Understanding Carbon Dioxide Sensors and their Applications - Robert E. Henderson

Safety professionals sometimes forget that carbon dioxide is a toxic gas with a strictly defined Occupational Exposure Limit. The safest approach is to measure CO2 directly when the gas is present in potentially dangerous concentrations.

Liquid and solid carbon dioxide (dry ice) are widely used as refrigerants, especially in the food industry. Carbon dioxide (CO2) is also used in many industrial and chemical industry processes. CO2 is particularly associated with the beer and wine making industries, where it is produced by yeast during the fermentation process. CO2 in the headspace of fermentation vessels can easily reach 50% by volume or even higher concentrations. CO2 is also widely used in the oil industry, where it is commonly injected into oil wells to decrease viscosity. It is also one of the most common atmospheric hazards encountered in confined spaces.

Carbon dioxide (CO2) is a byproduct of living organisms, and is naturally present in the earth’s atmosphere with an average concentration of about 350 ppm in fresh air. CO2 is a primary byproduct of
bacterial decomposition, and in many confined spaces there is a direct relationship between low concentrations of oxygen and elevated concentrations of CO2. If the low oxygen is due to bacterial action, a
concentration of 19.5% oxygen would be associated with an equivalent concentration of at least 1.4%, or 14,000 ppm CO2, which is almost three times higher than the occupational exposure limit.

The true concentration of CO2 can be substantially higher if the oxygen deficiency is due to displacement. Fresh air contains only 20.9% oxygen by volume. The balance consists mostly of nitrogen, with minor or trace concentrations of a wide variety of other gases including CO2. Because oxygen represents only about one-fifth of the total volume of fresh air, every 5% of a displacing gas that is introduced into a confined space reduces the oxygen concentration by only 1%. In the case of an oxygen deficiency due to the introduction of dry ice into an enclosed space, a reading of 19.5% O2 would not be indicative of 1.4% CO2, it would be indicative of 7% CO2, a concentration fourteen times higher than the toxic exposure limit!

CO2 is also heavier than air, with a density of 1.5 times that of fresh air. When carbon dioxide is released into an enclosed or confined space it tends to settle to the bottom of the space, reaching the highest concentration in the lowest parts of the space. Because of this tendency to settle, as CO2 is produced it can reach higher and higher concentrations in localised regions of the space (such as the head space immediately above the liquid in fermentation vats).

In spite of these considerations, in the past it was seen as adequate to simply measure the oxygen concentration. This attitude is changing as it becomes more feasible (and affordable) to directly measure CO2 by means of compact, portable multi-sensor gas detectors.