Functional test for gas sensors

A method for functionally testing a gas sensor operating according to the principle of thermal conductivity, the gas sensor designed to sense the presence of a target gas in a fluid medium, the gas sensor comprising a measurement chamber with a connection to a supply of a purging gas, a thermal conductivity sensor, disposed in the measurement chamber, for determining one or more measurement values, a temperature sensor, and a membrane that separates the measurement chamber from the fluid medium during operation, the membrane being permeable to the target gas;

• the method comprising the steps of:
• performing a calibration cycle, comprising the steps of:
• immersing the membrane of the gas sensor in a calibration medium that is a fluid medium with a known concentration of the target gas;
• conducting a purging procedure, comprising purging the measurement chamber with the purging gas;
• conducting a measuring procedure after the purging procedure, the measuring step comprising the step of using the thermal conductivity sensor to take a plurality of measurements of the gas in the measurement chamber, obtaining a plurality of time-related measurement values;
• and
• determining a calibration baseline from the plurality of time-related measurement values;
• performing a measurement cycle, comprising the steps of:
• immersing the membrane of the gas sensor in a measurement medium that is a fluid medium with an unknown concentration of the target gas;
• conducting a purging procedure, comprising purging the measurement chamber with the purging gas;
• conducting a measuring procedure after the purging procedure, the measuring step comprising the step of using the thermal conductivity sensor to make a plurality of measurements of the gas in the measurement chamber,
• obtaining a plurality of time-related measurement values;
• and
• determining a measurement baseline from the plurality of time-related measurement values;
• calculating a baseline comparison value taking account of the calibration baseline and the measurement baseline;
• and
• comparing the baseline comparison value to a predetermined baseline threshold value, and, if the baseline comparison value is greater than the baseline threshold value, generating a first error message, indicating a malfunction in the purging gas supply.