1. A manometer is a U-shape tube filled with some liquid (usually mercury).
  2. Manometer is a device used to measure gas pressure in a container.
  3. The pressure of the gas is equal to the sum of the atmospheric pressure and pressure due to the column of liquid.
Pgas = Patm + Pliquid


There are a few points we need to know when using a manometer
  1. Difference in gas pressure at difference level can be ignored.
  2. Pressure on the surface of liquid is equal to the gas pressure in contact.
  3. Pressure that cause by liquid = hρg.
  4. For a given liquid, the pressures at any point of the same level are the same.
  5. For different liquid with different density, pressure at two different level will be different.


The figure above shows a manometer containing mercury connected to a tank with methane liquid and gas. Find the pressure of the gas supply in the units cmHg and Pa.
[Density of mercury = 13.6 x 10³ kg/m³; atmospheric pressure = 76 cmHg]

Pressure of the gas in cmHg

P = 20 + 76 = 96 cmHg

Pressure of gas in Pa
The atmospheric pressure,
Patm =(0.76)(13.6× 10 3 )(10)
Patm =103,360Pa

Pressure of the gas,
P=hρg+ Patm
P=(0.2)(13.6× 10 3 )(10)+103,360
P =130,560Pa


Figure above shows the mercury levels in a manometer used to measure the pressure of a gas supply. How much is the gas pressure greater than the atmospheric?


Pgas = Pmercury + Patm

Pgas – Patm = Pmercury = (20 – 15) cmHg

Pmercury = 5 cmHg