Calculating the CO/CO2 ratio from analyser readings

Put the readings on the same basis

An analyser commonly reports carbon monoxide (CO) in parts per million and carbon dioxide (CO2) as a percentage. One per cent is 10,000 ppm. Divide the CO value by the CO2 value after converting them to the same units:

CO/CO2 ratio = CO in ppm ÷ (CO2 in per cent × 10,000).

Two worked readings

With CO at 100 ppm and CO2 at 5.0%, the ratio is 100 ÷ 50,000 = 0.002.

With CO at 240 ppm and CO2 at 4.0%, the ratio is 240 ÷ 40,000 = 0.006. This sample has more CO relative to CO2 than the first sample.

A calculation is not a pass certificate

Use the limits and procedure for the actual appliance, fuel and operating condition. Some manufacturer checks use a ratio below 0.004 together with a separate CO limit. Neither that ratio nor any other single number is a universal declaration of safety.

For a supplied condition of below 0.004, 0.002 meets the ratio condition, 0.006 does not, and exactly 0.004 does not. The absolute CO reading and remaining required checks still matter. Do not substitute heat output, flame colour or an acceptable-looking CO2 reading for those checks.

Use a suitable, calibrated analyser, correct sampling and stable operating conditions. Follow the manufacturer investigation instructions when a check fails; do not make an unsupported gas-valve adjustment.

Recall check: 100 ppm is 0.01%, so 0.01 ÷ 5.0 gives the same ratio of 0.002.