Pressure Compensation of a CO2 Sensor
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APPLICATION NOTE AN001: PRESSURE COMPENSATION OF A CO2 SENSOR ABSTRACT Non-dispersive infra-red (NDIR) sensors work by measuring the amount of IR light absorbed by the target gas in a fixed volume. Gas concentration is proportional to the amount of light absorbed as it passes through the gas. The amount of light absorbed by the target gas, and hence measurement accuracy, is influenced by external environmental factors. The absorption of IR light is dependent on the number of gas molecules present in the fixed volume. Three factors influence the number of molecules in this fixed space, the gas concentration, gas temperature and gas pressure. To accurately measure gas concentration, it is therefore important to understand the effects of temperature and pressure. The general behaviour of a gas, due to changes in temperature and pressure is based on well understood laws. However, the effects of temperature and pressure on the absorption of infra-red light by gas molecules and how they influence measurement accuracy are less well understood. This application note describes the physical effects on the gas molecules due to changes in temperature and pressure. Changes in ambient temperature and pressure will induce CO2 concentration measurement errors unless corrected. It also explains how CO2 sensor measurements are affected by temperature and pressure and how these effects can be compensated for to reduce their impact on sensor accuracy. Gas Sensing Solutions Ltd. Page | 1 For regular updates, sign up at https://gassensing.co.uk Revision 1.0, 3 May 2021 Copyright © 2021 Gas Sensing Solutions Ltd.

Gas Sensing Solutions Ltd. Page | 2 For regular updates, sign up at https://gassensing.co.uk Revision 1.0, 3 May 2021 Copyright © 2021 Gas Sensing Solutions Ltd.

APPLICATION NOTE AN001: PRESSURE COMPENSATION OF A CO2 SENSOR BASIC PRINCIPLES All NDIR sensors measure the concentration of CO2 in a fixed volume, which in turn is based on the number of molecules present. The more molecules there are in the fixed volume, the more IR radiation is absorbed. Both temperature and pressure influence the number of molecules present in the fixed volume and therefore, measurement accuracy will be affected unless steps are taken to compensate for these effects. ENVIRONMENTAL FACTORS The two environmental factors that decide the number of molecules present in a...

APPLICATION NOTE AN001: PRESSURE COMPENSATION OF A CO2 SENSOR Figure 1: CO2 Vibration Modes The symmetric stretch does not create a dipole and consequently is not able to absorb any photons. It therefore does not absorb infra-red radiation. The other three vibration modes produce a dipole and absorb photons although at different frequencies. The asymmetric stretch absorbs radiation at 4.26µm, whereas the bending modes absorb at 15µm. Most NDIR based sensors measure the absorption of photons at 4.26um, and this is due to the asymmetric stretch vibration mode. The proportion of molecules...

APPLICATION NOTE AN001: PRESSURE COMPENSATION OF A CO2 SENSOR ABSORPTION Absorption at 4.26μm due to the symmetric vibration mode is negligible. This is because the average position of the gas molecule is the same as symmetric bending mode case, with no dipole and hence little absorption. However, as the atoms vibrate asymmetrically and form different dipole strengths depending on the energy levels of the electrons, the gas molecules begin to absorb infrared radiation. The different energy levels of the molecules cause absorption at a series of distinct wavelengths as shown below in Figure...

APPLICATION NOTE AN001: PRESSURE COMPENSATION OF A CO2 SENSOR Figure 3: Absorption at 4.28µm vs. Pressure Figure 4: Absorption at 4.28µm vs. Temperature As the gas temperature is increased, the gas molecules vibrate more intensely and increase in energy. However, the absorption of infra-red radiation actually decreases, as shown in Figure 4 by the drop in the peak and narrowing of the absorption band. This is due to a reduction in the number of gas molecules in the fixed volume that are available to absorb the IR light. Gas Sensing Solutions Ltd. Page | 6 For regular updates, sign up at...

APPLICATION NOTE AN001: PRESSURE COMPENSATION OF A CO2 SENSOR COMPENSATION OPTIONS It has been shown that IR radiation is affected by temperature and pressure. Depending on CO2 measurement accuracy requirements, it may be necessary to compensate for these effects. Temperature Compensation Techniques Changes in temperature effect sensor accuracy in several different ways. As well as those effects that can be described by simple gas laws, the LED and photo-diode, the electronics and the mechanical parts of the sensor will be all be affected by temperature. There are several ways to mitigate...

APPLICATION NOTE AN001: PRESSURE COMPENSATION OF A CO2 SENSOR APPLYING PRESSURE COMPENSATION As discussed earlier, the shape of the absorption band changes with pressure mainly caused by increased molecule collisions. This effect is known as spectral broadening. As pressure increases, so do the number of collisions due to the higher density of molecules present, and as CO2 has a higher density than air, there is also an effect driven by CO2 concentration. Ignoring the effect of spectral broadening, the measurements can be corrected for pressure based on basic gas laws. Boyle’s Law states...

APPLICATION NOTE AN001: PRESSURE COMPENSATION OF A CO2 SENSOR Figure 5 below shows how sensor measurements are affected using a constant Y correction factor. Without applying the correction, the sensor measurement would increase erroneously with increasing pressure. Including this correction factor maintains the accuracy of the measurement with changing pressure. Pressure (mbar) Uncorrected Sensor Meaurement (ppm) -Corrected Measurement (ppm) Figure 5: Effects of Pressure on Gas Concentration Page | 9 For regular updates, sign up at https://gassensing.co.uk Copyright © 2021 Gas Sensing...