U-Tube Manometer
About the U-tube manometer
The U-tube manometer consists of a glass
vessel in the shape of a letter U. When the manometer is used for
measuring gauge pressure, both ends of the tube are open, with an
unknown pressure being applied at one end and the other end being open
to the atmosphere. The unknown gauge pressure of the fluid (P1)
is then related to the difference (h) in the levels of fluid in
the two halves of the tube by the expression:
P1 = hpg
where P is the specific gravity of the fluid.
In a third way of using the U-tube manometer, each open end of the
tube is connected to different unknown pressures (P1 and P2),
and the instrument thereby measures the differential pressure (P1
- P2) according to the expression:
(P1 -P2) = hpg
U-tube manometers are typically used to measure gauge and differential
pressures up to about 2 bar. The type of liquid used in the instrument
depends on the pressure and characteristics of the fluid being
measured. Water is a convenient and certainly cheap choice, but it
evaporates easily and is difficult to see. It is nevertheless used
extensively, with the major obstacles to its use being overcome by
using colored water and regularly topping up the tube to counteract
evaporation.
Situations where water is definitely not used as the U-tube manometer
fluid include the measurement of fluids which react with or dissolve
in water, and also where higher-magnitude pressure measurements are
required. In such circumstances, liquids such as aniline, carbon
tetrachloride, bromoform, mercury or transformer oil are used.
The U-tube manometer, in one of its various forms, is an instrument
commonly used in industry to give a visual measurement of pressure
which can be acted upon by a human operator. It is not normally
possible to transform the output of a U-tube manometer into an
electrical signal, however, and so this instrument is not suitable for
use as part of automatic control systems.

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