How to Measure Carbon Dioxide (CO₂)

The ideal gas law is useful for estimating the effect of temperature and pressure changes on CO2 measurement, and it allows readings to be compensated accordingly.

An ideal gas is a hypothetical gas consisting of randomly moving identical point particles, negligible in size and with negligible intermolecular forces. In practice, real gases don't behave exactly this way, but the approximation reasonably describes their behavior.

The law relates the state of a given amount of gas to its pressure, volume, and temperature:

Most gas sensors output a signal proportional to molecular density (molecules per volume of gas), even though the reading is expressed in parts per million (volume/volume). As pressure and/or temperature change, the molecular density of the gas changes according to the ideal gas law — and that effect is reflected directly in the sensor's ppm reading.

Typical CO2 instruments do not measure pressure and therefore cannot automatically compensate for its variations. When calibrated at the factory, instruments are usually set to sea-level pressure conditions (1013 hPa). When measuring at altitudes other than sea level, it's recommended to compensate for the pressure effect — either by entering the correct setting for internal compensation (constant pressure conditions) or by programming the compensation into an automation system or PC (changing pressure conditions). The same logic applies to the temperature effect, although more and more CO2 meters now measure and compensate for temperature internally, without requiring external compensation.

Table 1 shows, as a reference, how a CO2 sensor's reading (gas containing 1,000 ppm of CO2 at standard conditions) changes with temperature and pressure, according to the ideal gas law.

The molecules of a gas mixture occupy the same system volume at the same temperature. According to Dalton's Law of Partial Pressure, the total pressure of a gas mixture is the sum of the partial pressures of all the component gases.

This is relevant to understanding the influence of water vapor on a CO2 sensor's reading. When water vapor is added to a dry gas at constant pressure, temperature, and volume, water replaces some of the gas molecules in the mixture. Similarly, when a gas sample is drawn from a high-humidity environment and allowed to dry before entering the measurement chamber of a CO2 meter, the loss of water molecules changes the gas composition and affects the measurement.

This dilution effect can be estimated using Table 2. Knowing the CO2 concentration of the already-dried gas, it's possible to calculate the CO2 concentration of the original high-humidity environment — this requires knowing the dew point (Td at 1013 hPa) or water concentration (ppm) of both the wet and dry conditions.

Example: a gas sample is drawn from an environment with a dew point of 40°C (73,000 ppm of water) into an environment of 20°C Td (23,200 ppm of water). The measured CO2 concentration of 5.263% at 20°C Td translates to 5.00% in the 40°C Td environment (5.263% × 0.950 = 5.00%). The lower reading is caused by dilution resulting from the higher water content at 40°C Td.