Abstract: An apparatus for calibrating a spirometer controls the temperature and moisture content of the gas which it receives and accurately measures the flow rates of gas it receives and discharges. This apparatus includes a housing with an opening through which it receives and discharges the gas. This housing contains a piston which engages the inner surface of the housing to form a gas receiving chamber within the housing. The piston lies in sliding engagement in the housing and a user may move it to increase or decrease the volume of the chamber and pull gas into the chamber through the opening or force gas out of the chamber. A heating element disposed proximate the chamber heats the gas which the chamber receives; and a gas moisturizing element disposed in the housing adds moisture to the gas moving into the chamber and out of the chamber through the housing opening.