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WORKSHOP SCIENCE - CITS

Open tube manometer (Fig 2)

Fig 2

It is more suitable to measure pressure in vessels which is having little variation to atmospheric pressure. It is a ‘u’
shaped tube containing mercury having its one end connected to the vessel container in which the liquid is there
whose pressure is to be determined. The other end is open. The manometer will show a difference in both the
limbs of the tube when the pressure inside the vessel is more or less than the air pressure outside.
For a barometer reading with reference to an atmospheric pressure of 1 bar we have
P = 1 + P (shown in Fig 2)
o
a
P = 1 - P (shown in Fig 3)
a u
Fig 3

Example (Fig 4)
A manometer is connected to an air pressure tank and it indicates an over pressure of 615 mm pressure head.
The external air pressure is 1015 mbar. Calculate the absolute pressure in bar and in Pascal (Fig 4).

Fig 4

100

CITS : WCS - Mechanical - Exercise 10