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KAH Operation & Maintenance Guide
Page: 62 of 88
SECTION 4 - SERVICE & REPAIR
U-TUBE MANOMETER
Pressure is defined as force per unit area. The best way
to measure air pressure is to balance a column of liquid
of known weight against the air pressure, and measure
the height of liquid columns so balanced. The units of
measure commonly used are inches of mercury (in. Hg.),
using mercury as the fluid; and inches of water (inches
wg), using water or oil as the fluid.
Instruments employing this principle are called
manometers. The simplest form is the basic and well-
known U-tube manometers as shown in Figure 73 (below).
This device indicates the difference between two pressures
or between a single pressure and atmosphere, when one
side is open to atmosphere.
If a U-tube is filled to the halfway point with water, and air
pressure is exerted on one of the columns, the fluid will
be displaced. Thus, one leg of water column will rise and
the other will fall. The difference in height, h, which is the
sum of the readings above and below the halfway point
indicated the pressure in inches of the water column.
The U-tube manometer is a primary standard because the
difference in height between the two columns is always a
true indication of the pressure regardless of the variations
in the internal diameter of the tubing.
For use on all air units, the manometers should cover at
least a 26” range as shown in Figure 74 (right).
FIGURE 74 - SLACK TUBE MANOMETER
FIGURE 73 - U-TUBE MANOMETERS
0
1
2
3
4
1
2
3
4
4
h
3
2
1
0
1
2
3
4
4
3
2
1
0
1
2
3
4
PRESSURE
VACUUM
h
