How to measure carbon dioxide (CO₂)

Operation Principle of Infrared Sensors

Carbon dioxide and other gases consisting of two or more dissimilar atoms absorb infrared (IR) radiation in a characteristic, unique manner. Such gases are detectable using IR techniques. Water vapor, methane, carbon dioxide, and carbon monoxide are examples of gases that can be measured with an IR sensor. Their characteristic absorption bands are shown in Figure 1.

IR sensing is the most widely applied technology for CO₂ detection. IR sensors have many benefits over chemical sensors. They are stable and highly selective to the measured gas. They have a long lifetime and, as the measured gas doesn't directly interact with the sensor, IR sensors can withstand high humidity, dust, dirt, and other harsh conditions.

The key components of an IR CO₂ detector are light source, measurement chamber, interference filter, and IR detector. IR radiation is directed from the light source through the measured gas to the detector. A filter located in front of the detector prevents wavelengths other than that specific to the measured gas from passing through to the detector. The light intensity is detected and converted into a gas concentration value.

The Vaisala CARBOCAP® carbon dioxide sensor uses IR sensing technology to measure the volumetric concentration of CO₂. It features a unique electrically tunable Fabry-Perot Interferometer (FPI) filter for dual-wavelength measurement. This means that in addition to measuring CO₂ absorption, the CARBOCAP® sensor also performs a reference measurement, which compensates for any changes in the light source intensity as well as for dirt accumulation and contamination. This makes the sensor extremely stable over time. View the complete range of Vaisala products for CO₂ measurement at www.vaisala.com/CO2

Figure 1. IR absorption of CO₂ and some other gases.

Ideal Gas Law

The ideal gas law is useful when estimating the effect of temperature and pressure changes on CO₂ measurement. It can be used to compensate the CO₂ readings.

Ideal gas is a hypothetical gas consisting of randomly moving identical point particles that are negligible in size and possess negligible intermolecular forces. Ideal gas molecules are assumed to undergo elastic collisions both with each other and with the container walls.

In reality, gases do not behave exactly like ideal gases, but the approximation is often used to describe the behavior of real gases. The ideal gas law relates the state of a certain amount of gas to its pressure, volume, and temperature, according to the equation:

pV = nRT 
where 
p = pressure [Pa] 
V = volume of the gas [m3] 
n = amount of gas [mol] 
R = universal gas constant (= 8.3145 J/mol K) 
T = temperature [K]

Figure 2. The structure of the Vaisala CARBOCAP® CO₂ sensor.

Pressure Increase at Constant Temperature

Temperature Increase at Constant Pressure

Optimal Locations for CO₂ Transmitters

• Avoid locations where people may breathe directly onto the sensor. Also avoid placing sensors close to intake or exhaust ducts, or near windows and doorways.  
• In demand controlled ventilation wall-mounted sensors provide more accurate data on ventilation effectiveness than duct-mounted sensors. Duct-mounted sensors are best suited to single-zone systems and should be installed as close to the occupied space as possible to allow for easy maintenance access.
• When measuring CO₂ for the purposes of personnel safety, transmitters should be installed close to potential leakage points to enable early detection. The geometry, ventilation, and airflow of the monitored area need to be taken into account. The number and location of the CO₂ transmitters should be based on a risk assessment.

The Effect of Temperature and Pressure on CO₂ Measurement

Most gas sensors give out a signal proportional to the molecular density (molecules/volume of gas), even though the reading is expressed in parts per million (volume/volume). As the pressure and/or temperature changes, the molecular density of the gas changes according to the ideal gas law. The effect is seen in the ppm reading of the sensor.

The following illustrations visualize how an increase in pressure or temperature changes the state of the gas and how it affects CO₂ measurement.

The ideal gas law can be used to calculate the molecular density of a gas at a given temperature and pressure, when the gas density at Standard Ambient Temperature and Pressure (SATP) conditions is known. Replacing the amount of gas (n) with ρV/M, and assuming that the molar mass of the gas (M) is constant in the two different conditions, the equation can be written as in Equation 1.

where 
ρ = gas volume concentration [ppm or %] 
p = ambient pressure [hPa] 
t = ambient temperature [°C]

Equation 1. Calculation of gas concentration at given temperature and pressure.

The density formula can be used to estimate how gas sensor reading changes as temperature and/or pressure is changed.

The density formula can be used to compensate for temperature and pressure variations when measuring CO₂. Typical CO₂ instruments do not measure pressure and thus cannot automatically compensate for pressure variations. When calibrated at the factory, instruments are typically set to sea-level pressure conditions (1013 hPa). When measuring at altitudes other than sea-level, it is recommended to compensate for the pressure effect. This can be done either by entering the correct pressure settings for internal compensation (constant pressure conditions) or by programming the compensation into an automation system or PC (changing pressure conditions).

The same compensation rules apply to the temperature effect. However, there are more and more CO₂ meters available that both measure and compensate for temperature variations, and therefore do not require any external compensation.

Table 1 shows an example of the changes in the CO₂ sensor reading (gas contains 1,000 ppm of CO₂ at SATP) as temperature and pressure change, according to the ideal gas law.

Table 1. The ppm reading of a CO₂ sensor when measuring a gas with 1,000 ppm concentration 
under different temperature and pressure conditions.

Drying a Wet Gas Sample

Processing the ideal gas law further provides a way of understanding what happens when the composition of a gas mixture is varied at a constant pressure, temperature, and volume. This can be used, for example, to estimate the effect of changing humidity on the CO₂ reading.

The molecules of a gas mixture exist in the same system volume (V is the same for all gases) at the same temperature. The ideal gas law can be modified to:

where 

n_gas1 = amount of gas 1 [mol] 
n_gas2 = amount of gas 2 [mol], etc.

and

where

p = total pressure of the gas mixture 
p_gas1 = partial pressure of gas 1 
p_gas2 = partial pressure of gas 2, etc.

The second equation is called Dalton's Law of Partial Pressure. It states that the total pressure of a gas mixture is the sum of the partial pressures of all the component gases in the mixture.

This information is useful when taking into account the influence of water vapor on CO₂ sensor readings. 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 is allowed to dry before entering the measurement chamber of a CO₂ meter, the loss of water molecules changes the composition of the gas and has an effect on the CO₂ measurement.

This so-called dilution effect can be estimated using Table 2. The CO₂ concentration of the high-humidity environment can be calculated when the CO₂ concentration of the dried gas is known. In order to do this, the dew point (T_d at 1013 hPa) or water concentration (ppm) of the wet and dry conditions need to be known. The humidity condition of the high-humidity environment is chosen from the horizontal axis and the condition of the dried gas from the vertical axis.

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

Table 2. Dilution coefficients in gas sample drying.

Carbon Dioxide and Safety

Carbon dioxide is a non-toxic and non-flammable gas. However, exposure to elevated concentrations can induce a risk to life. Whenever CO₂ gas or dry ice is used, produced, shipped, or stored, CO₂ concentration can rise to dangerously high levels. Because CO₂ is odorless and colorless, leakages are impossible to detect, meaning proper sensors are needed to help ensure the safety of personnel.