APPLICATION NOTE www.vaisala.com How to Measure Carbon Dioxide Carbon dioxide measurement is required in many applications from building automation and greenhouses to life science and safety. This document covers the following topics: • Operation principle of infrared carbon dioxide (CO2 ) sensors • The ideal gas law and how to use it to compensate the CO2 measurement for environmental factors • Optimal locations for CO2 transmitters • Safety issues related to CO2 Operation Principle of Infrared Sensors Carbon dioxide and other gases the measured gas doesn't directly consisting of two or more interact with the sensor, IR sensors dissimilar atoms absorb infrared can withstand high humidity, dust, (IR) radiation in a characteristic, dirt, and other harsh conditions. unique manner. Such gases are detectable using IR techniques. The key components of an IR Water vapor, methane, carbon CO2 detector are light source, dioxide, and carbon monoxide measurement chamber, are examples of gases that can be interference filter, and IR detector. measured with an IR sensor. Their IR radiation is directed from the characteristic absorption bands light source through the measured are shown in Figure 1. gas to the detector. A filter located Figure 1. IR absorption of CO2 and in front of the detector prevents some other gases. IR sensing is the most widely wavelengths other than that applied technology for CO2 specific to the measured gas from detection. IR sensors have many passing through to the detector. benefits over chemical sensors. The light intensity is detected and They are stable and highly converted into a gas concentration selective to the measured gas. value. They have a long lifetime and, as VIM-G-How-to-measure-CO2-Application-Note-B211228EN-B.indd 1 03/06/2019 13.21 The Vaisala CARBOCAP® approximation is often used to carbon dioxide sensor uses IR describe the behavior of real sensing technology to measure gases. The ideal gas law relates the volumetric concentration the state of a certain amount of of CO2. It features a unique gas to its pressure, volume, and electrically tunable Fabry-Perot temperature, according to the Interferometer (FPI) filter for equation: dual-wavelength measurement. This means that in addition to pV = nRT measuring CO2 absorption, the where CARBOCAP® sensor also performs p = pressure [Pa] a reference measurement, which V = volume of the gas [m3] Figure 2. The structure of the ® compensates for any changes n = amount of gas [mol] Vaisala CARBOCAP CO2 sensor. in the light source intensity as R = universal gas constant well as for dirt accumulation and (= 8.3145 J/mol K) contamination. This makes the T = temperature [K] sensor extremely stable over time. View the complete range of Vaisala products for CO2 measurement at www.vaisala.com/CO2 Pressure Increase at Constant Temperature Ideal Gas Law Pressure increases at constant temperature. The ideal gas law is useful when estimating the effect of temperature and pressure changes IR sensor detects more CO2 molecules. on CO2 measurement. It can be used to compensate the CO2 readings. Ideal gas is a hypothetical gas consisting of randomly moving Temperature Increase at Constant Pressure identical point particles that are negligible in size and possess negligible intermolecular forces. Temperature increases Ideal gas molecules are assumed at constant pressure. to undergo elastic collisions both with each other and with the IR sensor detects container walls. fewer CO2 molecules. In reality, gases do not behave exactly like ideal gases, but the 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. VIM-G-How-to-measure-CO2-Application-Note-B211228EN-B.indd 2 03/06/2019 13.21 The Effect of Temperature and Pressure on CO p 298 2 (t, p) (25C,1013hPa) Measurement 1013 (273 t) Most gas sensors give out a signal where proportional to the molecular ρ = gas volume concentration [ppm or %] density (molecules/volume of p = ambient pressure [hPa] gas), even though the reading t = ambient temperature [°C] is expressed in parts per million (volume/volume). As the pressure Equation 1. Calculation of gas concentration at given temperature and and/or temperature changes, pressure. the molecular density of the gas changes according to the ideal gas law. The effect is seen in the ppm and pressure variations when Table 1 shows an example of the reading of the sensor. measuring CO2. Typical CO2 changes in the CO2 sensor reading instruments do not measure (gas contains 1,000 ppm of CO2 at The following illustrations visualize pressure and thus cannot SATP) as temperature and pressure how an increase in pressure or automatically compensate change, according to the ideal gas temperature changes the state for pressure variations. When law. of the gas and how it affects CO2 calibrated at the factory, measurement. instruments are typically set to Drying a Wet Gas Sample sea-level pressure conditions The ideal gas law can be used to (1013 hPa). When measuring at Processing the ideal gas law calculate the molecular density of altitudes other than sea-level, it is further provides a way of a gas at a given temperature and recommended to compensate for understanding what happens when pressure, when the gas density at the pressure effect. This can be the composition of a gas mixture Standard Ambient Temperature done either by entering the correct is varied at a constant pressure, and Pressure (SATP) conditions is pressure settings for internal temperature, and volume. This can known. Replacing the amount of compensation (constant pressure be used, for example, to estimate gas (n) with ρV/M, and assuming conditions) or by programming the the effect of changing humidity on that the molar mass of the gas (M) compensation into an automation the CO2 reading. is constant in the two different system or PC (changing pressure conditions, the equation can be conditions). The molecules of a gas mixture written as in Equation 1. exist in the same system volume The same compensation rules (V is the same for all gases) at the The density formula can be used to apply to the temperature effect. same temperature. The ideal gas estimate how gas sensor reading However, there are more and law can be modified to: changes as temperature and/or more CO2 meters available that pressure is changed. both measure and compensate RT p (ngas1 ngas2 ngas3 ...ngasn ) for temperature variations, and V The density formula can be used therefore do not require any where to compensate for temperature external compensation. ngas1 = amount of gas 1 [mol] ngas2 = amount of gas 2 [mol], etc. and p pgas1 pgas2 pgas3 ...pgasn where p = total pressure of the gas mixture pgas1 = partial pressure of gas 1 pgas2 = partial pressure of gas 2, etc. Table 1. The ppm reading of a CO2 sensor when measuring a gas with 1,000 ppm concentration under different temperature and pressure conditions. VIM-G-How-to-measure-CO2-Application-Note-B211228EN-B.indd 3 03/06/2019 13.21 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 Table 2. Dilution coefficients in gas sample drying. pressure, temperature, and volume, water replaces some of the gas molecules in the mixture. Similarly, when a gas sample high-humidity environment can Example: A gas sample is drawn is drawn from a high-humidity be calculated when the CO2 from an environment with a dew environment and is allowed to dry concentration of the dried gas point of 40°C (73,000 ppm of before entering the measurement is known. In order to do this, the water) to an environment of chamber of a CO2 meter, the loss dew point (Td at 1013 hPa) or water 20°C Td (23,200 ppm of water). of water molecules changes the concentration (ppm) of the wet The measured CO2 concentration composition of the gas and has an and dry conditions need to be of 5.263% at 20°C Td translates to effect on the CO2 measurement. known. The humidity condition of 5.00% in the 40°C Td environment the high-humidity environment is (5.263% × 0.950 = 5.00%). The This so-called dilution effect chosen from the horizontal axis lower reading is caused by dilution can be estimated using Table 2. and the condition of the dried gas resulting from the higher water The CO2 concentration of the from the vertical axis. content at 40°C Td. 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.