Gas sensor with two switchable filters and method for operating such a gas sensor

Adaptable filter system for a gas sensor, wherein the adaptable filter system comprises

• a first optical filter and
• a second optical filter,
• wherein the first filter is operable to switch between a first reference state in which the first filter passes light according to a first reference wavelength profile and a first measurement state in which the first filter passes light according to a first measurement wavelength profile,
• wherein the second filter is operable, independently of the first filter, to switch between a second reference state in which the second filter passes light according to a second reference wavelength profile and a second measurement state in which the second filter passes light according to a second measurement wavelength profile,
• wherein the first reference wavelength profile is different from the second reference wavelength profile, the first and second filters, thereby being switchable between four different composite states, the four composite states comprising:
• a first composite state in which the first and second filters, are in the first and second reference states respectively,
• a second composite state in which the first filter is in the first reference state and the second filter is in the second measurement state,
• a third composite state in which the first filter is in the first measurement state and the second filter is in the second reference state, and
• a fourth composite state in which the first and second filters, are in the first and second measurement states respectively.

A method of operating a gas sensor configured and arranged to detect at least one target gas, wherein the gas sensor comprises a light source configured and arranged to emit light, a measurement volume, a detector configured and arranged to receive light, and an adaptable filter system, wherein the adaptable filter system comprises a first filter and a second filter, the method comprising the steps of:

• providing the first filter to be operable to switch between a first reference state in which the first filter passes light according to a first reference wavelength profile and a first measurement state in which the first filter passes light according to a first measurement wavelength profile;
• providing the second filter is to be operable, independently of the first filter, to switch between a second reference state in which the second filter passes light according to a second reference wavelength profile and a second measurement state in which the second filter passes light according to a second measurement wavelength profile,
• wherein the first reference wavelength profile is different from the second reference wavelength profile;
• providing the first and second filters to be switchable between four different composite states, the four composite states comprising:
• a first composite state in which the first and second filters are in the first and second reference states respectively,
• a second composite state in which the first filter is in the first reference state and the second filter is in the second measurement state,
• a third composite state in which the first filter is in the first measurement state and the second filter is in the second reference state, and
• a fourth composite state in which the first and second filters are in the first and second measurement states respectively;
• with the light source emitting light, wherein at least a part of the emitted light passes along a light path from the light source through the measurement volume and the adaptable filter system to the detector;
• switching the adaptable filter system between at least three of the composite states;
• with the detector generating a respective signal corresponding to light received by the detector for every one of the at least three used composite states;
• and
• determining a presence of a contaminant other than the target gas in the light path between the light source and the detector automatically, by comparing the respective detector signals generated during at least three of the composite states with each other.