Question

At a certain point, the absolute pressure and atmospheric pressure is given by 850 mm of Hg and 700 mm of Hg respectively. What is the value of gauge pressure (mm of Hg) at that point ?

• A. 50
• B. 100
• C. 150
• D. 200

Answer 1

The correct option is C 150


Data given :

$Absolutepressure,P_{abs}=850mmofHg$

$Atmosphericpressure,P_{atm}=700mmofHg$

$P_{gauge}=P_{abs}-P_{atm}=850-700=150mmofHg$

Atmospheric Pressure

• It is the pressure exerted by atmosphere. Its value is taken as 1.013 bar at mean sea level. It is measured by a barometer. At mean sea level, it is equal to 10.3 m head of water or 76 cm head of mercury (Sp.Gravity = 13.6).
• $h_{meter}$ head of water is equivalent to ${\gamma }_{\omega }h$ pressure and $h_{meter}$ head of Hg is equivalent to $\left({\gamma }_{Hg}h\right)$ pressure.

Absolute Pressure :

• Pressure measured w.r.t to absolute zero or complete vacuum is called absolute pressure. It is also called the actual pressure at a given position. It is measured using an Aneroid Barometer.

Gauge Pressure

• It is the pressure measured w.r.t. local atmospheric pressure as datum. It is measured using manometer or Bourdon gauge.

Most of the pressure measuring devices are calibrated to read gauge pressure.