Convert CO and CO2 to the same units, divide, then compare the answer with the exact wording of the appliance's limit.
The method
- Read carbon monoxide (CO) in parts per million (ppm) and carbon dioxide (CO2) as a percentage.
- Convert the CO2 percentage to ppm. 1% = 10,000 ppm, so multiply the percentage by 10,000.
- Divide: CO/CO2 ratio = CO in ppm ÷ CO2 in ppm.
- Compare the answer with the exact wording of the limit in the appliance instructions, then check the CO reading against its own limit.
Example 1: 80 ppm CO and 8.0% CO2
- CO2 in ppm: 8.0 × 10,000 = 80,000 ppm.
- Ratio: 80 ÷ 80,000 = 0.0010.
Example 2: 100 ppm CO and 5.0% CO2
- CO2 in ppm: 5.0 × 10,000 = 50,000 ppm.
- Ratio: 100 ÷ 50,000 = 0.002.
Check it the other way: 100 ppm is 0.01%, and 0.01 ÷ 5.0 also gives 0.002.
Example 3: 240 ppm CO and 4.0% CO2
- CO2 in ppm: 4.0 × 10,000 = 40,000 ppm.
- Ratio: 240 ÷ 40,000 = 0.006. This sample has more CO relative to CO2 than Example 2.
A ratio written as "1 to 250"
Divide the first number by the second: 1 ÷ 250 = 0.004. This is a number conversion, not a universal pass mark.
Comparing with a limit
Suppose the instructions for the appliance require a ratio below 0.004:
- 0.002 meets the condition.
- 0.006 does not.
- Exactly 0.004 does not, because equal is not below. Where a limit is worded "at or below", the same reading would meet it.
Common mistakes
- Dividing ppm by a percentage without converting: 80 ÷ 8.0 gives 10, not 0.0010.
- Treating 0.004 as a universal pass mark. Use the limit for the actual appliance, fuel and operating condition.
- Stopping at the ratio. A ratio that meets its limit does not cancel a CO reading above the appliance's CO limit or the other required checks.
- Selecting or averaging readings to land within the limit. Record the true, stable figures from a calibrated analyser.