Understanding CO2 Sensors

1.0  Thermal Conductivity (TC) CO2 sensors.

Thermal Conductivity (TC) works by measuring the resistance between two thermistors, one exposed to the chamber and the other enclosed. The presence of CO2 in the chamber changes the resistance between the two thermistors. The level of CO2 is determined by measuring the resistance. One drawback of the TC system is that changes in temperature and relative humidity can affect the accuracy of the sensor. Door openings, which cause change in temperature and relative humidity in addition to CO2 levels, can affect the thermal TC sensor.

A schematic of a classic thermal conductivity detector design utilizing a wheatstone bridge circuit is shown. The reference flow across resistor 4 of the circuit compensates for drift due to flow or temperature fluctuations. Changes in the thermal conductivity of the column effluent flow across resistor 3 will result in a temperature change of the resistor and therefore a resistance change which can be measured as a signal.

2.0  Infrared (IR) CO2 sensors.

The infrared (IR) system was developed as an alternative to the TC sensor and is a more accurate controller of CO2 levels. The IR detects CO2 levels with an optical sensor. The chamber air passes over an IR emitter (a light source) and the sensor. The sensor detects a reduction in the IR from the emitter as the CO2 in the chamber air absorbs the IR. The amount of IR absorbed is relative to the levels of CO2 in the chamber air. The IR sensor is not effected by variations in temperature and humidity so it is more accurate than the TC sensor especially following door openings. This is more expensive method than the TC.

3.0  Time injection or constant flow type CO2 control.

In this method filtered air is injected from outside the chamber at a specified flow rate and CO2 is injected at the same flow rate. These are alternated on a time sequence. Different manufacturers use different timings. Example if you want to control 5% CO2. Air is injected for 95 seconds and CO2 is injected for 5 seconds. The door switch detects when the door is opened and a recovery time inject pure CO2 so that the unit recovers to set point.