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How to Measure Carbon Dioxide (CO2), Range, Accuracy, and Precision

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 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 at first seem logical that since we offer devices and sensors that measure up to 100% CO2, the simple answer would be “give me something that will measure everything!” The problem with this answer is that, in general, the higher 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 a % 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. So here’s a thought-experiment that might help. Imagine you were given a box with 1 million molecules of fresh air (assuming STP - standard temperature and pressure) and 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 400ppm - 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 10,000ppm
  • 1,000/1,000,000 = 0.1% or 1,000ppm
  • 400/1,000,000 = 0.04 or 400ppm

To compute the exact amount of CO2 required to raise the level to 1,000ppm use this equation: 

( Width x length x height x desired CO2 level in parts per million )

What instrument is used to measure CO2?

In order to measure carbon dioxide (CO2) a CO2 sensor will be used. The most common types of carbon dioxide sensors range from non-dispersive infrared, electrochemical, or metal oxide semiconductor. Most popular, however, are non-dispersive infrared (NDIR) technology due to their long life-span and wide measurement range. An NDIR CO2 sensor works by an infrared (IR) lamp directing waves of light through a tube filled with a sample of air. This air moves toward an optical filter in front of an IR light detector. The IR light detector measures the amount of IR light that passes through the optical filter.

Next, the band of IR radiation also produced by the lamp is very close to the absorption band of CO2. Because the IR spectrum of CO2 is unique, matching the light source wavelength serves as a signature or "fingerprint" to identify the CO2 molecule.

Today's newest generation of carbon dioxide sensors have even more optimized wavelengths, allowing a longer optical light path to be folded into an even smaller footprint. At CO2Meter, we provide a wide variety of carbon dioxide sensors that use NDIR technology, like the CozIR®-LP Miniature 5,000ppm CO2 Sensor

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

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

Accuracy vs. Precision

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

In 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 400ppm could report a single measurement anywhere between

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

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.

What Percentage of CO2 do you need?

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 400ppm and 2,000ppm CO2 by volume. This means that for projects measuring indoor air quality to measuring atmospheric CO2, a 0-1% (0-10,000ppm) 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.

Carbon Dioxide (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.

Carbon Dioxide CO2 Safety Levels
Overall, it is not that difficult to monitor carbon dioxide (CO2) 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, feel free to contact us directly at Sales@CO2Meter.com or call us at 877-678-4259

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