A manometer consists of a glass U-tube containing liquid such as coloured water or mercury. It is used to measure gas pressure by determining the difference between the gas pressure and atmospheric pressure based on the difference in liquid column levels.
See how the difference in liquid height h corresponds to the difference between gas pressure and atmospheric pressure at the same horizontal reference line.
Both arms are open to the atmosphere, so the liquid levels in both arms are equal.
Gas pressure pushes the liquid down on the gas side, while the open side rises by height h.
A manometer consists of a glass U-tube containing liquid such as coloured water or mercury. It measures gas pressure by comparing the gas pressure with atmospheric pressure.
At equilibrium, two points at the same horizontal level in the same continuous liquid have the same pressure. Therefore, \(P_{\text{gas}} = h + P_{\text{atm}}\).
The height, \(h\), of the liquid column represents the difference between gas pressure and atmospheric pressure: \(\Delta P = P_{\text{gas}} - P_{\text{atm}} = h\).
To convert liquid column height to pascals, use \(P = h\rho g\), where \(\rho\) is liquid density and \(g\) is gravitational acceleration.
Answer in your mind, then press “Check Answer”.
Why is mercury preferred over water in a manometer when measuring high gas pressures?
If the diameter of the glass tube of a manometer is doubled, what happens to the height difference, h, of the liquid columns?
A mercury manometer shows a liquid column height difference of 40 cm. Given that \(P_{\text{atm}} = 76\ \text{cm Hg}\), what is the pressure of the gas supply in cm Hg?
A wider manometer tube requires more force to push the liquid, so the height difference \(h\) will be smaller.
Pressure depends only on vertical depth, liquid density, and gravitational acceleration (\(P = h\rho g\)). It is independent of the surface area or diameter of the tube.
The height difference \(h\) directly equals the total gas pressure.
The height difference \(h\) represents only the difference between the gas pressure and atmospheric pressure (\(P_{\text{gas}} - P_{\text{atm}}\)). To find the total gas pressure, atmospheric pressure must be added.
Objective: Test your understanding of manometer principles, pressure calculations and key terms.
1 What happens to the height difference, \(h\), between the liquid levels in a manometer if the glass tube is replaced with one of larger diameter?
2 A mercury manometer connected to a gas tank has a mercury level difference of 25 cm. If atmospheric pressure is 75 cm Hg, what is the gas pressure in cm Hg?
3 In a manometer, the height difference \(h\) represents the difference between the gas pressure and pressure.
4 Digital manometers that use a pressure are more common nowadays because they are portable and provide accurate readings without using toxic mercury.
Match each manometer term or parameter with its correct description. If using a phone, tap the answer first, then tap the matching box.
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