A method of measuring a gas concentration, the method comprising: illuminating, with a light source, a volume of space that includes a gas, wherein the volume of space is defined by a sample gas tube within a housing configured to constrain the gas to flow through the volume of space; measuring, with a detector, a first illumination level being an amount of light from the light source after the light has propagated through a portion of the volume of space; and measuring, with the detector, a second illumination level of the volume of space with the gas removed from the volume of space; and determining, via a processor, the gas concentration in the volume of space based on a comparison of the second illumination level of the volume and the first illumination level of the volume; characterised in that the volume of space is configured to be in fluid communication with a gas recirculation flow path including a catalyst, the catalyst configured to substantially remove the gas from the volume of space, wherein the method further comprises: removing, via the catalyst, the gas from the volume of space; and wherein removing the gas from the volume of space comprises: closing one or more valves (236, 238) configured to shut off fluid communication between the sample gas tube, a fluid inlet, and a fluid outlet; opening one or more valves (236, 238) configured to open fluid communication between the sample gas tube and the gas recirculation flow path including the catalyst; causing, via a pump, the gas to circulate within the volume of space and the gas recirculation flow path including the catalyst; and removing the gas via chemical reaction of the gas by the catalyst. illuminating, with a light source, a volume of space that includes a gas, wherein the volume of space is defined by a sample gas tube within a housing configured to constrain the gas to flow through the volume of space; measuring, with a detector, a first illumination level being an amount of light from the light source after the light has propagated through a portion of the volume of space; and measuring, with the detector, a second illumination level of the volume of space with the gas removed from the volume of space; and determining, via a processor, the gas concentration in the volume of space based on a comparison of the second illumination level of the volume and the first illumination level of the volume; characterised in that the volume of space is configured to be in fluid communication with a gas recirculation flow path including a catalyst, the catalyst configured to substantially remove the gas from the volume of space, removing, via the catalyst, the gas from the volume of space; and wherein removing the gas from the volume of space comprises: closing one or more valves (236, 238) configured to shut off fluid communication between the sample gas tube, a fluid inlet, and a fluid outlet; opening one or more valves (236, 238) configured to open fluid communication between the sample gas tube and the gas recirculation flow path including the catalyst; causing, via a pump, the gas to circulate within the volume of space and the gas recirculation flow path including the catalyst; and removing the gas via chemical reaction of the gas by the catalyst.