Gas sensing device and a method for sensing gas

A gas sensing device for sensing a certain gas that is associated with a certain spectral band, the gas sensing device comprises:

• a passive gas sensor that is configured to generate passive gas sensor detection signals that are responsive to the certain spectral band;
• a passive dummy sensor that is configured to generate passive dummy sensor detection signals that are indifferent to the certain spectral band;
• and
• at least one circuit that is configured to detect a presence or absence of the certain gas within a certain volume that is located within a field of view of the passive gas sensor and a field of view the passive dummy sensor based on a comparison between the passive gas sensor detection signals and the passive dummy sensor detection signals;
• wherein a detection of the presence or absence of the certain gas within the certain volume is based on comparisons between passive gas sensor detection signals obtained during multiple measurements and passive dummy sensor detection signals obtained during the multiple measurements;
• wherein the multiple measurements comprise a first measurement and a second measurement;
• wherein the first measurement was taken when the gas sensing device was positioned at a first position (a) that is on a floor of a room and facing a ceiling and (b) in which the passive gas sensor has a first optical length;
• wherein the second measurement was taken when the gas sensing device was positioned at a second position (a) that is on a head of a user and facing the ceiling, and (b) in which the passive gas sensor has a second optical length that differs by a known difference from the first optical length.