Question

A manometer reads the pressure of a gas in an enclosure as shown in the fig. (a) when some of the gas is removed by a pump, the manometer reads as in fig (b). The liquid used in manometer is mercury and the atmospheric pressure is 76 cm of mercury, (i) Give absolute and gauge pressure of the gas in the enclosure for cases (a) and (b).

Answer 1

(i) In case (a) Pressure head, h = + 20 cm of Hg 

Absolute Pressure = P + h = 76 + 20 = 96 cm of Hg. 

Gauge Pressure = h = 20 cm of Hg. 

In case (b) Pressure Head h = – 18 cm of Hg 

Absolute Pressure = 76 – 18 = 58 cm of Hg 

Gauge Pressure = h = – 18 cm of Hg