- A manometer is a U-shape tube filled with some liquid (usually mercury).
- Manometer is a device used to measure gas pressure in a container.
- 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
Note:There are a few points we need to know when using a manometer
- Difference in gas pressure at difference level can be ignored.
- Pressure on the surface of liquid is equal to the gas pressure in contact.
- Pressure that cause by liquid = hρg.
- For a given liquid, the pressures at any point of the same level are the same.
- 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)
Pressure of the gas,
P=(0.2)(13.6× 10 3 )(10)+103,360