How we perform an end-to-end calibration of the sensors on a special calibration bench

2023-06-30 16:25

Electrochemical sensors are used to measure the concentration of gases in the air. They are quite small and can work continuously. But there is a nuance: the readings of electrochemical sensors depend not only on the concentration of the measured gases but also on temperature, humidity, and the concentration of some "neighboring" gases.

In#nbsp;addition, each specific electrochemical sensor is#nbsp;unique in#nbsp;a#nbsp;certain sense and has unknown characteristics. Therefore, a#nbsp;good solution is#nbsp;to#nbsp;use a#nbsp;whole set of#nbsp;sensors for different gases in#nbsp;one device and perform an#nbsp;end-to-end calibration of#nbsp;the sensors on#nbsp;a#nbsp;special calibration bench.

The calibration procedure consists of#nbsp;successive changes in#nbsp;the operating conditions (temperature, humidity, and gas concentration) of#nbsp;sensors placed in#nbsp;a#nbsp;special chamber. All sensor readings are recorded under each condition.

Then systems of#nbsp;linear equations are solved, and matrices are built that determine the most accurate linear relationship between "millivolts at#nbsp;a#nbsp;given set of#nbsp;sensors" and "real gas concentrations". It#nbsp;is#nbsp;noteworthy that the matrices depend only on#nbsp;temperature and humidity (and in#nbsp;some cases also on#nbsp;the rate of#nbsp;change in#nbsp;temperature and humidity).

In#nbsp;order to#nbsp;collect enough data for an#nbsp;accurate calibration, our bench will run for a#nbsp;full month. Therefore, before placing a#nbsp;large set of#nbsp;sensors in#nbsp;the chamber, they must be#nbsp;carefully checked to#nbsp;make sure that they all work properly.

In#nbsp;the photos, you can see the fairy lights of#nbsp;a#nbsp;set of#nbsp;sensors before the calibration procedure on#nbsp;our calibration bench. 35 sensors are placed in#nbsp;the sockets, where the necessary conditions are maintained: pressure, temperature, humidity, and concentration of#nbsp;the supplied gas mixtures.

Each of#nbsp;these lights refers to#nbsp;one socket. The wires on#nbsp;top are part of#nbsp;the electronic harness, which "interrogates" all the sensors and logs data for further processing.

Then, after comparing the sensor readings with the calibration system data, we#nbsp;will assign each sensor its own unique calibration factor.

As#nbsp;for the LEDs, they show that each sensor under the light is#nbsp;receiving power and successfully responding to#nbsp;requests.