Zeroing a CO2 Sensor
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APPLICATION NOTE AN002: Zeroing a CO2 Sensor ABSTRACT GSS sensors are fully calibrated and characterised over their full operating temperature range, their gas concentration range and tested for CO2 output measurement accuracy. All GSS sensors use a technique called non-dispersive infra-red (NDIR) sensing where light from a LED 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. The performance of an NDIR sensor can be compromised by the build-up of contaminants in the measurement chamber of the sensor, as well as other degradations in the optical components. The effect of these degradations can be fully eliminated by using one or more of the GSS zeroing routines. Zero-point setting is the process of modifying the CO2 measurement value read by the sensor and align it with an external reference set point. The set point can be any value within the concentration range of the sensor, but is typically 0ppm, or some other easily accessed reference gas level. In a typical ambient application, fresh air with a CO2 level of 400ppm is often used as a reference set point. The requirement to re-zero the sensor will depend on the application and the environment in which the sensor is used. There is no requirement GSS sensors to be re-set for the CO2 concentration span. A simple zero reset is sufficient to maintain accuracy over the full lifetime of the sensor. Gas Sensing Solutions Ltd. Page | 1 For regular updates, sign up at https://gassensing.co.uk Revision 1.1, 18 May 2020 Copyright © 2021 Gas Sensing Solutions Ltd.

Gas Sensing Solutions Ltd. Page | 2 For regular updates, sign up at https://gassensing.co.uk Revision 1.1, 18 May 2020 Copyright © 2021 Gas Sensing Solutions Ltd.

APPLICATION NOTE AN002: Zeroing a CO2 Sensor OVERVIEW OF ZERO SETTING 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. The GSS sensor has two methods to reset the reference value or ‘zero-point. Setting the zero-point can be done manually in either software or hardware on some models, or it can be done automatically using the sensor’s auto-zeroing settings. Regardless of which method is...

APPLICATION NOTE AN002: Zeroing a CO2 Sensor ZERO SET POINT METHODS All GSS sensors have three manual zero-point setting methods. ZERO IN A KNOWN GAS CONCENTRATION The sensor must be placed in CO2 gas with a known gas concentration level. The sensor needs to be instructed to run a zero-point setting cycle in a known gas concentration. The value of the known gas concentration level must be written to the sensor. Once the sensor has completed the measurement cycle, it will internally compare the measured result with the input gas concentration level and calculate an offset value. This offset...

APPLICATION NOTE AN002: Zeroing a CO2 Sensor ZERO-POINT ADJUSTMENT If the CO2 concentration and the sensor reported concentration are known, the zero point can be adjusted using the known concentration to fine tune the zero point. For example, if the sensor has been in an environment that has been exposed to outside air, and the sensor reading is known at that time, the zero point can be fine-tuned to correct the reading. This is typically used to implement automated zeroing routines. PRESSURE AND CONCENTRATION LEVEL COMPENSATION NDIR gas sensors detect the concentration of gas by measuring...

APPLICATION NOTE AN002: Zeroing a CO2 Sensor AUTO-ZERO SETTINGS By default, the sensor will automatically 'zero' using the measured CO2 level sampled during the auto-zeroing period. The user can alter the behaviour of the sensor as a result of the auto-zeroing process. SOFTWARE INSTRUCTION SEQUENCE To manually instruct a zero-point reset, follow the recommended sequence. This sequence is common to all GSS sensors. Page | 6 For regular updates, sign up at https://gassensing.co.uk Copyright © 2021 Gas Sensing Solutions Ltd.

APPLICATION NOTE AN002: Zeroing a CO2 Sensor HARDWARE INSTRUCTION SEQUENCE Some GSS sensors have a pin on the sensor that can be used to force a zero reset. There are two options, either to initiate a Zero in Nitrogen, or a Zero in Fresh Air. The functions are identical to the those initiated in software. The zero reset is initiated by forcing the pin low for a minimum of 3 seconds. The timing for FRESH_AIR_ZERO is identical to NITROGEN_ZERO. Page | 7 For regular updates, sign up at https://gassensing.co.uk Copyright © 2021 Gas Sensing Solutions Ltd.

APPLICATION NOTEAN002: Zeroing a CO2 Sensor To manually instruct a zero-point reset using the hardware pins, follow the recommended sequence. This sequence is common to GSS sensors with hardware zero reset pins. SUMMARY Resetting the 'zero' reference level is a simple process that will maintain sensor accuracy for the lifetime of the product. The reset can be done manually, or depending on the environmental conditions, set to be done fully automatically with no user intervention. If done correctly, the sensor will continue to maintain the original factory test accuracy. Page | 8 For regular...

APPLICATION NOTE AN002: Zeroing a 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...