How to measure Carbon Dioxide (CO2)?

Carbon Dioxide (CO2) is a compound that at room temperature, exists as an odourless colourless gas. The compound consists of one carbon atom and two oxygen atoms. How do we measure the amount of Carbon Dioxide (CO2) in the air around us then? In this article, we will go through:

  • Why is Carbon Dioxide (CO2) measured?
  • How is Carbon Dioxide (CO2) measured? – Different types of CO2 sensors,
  • Recommend various types of Carbon Dioxide (CO2) sensors,
  • Applications of CO2 sensors.

Why is Carbon Dioxide (CO2) measured?

Unlike other air pollutants such as tobacco or formaldehyde, carbon dioxide (CO2) is impossible to detect with your senses alone. As mentioned, carbon dioxide (CO2) is colourless and odourless. It is not toxic, however, it is toxic at higher concentrations. The main concern lies in the fact that carbon dioxide (CO2) is very harmful to the environment. 

Source: Wikipedia

Carbon dioxide (CO2) is a greenhouse gas. This means that it naturally traps heat energy emitted from the Earth’s surface, raising the surface temperature of the Earth leading to global warming. By measuring the amount of carbon dioxide (CO2) in the environment, actions and precaution measures can be implemented to curb the concentration of carbon dioxide (CO2) in the environment.

However, it is not all negativity when it comes to carbon dioxide (CO2). Carbon dioxide is also used for extinguishing fires, freezing food, producing carbonated beverages such as Coke or Sprite, and many more within the medical and agricultural industries. Measurement of carbon dioxide (CO2) will be required to implement these uses. More applications of carbon dioxide measurement implementations will be mentioned below!

How is Carbon Dioxide (CO2) measured?

Carbon Dioxide (CO2) is measured with a gas sensor specifically made to measure the concentration of carbon dioxide (CO2) in the air. There are three main types of Carbon Dioxide (CO2) sensors: Electrochemical sensors, Non-Dispersive Infrared (NDIR) Sensors, and Metal Oxide Semiconductor (MOS) Sensors. 

Electrochemical Sensors

Source: Smart Citizen

When the gas enters the sensors, it goes through a chemical reaction, either oxidation or reduction reactions. These reactions in the sensors generate the positive or negative current flow through the external circuit. Gas first finds its way into the outlet of the membrane on top of the sensor housing. Then, the three electrodes as shown in the picture. An electrochemical reaction (oxidation or reduction) will then occur. 

Oxidation will cause the flow of electrons to move to another electrode, the counter electrode. While reduction will adversely cause the flow of electrons to move from the counter electrode to the working electrode. Essentially, the gas is going through a chemical reaction to sort them into their respective electrodes. The sensor will then use the type and amount of electrical change to determine how much CO2 is present.

Pros

Electrochemical sensors are generally less vulnerable to humidity and temperature changes. 

Non-Dispersive Infrared (NDIR) Sensors

Source: CO2 Meter

Every element on Earth absorbs certain types of light. For example, orange will reflect orange light but absorb other colours. As the light is absorbed, our eyes can’t see them. When elements are broken down into atoms and molecules, we can accurately point out what kind of light each absorbs, even lights that are invisible to our naked eyes. With this knowledge, we can use sensors to test if certain elements are presents, which is how NDIR sensor work!

Air will enter the sensor, then a light set at the specific wavelengths for CO2 will be activated. The CO2 in the air will absorb some of the light beams, decreasing the amount of light. The more CO2 present, the more light will be absorbed, the amount of light on the other side of the sensor decreases./ The other side of the sensor will then measure how much light is left.

Pros

NDIR sensors have a very long life span. Other substances will also not interfere with readings. It works well at common CO2 ranges.

Metal Oxide Semiconductor (MOS) Sensors

Source: ResearchGate

There will be a sensing material in the sensor, for example, SnO2 (Tin) in the picture shown above. It will be exposed to the air you want to test. When carbon dioxide (CO2) from the air comes into contact with the sensing material, it will change the chemical composition either through reduction or oxidation reaction, depending on the gas. The resistance will then change, which determines the concentration of carbon dioxide. Depending on what kind of metal the sensing material is made of, different gases will react differently to the material. These sensors are better used at higher, less common CO2 concentrations (>2000ppm) as compared to common, lower concentrations. 

Pros

They are relatively easy to use due to their simple design.

Which one do you choose?

All three types of sensors are able to provide accurate results under most conditions. However, all three sensors have their little own “area of expertise”.

If your area of deployment has irregular temperatures, use Electrochemical sensors. Electrochemical sensors are less vulnerable to humidity and temperature changes.

If your area of deployment has many pollutants and unwanted substances, use non-dispersive Infrared (NDIR) sensors. NDIR Sensors are immune to other substances.

Lastly, if your area of deployment has higher, less common concentrations of CO2 (>2000ppm), use Metal Oxide Semiconductor (MOS) Sensors. MOS Sensors are usually better for higher concentrations of CO2.

Do note that, most CO2 sensors in the market are NDIR sensors due to their better reliability.

Recommended CO2 Sensors

Here are some CO2 Sensors that are available on Seeed’s website!

Grove – Carbon Dioxide Sensor(MH-Z16)

The Grove – CO2 Sensor module is an NDIR CO2 Sensor. It is a general-purpose, small sensor with a built-in temperature sensor as well! With its low price, it packs many features! Its built-in temperature sensor is equipped with temperature compensation as well, to ensure accurate temperature measurement. It also comes with a UART output and a Grove connector. 

This sensor is great for general-purpose HVAC and indoor air quality monitoring. It can also be used in agriculture, and industrial process monitoring.

What is Grove?

Grove is an open-source, modulated, and ready-to-use toolset. It takes a building block approach to assemble electronics. Compared with the traditional, complicated learning method of using a breadboard and various electronic components to assemble a project, Grove simplifies the learning process significantly.

The Grove System consists of a Base Shield and various modules with standardized connectors. The Base Shield allows for easy connection of any microprocessor input or output from the Grove modules, and every Grove module addresses a single function, such as a simple button or a more complex sensor. There are already more than 300 Grove modules and each one comes with clear documentation and demo code to help you get started quickly.

Do note that this sensor only reflects the approximated amount of gas concentration in a permissible error range. For an exact gas concentration, a more precise and costly instrument will be required.

Grove – CO2 & Temperature & Humidity Sensor for Arduino (SCD30)

This Grove – SCD30 is a 3-in-1 sensor for Arduino that can measure CO2, temperature, and humidity. It is also an NDIR carbon dioxide sensor, with high precision and wide measurement accuracy! It also comes with a built-in Grove Connector for easier connectivity. The embedded Sensirion SCD30 is able to measure CO2 with high precision and a wide measurement accuracy at ±(30 ppm + 3%) between 400ppm to 10’000ppm.

This sensor will be an ideal choice for your Arduino projects that requires a highly precise measurement such as an Arduino weather station.

SenseCAP SOLO CO2 5000 (A1) – NDIR CO2 Sensor, MODBUS-RTU RS485 & SDI-12

This digital SenseCAP NDIR CO2 concentration sensor has high accuracy, fast response and superior stability. It can measure at an accuracy of 400ppm ~ 5,000 ppm, ± (50ppm+5%*MV). It has built-in Modbus-RTU/ASCII RS485 and SDI-12 communication protocols to allow it to easily integrate into automation systems. Find out more about RS485 here.

It is widely applicable in greenhouses, transportation stations and many professional scenarios where CO2 measurements are needed. You won’t go wrong with this CO2 sensor!

SenseCAP Wireless CO2 Sensor

This Sensecap Wireless CO2 Sensor comes with an NDIR CO2 Sensor and a custom battery. It’s designed to be connected wireless via a LoRa channel. There are two options for this sensor, one with EU868MHz, and another one with US915MHz.

This sensor packs many features! It has an industrial design that is dust-proof and suitable for outdoor harsh conditions. It features ultra-wide-distance wireless data transmission with low-power consumption. It is compatible with Open API various cloud services for further development as well.

Its battery life can last more than 1.5 years and data transmission of up to 10km! It is a perfect choice for smart facilities such as smart agriculture, smart building or environmental monitoring. You cannot go wrong with this sensor!

Current Real-World Applications

Modified Atmosphere Packaging

Source: Generon

Modified Atmosphere Packaging is to control the atmosphere of the environment where foods are packaged to ensure the optimum freshness to maintain visual and nutritional appeal. Precise control of the package’s gaseous environment is required, so the shelf life of the product can be extended without adding preservatives. 

Carbon dioxide sensors are used to measure the concentration of carbon dioxide in that atmosphere.

Indoor Air Quality Monitoring 

Source: Umweltbundesamt

Precise control of the significant indoor environment by a carbon dioxide monitor is essential for the well-being and comfort of building occupants. Carbon Dioxide is one of the greatest variables affecting indoor air quality amongst the multitude of gases. It is a good indication of the general level of pollutants within buildings. This system can be found in many big hotels, offices, classrooms and also homes.

Excess carbon dioxide can lead to tiredness and a lack of concentration. It can lead to symptoms such as headaches, eye, nose and throat irritation. 

Water Quality Measurement (Total Organic Carbon)

Source: Martek Marine

Unknown to many, water quality can be measured with a carbon dioxide sensor. It is done by measuring the total organic carbon (TOC), the amount of carbon found in an organic compound in the water. 

TOC is measured by a chemical reaction (oxidation) with the organic carbon to produce CO2, which can be quantified by measuring with a CO2 sensor. The value, when subtracted from the ‘inorganic compound’, is converted into a TOC measurement. This then tells us the amount of contamination in the body of water. This process is significant as pure water is critical for healthcare industries, manufacturing industries and also water supply industries. 

Summary

It is not that difficult to measure carbon dioxide. There are three different kinds of sensors, but NDIR is the most widely-used one. I hope you have learnt plenty about carbon dioxide sensors in this article. Carbon dioxide measurements are required in many scenarios.

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