How to Accurately Measure CO2 in Life Science Incubators

Conditions inside these incubators typically have a:

CO2 concentration of 5% – a higher concentration of 10% is occasionally used in some applications; however, measurement and control capability typically ranges between 0 and 20%
Temperature of 37 °C (98.6 °F)
Relative humidity of over 90%

Typically, the measurement and control of relative humidity is required in high-end incubators but it is optional in CO2 incubator configurations.

Controlling Conditions

The conditions inside incubators must be controlled very carefully. Temperature control is vital to ensure even heating of an incubator, typically provided by a heater and a fast-rotating fan that continuously mixes the air inside the cabinet. This also helps to generate an even moisture and CO2 concentration in the air volume.

Additionally, relative humidity plays an important role in protecting specimens from drying out. In an ideal case the humidity inside the cabinet would be close to 100% in order to keep the samples fresh and alive. In reality, though, relative humidity is often closer to 90% because of temperature gradients and the limited accuracy of moisture control.

The simplest incubators contain a water pan on the cabinet floor to allow water to evaporate in the air. If the water is slightly colder than the rest of the incubator condensation can be avoided, but in reality it is challenging to maintain the humidity at an optimal level. For example, opening the door will cause the humidity level to fall, and it can take a long time to recover. Still water also presents a potential contamination risk if impurities appear in the water pool. Another solution is to use a humidity controlled atomizer to generate an exact amount of water particles, which requires an extremely accurate humidity measurement.

However, the most demanding element of parameter control is when pure carbon dioxide is fed from a bottle and mixed inside the incubator with the cabinet air to achieve the required concentration. This is why a good quality sensor for measuring CO2 is an essential part of the working system. The sensor location and the control algorithm are also both very important for the overall system.

Challenge of Accurate CO2 Measurement

Successful measuring depends on several aspects. A high-quality instrument must be installed correctly for it to work properly. It must also be suitable for the target application. Users must know how to use the measurement equipment and the data it generates. It is also important to remember that even the highest quality instruments need regular maintenance and calibration. It is also a good idea to validate the instrument’s measurements to ensure it is performing according to expectations.

How Does Technology Choice Affect CO2 Measurement?

There are two types of carbon dioxide measurements: sampling and in-situ. Sampling measurement refers to, for example, pumping an air sample from the cabinet into the sensor with a help of tubing and a pump. The sampling method may suffer from water condensing into the sample tubing, which may itself also cause an increased risk of contamination as tubing is hard to keep clean whereas in the in-situ method the sensors can be removed from the incubator for cleaning. The sampling pump may generate sharp pressure shocks for the CO2 sensor and the measurement output may fluctuate and be interrupted by pressure bursts.

When the sensor is located inside the cabinet, it is an in-situ measurement, which is often the preferred option because it requires fewer components and it has a faster response time. The sensor, though, must withstand high humidity and temperature at all times.

Depending on the technology used, there are also other parameters that affect the carbon dioxide measurement. Infrared measurement is based on a calculation of actual carbon dioxide molecules, when the gas density has to do with the measurement. Therefore, pressure and temperature are parameters that affect measurement accuracy. Based on the ideal gas law, the sample gas expands at a higher temperature and therefore shows a lower CO2 concentration than the same sample at a lower temperature. In addition, the same CO2 concentration at lower pressure shows too low readings, which is relevant for incubators operated in high-altitude locations. Humidity and oxygen have a minor effect on measurement accuracy, but usually this is not significant because they outbalance each other. Vaisala has introduced several ways to automatically compensate for all of the above mentioned phenomena.

Why Does Proper Installation Matter?

It is important to locate the CO2 sensor in a representative, open place, away from the CO2 gas inlet. It should be located away from hot spots like cabin heating, and cold spots like a water pan or humidifier. Incubator corners may not be suitable due to insufficient air circulation.

If the CO2 measurement sensor is properly installed, allow for easy access into the device when needed, for example, during incubator cleaning. It is advisable to locate the sensor in such a way that items placed inside the incubator do not prevent its operation and there must be some space around the sensor. It is also essential to seal the sensor thermally from the external environment (the outside areas of the chamber) using a feedthrough seal. If it is not properly sealed it will form a cold spot, causing humidity condensation to form inside the sensor and generate unwanted water. In general, it is important to isolate the cabinet from the external environment to avoid cold spots and free water. Poor sealing also offers a potential surface for contamination growth and hampers the cleaning.

As a manufacturer of CO2 sensors, we believe that the CO2 control of an incubator is only as good as the CO2 measurement.