In order to measure CO2 gas accurately, you must know how much CO2 you want to measure.
With all the different carbon dioxide meters, monitors and sensors we offer, it may feel overwhelming to choose one. That’s why one of the first questions we ask is, “What is your application?” This question helps us determine the range of CO2 levels you need to measure, which narrows down the list of products we offer that can would work best for you.
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 the wider the range of CO2 levels to be measured, the lower the accuracy. Conversely, the narrower the range, the more accurate the CO2 level reported 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).
What does ppm - parts per million stand for?
Parts-per-million (abbreviated ppm) is the ratio of one gas or other molecule to another. For example, 1,000 ppm of CO2 means that there are 1,000 molecules of CO2 and 999,000 molecules of other gases or water vapor.
In other words, 1 ppm = 0.0001% gas.
Learn more about parts per million here.
Measuring CO2 in parts per million
While percentages are easy to understand, ppm can seem a bit more confusing. Here is a example that can help.
Imagine you were given a box filled with 1 million marbles, each representing a molecule of the different gases in typical outdoor air. You're asked to count the marbles by hand.
Remember, this is a thought-experiment!
By the time you got to the bottom of the box, you'll have counted about 780,000 (or 78%) nitrogen marbles, 209,000 (or 20.9%) oxygen marbles, and 9,000 (or 0.9%) marbles of argon gas. Water vapor (H20) could also account for some of the marbles, but let's ignore it for this example.
After counting 998,000 (99.8%) of the nitrogen, oxygen and argon marbles, you'll still have 2,000 marbles left in several small piles. These will be other gases like CO2, neon, methane and helium.
The biggest of these small piles would be the CO2 marbles. There will 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 marbles, or 400 parts-per-million. Scientists abbreviate this as ppm.
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% - pure CO2
- 100,000/1,000,000 = 10% - deadly level of CO2
- 10,000/1,000,000 = 10,000 ppm (1%) - stale indoor air
- 1,000/1,000,000 = 1,000 ppm (0.1%) - typical indoor air
- 400/1,000,000 = 400 ppm (0.04%) - outdoor fresh air
What percentage of CO2 do you need to measure?
CO2 sensors can measure anywhere between 0 ppm and 100% CO2 in a gas by volume. In order to know what percentage of carbon dioxide you need to measure, you need to understand your application's requirements. 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 for your application. For example:
- For applications that require as little as 0 ppm CO2 like welding, you need a 0-2,000 ppm CO2 sensor.
- Indoor and outdoor air has between 400 ppm and 2,000 ppm CO2 by volume. This means that for projects measuring indoor air quality or for measuring atmospheric CO2, a 0-2,000 ppm or 0-10,000 ppm CO2 sensor is your best solution.
- For applications like indoor CO2 safety monitoring for restaurants, breweries & wineries, indoor agriculture or factories, a 5% CO2 sensor is used. This is because OSHA requires CO2 alarms 1.5% and 3% CO2.
- Other applications like modified atmosphere packaging, cryogenics, pest elimination, or fire suppression require sensors that measure between 5% and 100% CO2.
How is CO2 measured?
In order to measure carbon dioxide in air, an NDIR CO2 sensor is used. Non-dispersive infrared CO2 sensors are the standard worldwide due to their long life-span, low power requirements, speed, and most importantly, low cross-sensitivity to other gases in the sample.
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. This allows the sensor to measure the amount of CO2 by volume in an air sample inside the sensor.
Note that NDIR CO2 sensors are used to measure CO2 in air or gas. There are other ways to measure CO2 gas dissolved in blood, in water or in beer.
How is CO2 sensor accuracy defined?
The accuracy of a CO2 sensor is defined as how close the measurement is to a reference gas, expressed as either a ± (plus-minus) value in parts-per-million (ppm) or as a percentage of the measured value, or a combination of both. For example, if I have a tank of CO2 known to be exactly 10,000 ppm, and I test dozens of identical sensors hundreds of times to discover that all the readings are between 9,900 ppm and 10,100 ppm, I can report that the sensor's accuracy is ±100 ppm or ±2% (200/10,000). This is how you'll typically see accuracy listed for a CO2 sensor.
In other words, accuracy is determined by repeatedly testing a sensor against a known reference gas, and recording the range of values reported. The wider the range, the lower the accuracy. The testing is first done by the manufacturer at the factory when the sensor is designed. During production, sensors are calibrated against a reference gas, then ran for a period of time. If they perform within the pre-determined accuracy parameters, they are considered "accurate" and shipped.
Want to learn more? We have an app note here.
What can change CO2 sensor accuracy?
Keep in mind that a sensor's accuracy changes at different CO2 levels. A 10,000 ppm CO2 sensor might have one accuracy specified from 0-2,000 ppm, and a different accuracy specified between 2,001 ppm and 10,000 ppm. This is common for sensors designed for indoor air quality, where most CO2 level measurements will be between 400 ppm (fresh air) and 1,200 ppm (stale air).
In addition, the smaller the range of CO2 levels to measure, typically the greater the accuracy. A 10,000 ppm (1% CO2) sensor will typically be more accurate than a 100,000 ppm (10% CO2) sensor, which will be more accurate than a 100% CO2 sensor. This means that for greatest accuracy, you should use a CO2 sensor with the smallest range that still fits the intended purpose.
Accuracy also changes if there are changes in temperature, pressure, or humidity. Therefore, unless otherwise stated, you can assume the accuracy was measured at the factory at SATP (standard ambient temperature and pressure).
CO2 Measurement Range vs. Accuracy
So what does range 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 100% CO2 sensor would probably not record the 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 gas sample of CO2, a sensor can be said to be precise if all the measurements are close to each other, while the sensor can be said to be accurate if their average is close to the true sample of CO2 being measured. The two concepts are independent of each other, so a particular sensor can 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 stored in the sensor's memory 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
- 400 ppm - (30 + (400 * 0.03) = 358 ppm
- 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.
Why do different CO2 sensors give different results?
A customer recently wrote us and asked, "I am testing a SenseAir K-30 CO2 Sensor next to a desktop CO2 meter. The K-30 is reading 778 ppm, while the desktop meter shows 846 ppm. Which one is correct?" The simple answer is that they both are! Yes, they are different, but both sensors are working within their rated accuracy.
According to the specifications, the SenseAir K-30 sensor has an accuracy of ±30 ppm or ±3%, whichever is greater. At a reading of 778 ppm, the actual amount of CO2 could be between 748-808 ppm (30 ppm > 3% of 778 ppm). The TIM10 CO2 meter has an accuracy of ±50 ppm and ±5%, so at a reading of 846 ppm, the actual amount of CO2 could be between 796-896 ppm (50 ppm > 5% of 846 ppm). Since the 2 sensors accuracy ratings overlap between 796 ppm and 808 ppm, you can say they are both correct within their rated accuracy.
While this sounds like theory instead of science, it works in the real world. If you were to measure CO2 levels with 10 of the most accurate CO2 sensors available, they would all report slightly different results from one reading to the next. That's why at the parts-per-million level measuring an analog voltage proportional to the number of CO2 molecules, no CO2 sensor's single reading can ever be perfectly accurate.
However, from the accuracy ratings on the datasheets, we can see that the K-30 sensor is more accurate than the TIM10. Based on this information alone, I would say that while both devices are accurate, the K-30 sensor's readings over time is closer to the actual CO2 level.
CO2 Measurement Is Important
It is not that difficult to monitor carbon dioxide gas when you have the proper technology, monitor, or sensor. While carbon dioxide monitoring is required in many different scenarios and industries, we ensure our customers select the best solution to fit their application.
For more information on carbon dioxide sensors, contact us.
