A portable device for measuring the concentration of at least one component in a gas exhaled by a breath fluid, including a first chamber located upstream of a measuring vessel and including an inlet through which the exhaled breath fluid enters the first chamber, a second chamber located upstream of the measuring vessel, the second chamber including an inlet leading into the first chamber, an outlet in communication with the measuring vessel and through which some of the exhaled breath fluid passes, and an outlet leading to the ambient air, through which some of the exhaled breath fluid is expelled into the ambient air. Further, the second chamber is located inside or adjacent the first chamber and the outlet to the ambient air leads to the first chamber so some of the exhaled breath fluid enters the second chamber through the inlet of the second chamber.