Auto-Zero Setting CO2 Sensor
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APPLICATION NOTE AN007: Auto-Zero Setting CO2 Sensor ABSTRACT All GSS sensors use a technique called non-dispersive infra-red (NDIR) sensing where light is injected into the optical measurement chamber, which contains the gas which has been allowed to enter it. The light that passes through the optical cavity is detected by the photo diode. The signal from the photo-diode is digitised by the microcontroller and compared with a reference level stored in memory. The microcontroller can then calculate the level of CO2 in the optical measurement chamber. All GSS sensors are 100% tested for measurement accuracy at multiple, different gas concentrations before leaving the factory. In use, and dependent on the conditions, the CO2 concentration value measured by the sensor may vary from the reference value. GSS sensors have a typical operational lifetime of more than 10 years. However, during use, the sensor optical path will change very slowly over time due to the degradation of the optical surfaces caused by environmental contamination, and reduced LED output. This application note describes potential strategies to correct for these changes and ensure the sensor continues to provide highly accurate CO2 measurement results. The GSS sensor has a built-in automatic method to correct for low rate drift of the sensor ‘zeropoint’. Alternatively, the user can make manual adjustments to the zero value of the sensor. Gas Sensing Solutions Ltd. Page | 1 For regular updates, sign up at https://gassensing.co.uk Revision 1.8, 1 June 2020 Copyright © 2021 Gas Sensing Solutions Lt

Gas Sensing Solutions Ltd. Page | 2 For regular updates, sign up at https://gassensing.co.uk Revision 1.8, 1 June 2020 Copyright © 2021 Gas Sensing Solutions Ltd.

APPLICATION NOTE AN007: Auto-Zero Setting CO2 Sensor BACKGROUND TO ZERO-POINT SETTING All GSS sensors are calibrated for accuracy at the factory at multiple different concentration levels. 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 photo-diode in the sensor. The relationship between CO2 concentration and measured CO2 remains linear...

APPLICATION NOTE AN007: Auto-Zero Setting CO2 Sensor AUTO-ZERO SETTING PRINCIPLE OF OPERATION All GSS sensors are calibrated using a variety of different CO2 gas concentrations during factory calibration, typically calibrated mixtures of nitrogen and CO2. In practice, it is difficult in use to replicate these known gas calibrations to reset the ‘zero-point’ of the sensor. The built-in auto-zero setting process relies on resetting the zero-point of the sensor using the measured ‘fresh air’ CO2 concentration value. Therefore, to use the auto-zero setting function, the sensor must be exposed...

APPLICATION NOTE AN007: Auto-Zero Setting CO2 Sensor AUTO-ZERO INTERVALS Depending on sensor type, the intervals between auto-zero events can be programmed either based on time, or on the number of power cycles. The auto-zero time period or the number of power cycles can be programmed by the user. With the exception of the CozIR-Blink, all GSS sensors can be programmed to undertake an initial auto-zero after power-on. This means the sensor must be exposed to ‘fresh air’ at least once during this initial period. Thereafter, the auto-zero period can be set independently of the start-up...

APPLICATION NOTE AN007: Auto-Zero Setting CO2 Sensor AUTO-ZERO SETTINGS By default, the sensor will automatically ‘zero’ using the measured CO2 level sampled during the auto-zero period. The auto-zero function can be enabled to operate automatically, disabled, or can be forced. The user can also independently adjust the CO2 level used for auto-zeroing. All GSS sensors have a zero-setting function that allows the user to set the measurement value of ‘fresh air’. This is called the ZERO IN FRESH AIR reset value. Typically, the auto-zero reset value is set to the same value as the ZERO IN...

APPLICATION NOTE AN007: Auto-Zero Setting CO2 Sensor IMPORTANT NOTICE Gas Sensing Solutions Ltd. (GSS) products and services are sold subject to GSS’s terms and conditions of sale, delivery and payment supplied at the time of order acknowledgement. GSS warrants performance of its products to the specifications in effect at the date of shipment. GSS reserves the right to make changes to its products and specifications or to discontinue any product or service without notice. Customers should therefore obtain the latest version of relevant information from GSS to verify that the information is...

APPLICATION NOTE AN007: Auto-Zero Setting CO2 Sensor ADDRESS Gas Sensing Solutions Ltd. Grayshill Road Cumbernauld G68 9HQ United Kingdom Page | 8 For regular updates, sign up at https://gassensing.co.uk Copyright © 2021 Gas Sensing Solutions Ltd.

APPLICATION NOTEAN007: Auto-Zero Setting CO2 Sensor REVISION HISTORY DATE Page | 9 For regular updates, sign up at https://gassensing.co.uk Copyright © 2021 Gas Sensing Solutions Ltd.