3-4. Sighting the mercury level.

lists constant values of units of measurement used in

manufacturing and utilizing mercury barometers.

c. Constant Value of Units

Table 3-1. Constants Used in Construction and Calibration of Mercurial Barometers.

DATA

VALUE

1 mm of mercury

0.019337 psi

10-1 cm - 10-3 m

1 mm

0.039370079 in.

1 in.

25.4 mm . 2.54 cm

1 mb

10 dynes cm-

1 mm

1 torr

1 micron

1 millitorr

3-9