There is provided a device for measuring a user's oxygen-consumption. The device includes a tubular member with a first tapered portion through which an exhalation of air enters into the device, a second tapered portion, and a constriction between the portions thereof. The devices includes a flow sensing mechanism in communication with the first tapered portion of the tubular member. The device includes an oxygen sensor in communication with the first tapered portion of the tubular member. The device is configured such that the oxygen sensor is passively supplied a portion of the exhalation of air by means of positive or negative differential pressure referenced between the first tapered portion and at least one of ambient air and the constriction of the tubular member.