Carbon Dioxide (CO2) Sensor Technical Notes

This article provides an overview of the calibration and operation of the Carbon Dioxide (CO2) gas sensor.

How the Carbon Dioxide Sensor Works

The Carbon Dioxide sensor is a digital sensor available in Blackline Safety’s sensor portfolio. The sensor has a wide concentration detection range, long operating life, and a high degree of selectivity for carbon dioxide gas over other possible airborne gases.

Along with oxygen and nitrogen, earth’s atmosphere has a known natural background carbon dioxide concentration of 400 to 450 ppm outside. In occupied indoor spaces with acceptable air volume exchanges, this background concentration can range from 400 to 1,000 ppm. At concentrations of 1,000 to 5,000 ppm, carbon dioxide may be associated with overall poor air quality, drowsiness and headaches.  

At concentrations above 5,000 ppm, the severity of exposure symptoms increases. At concentrations above 40,000 ppm, carbon dioxide is considered immediately dangerous to health and life.

Sensor Calibration

Proper sensor calibration is key to ensuring each gas sensor is accurate. Due to the background concentration of carbon dioxide, it is strongly recommended that you verify that your calibration gas source is not at or near empty before calibrating your device. This ensures that the sensor calibration is completed correctly, and the sensor response is verified accordingly.

Carbon dioxide sensors can be calibrated using a gas mix that includes CO2 at 5,000 ppm, or a single gas cylinder containing 5,000 ppm CO2. Blackline Safety recommends that the balance gas for a gas mixture is an O2 composition of 18% and N2 for balance. For a single gas cylinder, the balance gas is synthetic air with an O2 composition of 20.9% and N2 for balance.

Failure to apply the correct gas concentration can result in a sensor baseline offset which affects the normal clean air concentration measured by the device and can cause inaccurate reporting of higher concentrations that could impact health and life.

Notes on Sensor Response

As pointed out above, carbon dioxide is always present in the environment. The processes or equipment operating at the user’s location will likely impact what gas concentration displays on their device.

Concentration baseline values of 400 to 450 ppm, using a properly calibrated sensor, are correctly displayed in a clean air environment. When activity increases in the area—for example, combustion gases from engines, an increased number of people, or increased use of other secondary sources of CO2—the sensor will respond to the presence of elevated CO2 gas concentrations and report those increases. This is expected and the sensor is operating correctly.

Based on Blackline’s review of how this sensor operates in different nonindustrial situations, we recommend that CO2 values above 1,200 ppm should be interpreted as an indicator of increased CO2 exposure and the correct steps followed to protect all individuals in the immediate or surrounding area. Values between 450 ppm and 1,200 ppm can be considered as the local background concentration values and noted, if necessary, but otherwise taken as nonhazardous for that location. Please consult your local Health and Safety group or an Industrial Hygienist for further information and guidance at your location.