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How to Measure Carbon Dioxide (CO2) Accurately?

How to measure carbon dioxide gas?

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 is, “What are you trying to measure?” This question helps us determine the range of CO2 levels, which narrows down the list of products we offer that can work for a particular application.

Since we offer devices and sensors that measure up to 100% CO2, it seems like the simple answer would be “give me a sensor that will measure everything!” The problem with this answer is that, in general, the wider the range of CO2 levels measured, the lower the accuracy. Conversely, the narrower the range, the more accurate each measurement will be.

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

Measuring CO2 in parts per million

While percentages are easy to understand, ppm can seem a bit more confusing. Here is a thought-experiment that may help.

Imagine you were given a box with 1 million molecules of fresh air assuming STP (standard temperature and pressure) and you are asked to count them by hand.

Remember, this is a thought-experiment!

By the time you got to the bottom of the box, you'd find that the majority of the molecules would be nitrogen (780,000 or 78%), oxygen (209,000 or 20.9%), and argon (9,000, or 0.9%). Water vapor (H20) could also account for some of the molecules, but let's ignore it for this example.

Once you've added up 998,000 (99.8%) of the nitrogen, oxygen and argon molecules, you'd still be left with about 2,000 remaining molecules in several small piles. These will be other gases like CO2, neon, methane and helium.

One of the biggest of these small piles would be the CO2 molecules. There should be about 400 of them, or 0.04% of the total. But instead of saying “four one-hundredths of a percent,” you would say, you counted 400 CO2 molecules, or 400 ppm - parts-per-million.

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

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

What percentage of CO2 do you need to measure?

In order to know what percentage of carbon dioxide you need to measure, you first need to understand the application or industry you are in. From there, our team can best determine the range, accuracy, or precision you will need to recommend the proper carbon dioxide sensor that is best suited.

To provide some perspective:

  • Indoor and outdoor air has between 400 ppm and 2,000 ppm CO2 by volume. This means that for projects measuring indoor air quality to measuring atmospheric CO2, a 0-1% (0-10,000 ppm) CO2 sensor is your best solution. 
  • For applications like CO2 safety monitoring for restaurants, breweries, indoor agriculture or industrial settings, a 5% CO2 sensor is used. This is because OSHA requires safety monitoring sensor devices to be present at 1.5% and 3% CO2. 
  • Other applications like modified atmosphere packaging, laboratory, cryogenics, or fire suppression require sensors between 5 and 100% CO2.

How is CO2 measured?

CO2 Sensor

In order to measure carbon dioxide, a CO2 sensor is used. One of the most common types is the NDIR non-dispersive infrared sensor. It is popular due to its long life-span, speed, and low cross-sensitivity to other gases.

An NDIR CO2 sensor works by measuring infrared light in an air sample. The amount of infrared light absorbed by the molecules of carbon dioxide is proportional to the number of CO2 molecules in the air sample. This allows the sensor to measure the amount of CO2.

An example of a CO2 sensor is the S8 Miniature 5% CO2 Sensor. It can measure from 400 ppm to 5% CO2, yet it has less than a 30x20x10mm footprint and runs on only 30mA.

CO2 Measurement Range vs. Accuracy

So what does carbon dioxide sensing have to do with accuracy? As a rule, the narrower the range of CO2 measured, the higher the accuracy of the sensor.

For example, if you were measuring 400 ppm CO2 (fresh air) using a 100% CO2 sensor and the CO2 level increased to 500 ppm, the change would be from 0.04% to 0.05%. This 0.01% (100 ppm) change is outside the 100-300 ppm range of accuracy of a typical 100% CO2 sensor. The sensor would probably not record any change.

However, the accuracy of a 10,000 ppm NDIR sensor is around 50 ppm (0.005%). This means that while you might not see an exact 100 ppm change, you'd see a rise between 50 ppm and 150 ppm CO2. Depending on the precision of the sensor, the rise would trend toward 100 ppm, which is what you originally wanted to measure.

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

CO2 Measurement Accuracy vs. Precision

Did you notice that the word precision was mentioned above? Accuracy and precision mean two different things when it comes to carbon dioxide sensors.

In the simplest terms, given a set of data points from repeated measurements of the same quantity, the set can be said to be precise if the values are close to each other, while the set can be said to be accurate if their average is close to the true value of the quantity being measured. The two concepts are independent of each other, so a particular set of data can be said to be either accurate, or precise, or both, or neither.

For NDIR CO2 sensors, accuracy is the difference between the amount of CO2 measured and the theoretical exact amount of CO2 in a gas sample. Precision isn't measured. Instead, NDIR CO2 sensors rely on increasing the sample size and average CO2 reported readings to increase the precision.

For example, sensor manufacturers list accuracy as ± ppm ± a percentage of the reading. Take the K30 10,000ppm sensor, which lists it's accuracy as ± 30 ppm ± 3%. This means that a K30 calibrated at 400 ppm could report a single measurement anywhere between

  1. 400 ppm - (30 + (400 * 0.03) = 358 ppm 
  2. 400 ppm + (30 + (400 * 0.03) = 442 ppm 

NDIR CO2 sensors take a new reading anywhere from 20 times a second to once every 30 seconds. As each reading is taken the sensor's on-board software takes into account all the previous measurements and report the average. This solves the problem of precision. If you want a more precise CO2 reading, look for a sensor with a higher frequency of measurements.

CO2 Measurement and Safety

Because carbon dioxide is an odorless, colorless, and tasteless gas; too much exposure can result in negative health effects or fatality. Whenever carbon dioxide gas is used, stored, or produced in high volumes; dangerously high concentrations can occur. Because these dangerous concentrations are hard to monitor with the naked eye, carbon dioxide sensors are used vastly throughout the world to mitigate the potential issue of a carbon dioxide leak to occur. These CO2 sensors can quickly and accurately monitor indoor ambient levels to protect employees, staff, and establishments from injury or hazard.

    400 ppm Typical atmospheric concentration
    600-800 ppm Acceptable indoor air quality
    1,000-1,200 ppm Tolerable indoor air quality
    5,000 ppm Maximum average exposure limit over 8 hours (OSHA)
    6,000-30,000 ppm Concern, short-term exposure only
    3-8% Increased respiration rate, headache
    > 10% Nausea, vomiting, unconsciousness
    > 20% Rapid unconsciousness, death

    Overall, it is not that difficult to monitor carbon dioxide gas when you have the proper technology, monitor, or sensing solution. While carbon dioxide monitoring is required in many different scenarios and industries, we provide further support to ensure our customers are selecting the proper solution to best fit their needs and environment.

    For more information on carbon dioxide sensors, contact us directly at Sales@CO2Meter.com or call us at 877-678-4259.

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