Measuring carbon dioxide levels
Answered by Bob Henderson, president of GfG Instrumentation Inc., Ann Arbor, MI.
Safety professionals sometimes forget that carbon dioxide is a toxic gas with a strictly defined permissible exposure limit. The safest approach is to measure CO2 directly when the gas is present in potentially dangerous concentrations.
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 byproduct of bacterial decomposition and, in many confined spaces, a direct relationship exists between low concentrations of oxygen and elevated concentrations of CO2. If the low oxygen is due to bacterial action, a concentration of 19.5 percent oxygen would be associated with an equivalent concentration of at least 1.4 percent, or 14,000 ppm CO2, which is almost 3 times higher than the PEL.
The true concentration of CO2 can be a lot higher if the oxygen deficiency is due to displacement. Fresh air contains only 20.9 percent oxygen by volume. Because oxygen represents only about one-fifth of the total volume of fresh air, every 5 percent of a displacing gas that is introduced into a confined space reduces the oxygen concentration by only 1 percent. It could take as much as 7 percent CO2 (a concentration 14 times higher than the toxic exposure limit) to displace enough oxygen for the O2 sensor to go into alarm at 19.5 percent.
CO2 also is heavier than air, with a density 1.5 times that of fresh air. When CO2 is released into a confined space it tends to settle at the bottom of the space, reaching the highest concentration in the lowest parts. Because of this, as CO2 is produced, it can reach higher and higher concentrations in localized regions of the space.
In spite of these considerations, in the past it was believed 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 multisensor gas detectors.
CO2 is listed as a toxic contaminant with a strictly defined PEL in every state. The most widely recognized exposure limits for CO2 reference an 8-hour time-weighted average of 5,000 ppm.
Besides displacing the oxygen in fresh air, high concentrations of CO2 can worsen the symptoms related to oxygen deficiency and interfere with successful resuscitation. Even moderately elevated concentrations can produce physiological symptoms, and concentrations of 40,000 ppm or higher should be regarded as immediately dangerous.
How nondispersive infrared CO2 sensors detect gas
The most widely used technique for real-time CO2 measurement is nondispersive infrared sensors that measure gas as a function of the absorbance of infrared light. In the past, infrared-based instruments tended to be bulky and expensive, and required a high level of operator expertise to obtain accurate readings. A new generation of miniaturized NDIR sensors has permitted the development of infrared-based instruments for an ever-widening variety of atmospheric hazards, including combustible gas and CO2 detection.
The regulations already are changing. Recent accidents have heightened concerns and increased the obligation for direct CO2 measurement during workplace procedures that may expose workers to this contaminant. With more availability and the increasingly affordable cost of miniaturized NDIR CO2 sensors, more atmospheric monitoring programs will include the direct measurement of this dangerous atmospheric contaminant.