CO Cross-sensitivity and XplorIR

BRANDON GAYLE, Gayle Training Solutions, LLC

Carbon Monoxide electrochemical sensors are commonly installed in 4 or 5-gas air monitors and are calibrated using CO at 50ppm. The issue with these and many other electrochemical sensors is that there are many cross-sensitive gases and vapors that can either give false readings or inhibit the accurate measurement of the target gas.

The only way for an operator to know that the readings on a CO sensor are accurate is to locate and identify the source of CO. Unfortunately, many inexperienced operators have a tendency to believe that if a reading is obtained on a CO sensor, then CO must be present. This is not the case and sometimes can be a dangerous assumption for both the responder and any future occupants of the structure.

Each manufacturer provides a list of known cross-sensitivities to their electrochemical sensors. One example is TN-114 from Honeywell RAE. This tech note lists 20 gases and vapors that are cross-sensitive on a CO sensor with varying responses for each. For example, Acetylene has a 1:1 response on a CO sensor and NO2 can actually have a negative response. Many alcohol vapors are also considered cross-sensitive and can potentially inhibit a CO sensor. While correction factors are available for many electrochemical cells, they tend to have high rates of error due to the non-linear response of many cross-sensitivities. Therefore, care should be taken when using the calculated concentrations, always erring on the side of safety. It is prudent for an operator to investigate the actual source of any readings that are obtained so that appropriate actions may be taken.

The RedWave XplorIR would be a logical pairing with a CO sensor to identify the source of the readings and ensure that appropriate actions are taken. The XplorIR can identify Carbon Monoxide if it is present, but also identify all but one of the listed cross-sensitive gases, that being H2 due to the fact that it is a homonuclear diatom.

As long as the operator is using a 4-gas monitor with a Methane calibrated LEL sensor, H2 has a 1.0 correlation factor and the operator will see LEL readings well before danger is encountered. This means that an XplorIR paired with a 4 or 5-gas air monitor will increase the effectiveness and safety of a responder and the public.