Don’t Use Oxygen Detectors for CO₂ Monitoring, Advises Manufacturer IGD

It may be tempting for users of CO₂ (R744) refrigeration to use oxygen (O₂) gas detectors to monitor for high levels of CO₂ in a confined space instead of a CO₂ detector. But U.K. manufacturer International Gas Detectors (IGD), advises against it in an article on its website.

IGD points out that CO₂ is present in the atmosphere at around 450ppm (0.04%). However, at higher concentrations in enclosed spaces, CO₂ becomes problematic, generating occupational exposure alarms: the “first alarm” takes place at 0.5% (5000ppm) concentration, which is the long-term exposure limit (over an 8-hour period); the “second alarm” occurs at 1.5% (15,000ppm) concentration, the short-term exposure limit (15 minutes).

Given this data, “at first glance it seems that it would be feasible to set a 0.5% depletion alarm on an O₂ gas detector to monitor for CO₂,” noted IGD. “However, this would be dangerous, misguided and potentially fatal.”

The reason for this, explains IGD, stems from the fact that oxygen accounts for 20.9% of air by volume, while nitrogen makes up most of the balance at 78% (with 1% as argon and a mix of other trace gases, including CO₂).

First-alarm levels on an O₂ gas detector are usually set to 19.5%, IGD says. The oxygen level will be depleting either because another gas is leaking into the area, causing dilution/displacement of the oxygen, or because something is consuming the oxygen (like a fire).

Since 78% of the atmosphere is nitrogen, in the event of a CO₂ leak, most of the gas displaced is nitrogen. A much smaller percentage of the oxygen is displaced. “The increase in CO₂ is therefore not matched by a correlating decrease in oxygen levels,” IGD notes.

For example, if CO₂ is present at 4% in the atmosphere, the corresponding oxygen level would still only be 20%. “Therefore, you would have exceeded both the long-term and short-term exposure limits for CO₂ by a factor of eight and still not trigger an alarm,” the company explains. Thus the need for a CO₂ detector.

Risk of excess CO₂

IGD also explains why excess levels of CO₂ in the environment are harmful. People normally exhale air that contains 4–5% CO₂ produced by cellular metabolic processes. This metabolic function balances the acidity of the bloodstream, ensuring it is kept at a neutral level. However, areas with elevated levels of CO₂ make it difficult for the body to disperse CO₂, making blood more acidic, a condition called acidosis, which can harm organs if left unchecked.

This, rather than asphyxiation (displacement of oxygen), is why excess CO₂ is harmful, says IGD.

Established in 1917, IGD has been manufacturing CO₂ detectors since before the Second World War. In 2021the company introduced a personal CO₂ monitor designed to keep staff and technicians safe from the dangers of elevated carbon dioxide levels in confined spaces.

Last year, IGD secured a contract to supply Stockport, U.K.-based Robinsons Brewery with an iGAS CO₂ portable gas detector.

“At first glance it seems that it would be feasible to set a 0.5% depletion alarm on an O₂ gas detector to monitor for CO₂. However, this would be dangerous, misguided and potentially fatal.”

International Gas Detectors