In a gas mixture each component contributes a partial pressure equal to its fraction times the total pressure. Many sensors respond to partial pressure or to the number of molecules per volume, so a change in pressure or temperature can shift a reading even when the gas is unchanged. Air is 20.95 % oxygen, about 212 hPa of oxygen at sea level.
- Use the ideal gas law qualitatively to predict the effect of pressure and temperature
- Explain partial pressure and mole fraction
- State the composition of dry air
- Explain why a sensor can read differently at the same concentration
The ideal gas law in one line
For most sample gases near atmospheric pressure, volume, pressure and temperature are linked by a simple rule: the pressure of a fixed amount of gas rises with temperature and falls when the volume grows.
P x V = n x R x T
Two practical lessons follow. First, a given volume of gas at higher pressure holds more molecules, so it is denser and an analyser that counts molecules in a fixed volume reads proportionally more. Second, gas at a higher temperature is less dense, so the same volume holds fewer molecules.
Mole fraction and partial pressure
A concentration given in percent or ppm by volume is a mole fraction: the share of molecules that are the target gas. It does not change when you compress or heat the gas.
The partial pressure of a component is its mole fraction times the total pressure. Air at one standard atmosphere (1013.25 hPa) is 20.95 % oxygen, so the partial pressure of oxygen is about 212 hPa.
partial pressure = mole fraction x total pressure
A sensor that responds to partial pressure does not read the mole fraction directly. If the total pressure at the sensor changes, the reading changes in proportion, even though the gas mixture is identical.
The composition of dry air
| Gas | Share of dry air |
|---|---|
| Nitrogen | 78.08 % |
| Oxygen | 20.95 % |
| Argon | 0.93 % |
| Carbon dioxide | about 0.04 % (roughly 420 ppm) |
Oxygen at 20.9 or 20.95 % is the natural calibration point for oxygen analysers, and it is the reference for zirconia cells. See How a zirconia oxygen analyser works.
What this does to a reading
- Pressure. A partial-pressure sensor reads about 1 % lower for a 1 % fall in absolute pressure. This is why purity analysers compensate for ambient pressure: Why ambient pressure affects oxygen purity readings.
- Temperature. Changes sensor output and gas density. Good analysers measure it and correct.
- Sample line pressure. Gas in a pressurised line packs more molecules into the same tube, so it takes longer to flush: Gas sample line lag time calculator.
- Water vapour. Water is a gas too and takes its share of the total. Wet gas shows lower oxygen than the same gas dried: Dry basis vs wet basis in gas measurement.
- Percent and ppm by volume are mole fractions and do not change with pressure.
- Partial pressure equals mole fraction times total pressure.
- Some sensors respond to partial pressure, so pressure changes shift their reading.
- Dry air is 78.08 % nitrogen, 20.95 % oxygen and 0.93 % argon.
Check your understanding
Answer in your head or on paper first, then open the answer. Recalling an answer teaches more than rereading.
What is the partial pressure of oxygen in air at 1013.25 hPa?
About 212 hPa: 20.95 % of 1013.25.
Does compressing a gas change its percentage composition?
No. Composition as a mole fraction is unchanged; only the partial pressures change.
An oxygen reading falls 3 % when a storm arrives. What is the likely cause?
Barometric pressure fell about 3 %, and the sensor responds to partial pressure. A compensated analyser removes the effect.
Key terms
Frequently asked questions
Why does gas composition not change when I compress it?
Compression squeezes all components equally, so each keeps its share of the molecules.
What is absolute pressure?
Pressure measured from a perfect vacuum, as opposed to gauge pressure, which is measured from the surrounding atmosphere.
Why do analysers need a pressure reading?
Some sensors respond to partial pressure, so the analyser measures ambient pressure to correct the reading.
Is air really 21 % oxygen?
Dry air is 20.95 %, usually rounded to 21 % or 20.9 %.
