A non-electrical device intended to be used with a mainstream carbon dioxide (CO2) sensor for the determination of CO2 concentration in gases expired from a patient. It is designed as a small, plastic, open-ended vessel with a clear optic window (cuvette) for the acceptance of an infrared sensor [not included] that is slotted over the outside of the window section; some types may additionally feature an adaptor for the acceptance of a gas flow sensor. It typically has connector ports that enable it to be connected into a breathing system (e.g., an anaesthesia or critical care ventilation system). This is a single-use device.