A method for determining a characteristic such as partial pressure or percentage of a gaseous constituent in a first gas mixture flow (32) in a flow chamber (30) where flows alternatively said first gas mixture flow (32) and a second gas mixture flow (34) comprising the following steps: a) introducing the first gas mixture flow (32) into a sensing chamber (40) when the first gas mixture flow (32) flows in the flow chamber (30), b) preventing introduction of gas from the flow chamber (30) into the sensing chamber (40) at least when the second gas mixture flow (34) flows in the flow chamber (30), c) sensing said characteristic of the first gas mixture flow (32) in the sensing chamber (40).
