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3.3.3 Measuring Gas Pressure

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Instruments Used to Measure Gas Pressure

  1. The pressure of the gas in a container can be measured by using
    1. Bourdon gauge
    2. Manometer
  2. In SPM, almost all calculation question about using instruments to find gas pressure in a container are related to manometer. Therefore it's important for you to know the concept behind this instrument.
  3. For the Bourdon Gauge, you need to know its working mechanism.

Bourdon Gauge


  1. A Bourdon gauge is used to measure to gas pressure in a container.
  2. There are 3 important components in a Bourdon gauge
  3. the copper tube
  4. the lever system
  5. the pointer

Working Mechanism of a Bourdon Gauge

  1. When the gauge is connected to a gas supply, the gas pressure will straighten the copper tube.
  2. The copper tube exerts a force on the lever system and hence move the pointer. The movement of the copper tube is magnified by the lever system
  3. The pointer rotates and give a reading (in unit of Pascal).

Manometer


  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

Note:

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.

Example 1:

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]

Answer:
Pressure of the gas in cmHg

P = 20 + 76 = 96 cmHg

Pressure of gas in Pa
The atmospheric pressure,

Pressure of the gas,



Example 2:

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?

Answer:

Pgas = Pmercury + Patm

Pgas - Patm = Pmercury = 5 cmHg

  1. The pressure of the gas trapped in a capillary tube depends on the position of the tube.
  2. Figure below shows the pressure of the gas when the capillary tube is horizontal, vertical and vertically upside down.

Example 3:

Figure above shows 3 identical capillary tubes with one end sealed and containing a column of mercury. PA, PB and PC are the gas pressure in the capillary tubes respectively. Find the value of PA, PB and PC. [Atmospheric pressure = 76cmHg]

Answer:

PA = 76cmHg

PB = 76cmHg + 2cmHg = 78cmHg

PC = 76cmHg - 2cmHg = 74cmHg


Example 4:


Figure above shows some air trapped in a J-tube. Find the pressure of the trapped air. [Density of water = 1000 kg/m³; Atmospheric pressure = 100,000 Pa]
Answer: