A gas exhaust by-product measurement system is provided. A gas chamber is configured to receive exhaust from the exhaust output. A light source, light detector, and at least one optical element are positioned so that a light beam from the light source is directed to the at least one optical element a plurality of times before reaching the light detector. At least one heater provides heat to the at least one optical element. A plurality of purge gas nozzles are in fluid connection with the optical cavity. A high flow line is in fluid connection between a purge gas source and the plurality of purge gas nozzles. A low flow line is in fluid connection between the purge gas source and the plurality of purge gas nozzles. At least one flow controller manages a plurality of flow rates including a high flow and a low flow.