CO2 Measurement Range - Why it Matters

co2 percentage vs ppm

With all the various carbon dioxide meters, monitors and sensors we offer, it may feel overwhelming to choose between them. That’s why one of the first questions we ask a client who calls is, “What are you trying to measure?” This question helps us determine the range of CO2 levels you’ll want to measure, which narrows down the list of products we offer that can meet your needs.

It may seem logical that since we offer products that measure up to 100% CO2 levels, the simplest answer would be “give me a product that will measure anything!” The problem with this is that in general, the higher the range of CO2 levels measured, the lower the accuracy. Conversely, the narrower the range, the more accurate the sensor will be for your purposes.

It may help to start out by describing how CO2 is measured. For most sensors, the CO2 level is measured as a percentage of a volume of air either as a % or as parts-per-million (ppm).

Counting a Million Gas Molecules

While percentages are easy to understand, ppm can seem a bit more confusing. So here’s a thought-experiment that might help. Imagine you were given a sample of 1 million molecules of fresh air and asked to count them (assuming standard temperature and pressure), the majority of the molecules would be oxygen (209,000, or 20.9%), nitrogen (780,000, or 78%) and argon (9,000, or 0.9%). Water vapor (H20) could also account for 1-5% of the molecules, but I’m going to ignore it for this example.

Once we have counted the 998,000 (99.8%) of oxygen, nitrogen and argon molecules, we’re left with 2,000 remaining molecules in the sample. These will be other gases like CO2, neon, methane, helium, etc. CO2 accounts for 0.04% of them. But instead of saying “four one-hundredths of a percent,” you’d simple say you counted 400 CO2 molecules, or 400ppm.

In general, percentages of gas in air samples below 1% are measured in parts-per-million. Here’s a table that shows how it works:

  • 1,000,000/1,000,000 = 100%
  • 100,000/1,000,000 = 10%
  • 10,000/1,000,000 = 1% (10,000ppm)
  • 1,000/1,000,000 = 0.1% (1,000ppm)
  • 400/1,000,000 = 0.04 (400ppm)

Measurement Range vs Accuracy

So what does all this have to do with accuracy? As a general rule, the narrower the range of CO2 measured, the higher the accuracy of the sensor. If you were measuring 1% CO2 using a 100% CO2 sensor and the CO2 level decreased by 500ppm, the change would be from 1% to 0.95%. This is outside the range of accuracy of a 100% CO2 sensor. You might not even see the change. But if you were using a 10,000ppm sensor, the decrease to 9,500ppm is within the range of accuracy and would be seen by the sensor.

By knowing the range of the CO2 levels you need to measure, we can easily narrow down the list of products we offer that will be the most accurate for your application.

Posted by CO2 Meter on May 17, 2016.
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