Question

A U-tube mercury manometer with one arm open to atmosphere is used to measure pressure in a steam pipe. The level of mercury in open arm is 97.5 mm greater than that in the arm connected to the pipe. Some of steam in the pipe condenses in the manometer arm connected to the pipe. The height of this column is 34 mm. The atmospheric pressure is 760 mm of Hg. Find the absolute pressure of steam.

Answer 1

Equating the pressure in mm of Hg on both arms above the line XX ), 

we get

p_abs. + p_water = p_Hg + p_atm.

Now, p_water = \(\cfrac{34}{136}\) = 2.5 mm of Hg.

∴ p_abs + 2.5 = 97.5 + 760

p_abs = 97.5 + 760 – 2.5

= 855 mm of Hg.

= 855 × p_Hg × g × 10^–5 bar

= \(\cfrac{855}{1000}\) (m) x (13.6 × 1000) (kg/m³) × 9.81 × 10^–5

= 1.1407 bar.