Monitoring Indoor CO₂ Levels

Introduction

Although monitoring indoor CO₂ levels has a long track record in regulating ventilation in Heating Ventilation and Air Conditioning (HVAC) systems, it became highly topical during the COVID-19 pandemic. Humans (and mammals in general) exhale CO₂ as part of the respiratory process. In indoor environments the absolute CO₂ levels reflect a balance between the rate of CO₂ generated by occupants (in the absence of other CO₂ sources) and the rate of exchange of indoor air with outside air (ventilation rate). Under steady state conditions (constant CO₂ generation and ventilation rates), an equilibrium level of CO₂ will be reached. For a given ventilation rate, the greater the CO₂ generation rate (number of occupants), the greater the equilibrium CO₂ level. The CO₂ concentration can be viewed as the fraction of ‘rebreathed air’ – hence its importance in relation to airborne disease transmission. As a very loose rule of thumb, it is desirable to keep indoor CO₂ levels below 1000 ppm.

Different Types of CO₂ Sensors

Semiconductor – equivalent CO₂

A semiconductor sensor, such as the SparkFun ENS160 Indoor Air Quality Sensor, relies on the fact that, in addition to CO₂, mammals also exhale small quantities of volatile organic compounds (VOCs) and Hydrogen. By measuring concentrations of VOCs and/or Hydrogen in the air, the sensor can use their values as a proxy for exhaled CO₂ on the assumption that the concentrations of VOCs/Hydrogen measured are solely a result of respiration. This type of sensor is cheap and compact and can work well in some circumstances for monitoring indoor CO₂ levels. However, if other non-respiratory sources of VOCs/Hydrogen are present in the local space, then misleading results will be obtained. For example, cracking open a can of beer will elevate local VOC/Hydrogen levels, as will lathering on hand sanitiser.

Figure 1. Spark Fun ENS160 air quality sensor including equivalent CO₂ (CC BY 2.0)

NDIR – actual CO₂

A better solution for measuring actual CO₂ levels (rather than equivalent CO₂ levels) is to use a Non-Dispersive Infra-Red (NDIR) CO₂ sensor. This type of sensor relies on the fact that gaseous CO₂ has a strong optical absorption band in the infrared spectrum. The sensor package consists of an infrared light source and a narrow-band optical filter tuned to the CO₂ absorption band along with a photodiode detector. As the concentration of CO₂ increases, so does the absorption of the infrared light source, leading to a reduction in the amount of infrared light which reaches the photodiode. Calibration of the photodiode signal allows a direct estimate of actual CO₂ levels.

A few examples include the following:

Telaire T6713 CO₂ Sensor

The Telaire T6713 CO₂ sensor features automatic calibration, provided the sensor is exposed to ambient outdoor CO₂ levels (400 ppm) once a week. Automatic background calibration is convenient for environments that regularly decay to ambient CO₂ levels, such as office spaces that are unoccupied outside of office hours, but may not be suitable for domestic environments unless measures are taken ensure ambient CO₂ levels are achieved at least once a week (e.g. by opening windows). Interfacing to the sensor is over I2C or UART. It retails for £80 – £90 depending on supplier.

Figure 2. Telaire T6713 CO₂ Sensor

Sensirion SCD30

This sensor supports automatic calibration as well as single-point calibration when exposed to ambient CO₂ levels. The single-point calibration feature makes this sensor useful for sensing environments that don’t regularly decay to ambient. It is often sold as a breakout board incorporating additional temperature and humidity sensors. Interfacing to the sensor is over I2C. It retails for around £80 depending on supplier.

Figure 3. Sensirion SCD30 CO₂ Sensor (CC BY 2.0)

Off-the-Shelf Solutions

There are a variety of off-the-shelf solutions often incorporating multiple sensors. One popular solution is the Aranet4 CO₂ monitor. This battery powered device employs an NDIR sensor and also reports temperature, humidity and barometric pressure. It supports automatic calibration or single-point calibration (recommended) at ambient CO₂ levels. Due to its small and lightweight size, it is easy to calibrate the device in outdoor air using the accompanying mobile app. It retails for £150 – £170 depending on supplier.

Figure 4. Aranet 4 CO₂ sensor

Summary

There are a variety of solutions available for measuring indoor CO₂ at various price points. Cheap semiconductor equivalent CO₂ sensors may provide a satisfactory option but are subject to artefacts particularly in environments where exposure to alcohol containing products is likely. For serious studies of indoor CO₂, NDIR-type sensors are recommended.