JOHNSON CONTROLS
80
FORM 102.20-OM2
ISSUE DATE: 6/01/2015
SECTION 4 - SERVICE AND REPAIR
Instruments employing this principle are called ma-
nometers. The simplest form is the basic and well-
known U-tube manometers as shown in Figure 85 on
page 80. This device indicates the difference be-
tween two pressures or between a single pressure and
atmosphere, when one side is open to atmosphere.
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
h
FIGURE 85 - U-TUBE MANOMETERS
LD06364a
The U-tube manometer is a primary standard because
the difference in height between the two columns is al-
ways 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 in. range as shown in Figure 86 on page
80.
LD27928b
FIGURE 86 - SLACK TUBE MANOMETER
DUCT PRESSURES
Velocity
When air moves at a given velocity (V) in FPM in a
duct, it creates a pressure corresponding to the veloc-
ity, which is a measure of the kinetic energy in the fluid
and is known as the velocity pressure (VP). VP is al-
ways exerted in the direction of air flow. The relation-
ship between the velocity and the VP may be expressed
by the following formulas:
Vp =
V
4005
V = 4005 Vp
(
)
2
It is a simple matter to determine the velocity (FPM)
of an the air stream so the VP can be measured. For ex-
ample, if a Pitot Tube manometer hook-up reads 0.25
inches water, the previous equation should be substi-
tuted:
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.
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