pounds per square inch. Very frequently manometer scales are graduated
to read directly the desired units of pressure.
(4) The importance of the effect of the density of liquid used in the
manometer is apparent from the basic equation. For the same pressure,
a larger deflection (h) is obtained with a low-density liquid compared
with that from a high-density liquid.
For low pressures, greater
substantially all of the useful range is used.
For instance, if the
pressure is approximately 2 pounds per square inch, the deflection
using water is 55.4 inches, whereas using mercury, it would be 4.1
inches. Specific gravity is the ratio of density of a substance to the
density of water at 4 C. (39.2 F.). Therefore, the density of water
is used as a reference when determining the specific gravity of liquids
used in a manometer.
Now that you know the principle on which the
manometer operates, let's discuss the manometer pressure measurement
process and some related considerations.
e. Manometer Pressure Measurements.
A simplified system of basic test
connections is shown in Figure 21. You should realize that the system in Figure 21
should be improved by the addition of shutoff and vent valves.
checks, adjustments, and considerations which you should make are as follows:
(1) Before taking readings, check the manometer liquid level.
(2) Vent both the high- and low-pressure connections and check the liquid
against the corresponding fill mark.
(3) Turn the scale indicator knob (the larger of the two knurled knobs
under the manometer channel) until the proper scale (gage pressure or
vacuum-absolute pressure) is in position, visible through the glass
(4) If gage pressure or vacuum is to be determined, turn the zero set knob
(the smaller knurled knob just below the scale indicator knob) in
either direction as required to bring the zero mark in line with the
liquid meniscus in the tube.
(5) If absolute pressure readings are made, position the scale to indicate
the current barometric pressure in line with the liquid meniscus in the
tube, making certain that the manometer is vented at the time of