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Thermal Conductivity versus Infrared CO2 Sensors

By Aimee O'Driscoll, 06 October 2018

When growing cell cultures, in order to ensure proper growth, temperature, humidity, and CO2 levels need to be controlled. CO2 levels are of importance because they help to control the pH of the culture medium. If there’s too much CO2, it will become too acidic. If there’s not enough CO2, it will become more alkaline.

In your CO2 incubator, the level of CO2 gas in the medium is regulated by the supply of CO2 in the chamber. The question is, how does the system “know” how much CO2 needs to be added? This is where CO2 sensor technologies come into play. There are two main types, each with its pros and cons:

  • Thermal conductivity uses a thermal resistor to detect gas composition. It’s the less expensive option but it’s also less reliable.
  • Infrared CO2 sensors use infrared light to detect the amount of CO2 in the chamber. This type of sensor is more expensive but more accurate.

In this post, we’ll explain these two types of sensor in more detail and discuss the practical implications of each.

Thermal Conductivity

Thermal conductivity works by measuring electrical resistance through the atmosphere. The sensor will typically comprise two cells, one of which is filled with air from the growth chamber. The other is a sealed cell that contains a reference atmosphere at a controlled temperature. Each cell contains a thermistor (a thermal resistor), the resistance of which changes with temperature, humidity, and gas composition.

When the temperature and humidity is the same for both cells, the difference in resistance will measure the difference in the gas composition, in this case reflecting the level of CO2 in the chamber. If a difference is detected, the system is prompted to add more CO2 into the chamber.

 

A diagram of a CO<sub>2</sub> sensor.

A representation of a thermal conductivity sensor.

Thermal conductors are an inexpensive alternative to IR sensors, which we’ll discuss below. However, they don’t come without their drawbacks. Because the resistance differential can be affected by other factors than just CO2 levels, the temperature and humidity in the chamber should always be constant for the system to work properly.

This means that every time the door opens and the temperature and humidity fluctuates, you’ll end up with inaccurate readings. In fact, the readings won’t be accurate until the atmosphere stabilizes, which could take half an hour or more. Thermal conductors might be okay for long-term storage of cultures, but they’re less suitable for situations where door openings are frequent (more than once per day).

Infrared CO2 Sensors

Infrared sensors detect the amount of gas in the chamber in an entirely different manner. These sensors rely on the fact that CO2, like other gasses, absorbs a specific wavelength of light, 4.3 μm to be precise.

 

Incubators with IR sensors.

The CO2Cell Incubator and the SCO Air Jacket CO2 Incubator both utilize IR sensors.

The sensor can detect how much CO2 is in the atmosphere by measuring how much 4.3 μm light passes through it. The big difference here is that the amount of light detected is not dependent on any other factors, such as temperature and humidity, as is the case with thermal resistance.

This means you can open the door as many times as you like and the sensor will always deliver an accurate reading. As a result, you’ll have a more consistent level of CO2 in the chamber, meaning better stability of samples.

The downside? Although the price of infrared sensors has gone down, they still represent a pricier alternative to thermal conductivity. However, if you consider the cost of the lack of productivity when using a thermal conductivity sensor, you may have a financial case for going with the IR option.

Single and Dual Beam IR Sensors

You may notice that some incubators come with dual beam IR sensors. The main difference between single and dual beam IR sensors is calibration. A single beam uses air as the reference and calibrates at regular intervals. A dual beam sensor uses the second filter as a reference and can calibrate in real time.

With more frequent calibration, you get greater accuracy and faster recovery of CO2 levels after door openings. These factors make a dual beam sensor more suitable for applications involving more sensitive cell cultures, such as embryos or stem cells.

 

Incubators with dual beam sensors.

The MyTemp Mini CO2 Digital Incubator and the Incu-Shaker™ CO2 Mini both come with dual beam IR sensors.

Conclusion

Both types of sensors are able to detect the level of CO2 in the incubator chamber. The main difference between the two is that a temperature sensor can be affected by multiple factors, whereas as an IR sensor is affected by the CO2 level alone.

This makes IR CO2 sensors more accurate, so they are preferable in most situations. They do tend to come with a higher price tag, but they are getting less expensive as time goes on.