Instrument, system and methods for use in respiratory exchange ratio measurement

The instrument has sensors for sensing oxygen and/or carbon dioxide content in exhaled air received in a receiving area in front of a mouth, an air flow rate sensor for sensing exhaled air flow rates in a flow rate sensing location and an air shield for shielding the receiving area and the flow rate sensing location from air flows from the environment. The air shield leaves a space between the air shield and the mouth of the person in open communication with the environment. The air flow rate sensor senses air flow speed in a location spaced from the exhaled air receiving area, rearward of a front end of the exhaled air receiving area and above a lower end of the exhaled air receiving area. In another embodiment a sensor for sensing ambient wind is provided