GSS AN: Operation of Auto Zero

All GSS sensors are calibrated for accuracy at the factory at multiple different gas concentrations and temperatures.

In use, the sensor reference level will change, due to changes in the optical surfaces, accumulation of dirt in the sensor and other degradations. Although the wavelength of the light being emitted by the LED is not affected, the impact of these changes will be to reduce the signal level received by the photodiode in the sensor.

The relationship between CO2 concentration and measured CO2 remains linear over time. However, the reference levels may change compared to those stored in the sensor when it was shipped from the factory. The change in reference level of the sensor can be cancelled out using a process known as zero-setting, sometimes known as baseline correction. This process resets the sensor to a defined concentration level.

The sensor zero-point can be reset by the user, or in some circumstances, the sensor can operate fully autonomously and periodically ‘auto-zero’ without user intervention.

This application note describes how the auto-zero function works in the CozIR family of CO2 sensors. Each member of the family has an auto-zero function that is enabled by default.