Spirometer

The device commonly used to measure lung volumes and capacities is the spirometer. The spirometer consists of two cylinders, both closed at one end, that are fitted one inside another. The inner cylinder is converted to closed chamber by filling the outside cylinder with water. Gas can then be inhaled from or exhaled into inner cylinder and the volume change measured. The volume of a gas varies according to the temperature, pressure, and presence of water vapor in the gas. Thus the exhaled gas volume is measured at ambient temperature and pressure and saturated with water vapor. Such measurement conditions are referred as ATPS (ambient temperature, pressure, saturated) conditions. To determine the gas volume present in the lung, the ATPS volume (spirometer) must be converted to body temperature, pressure saturated (BTPS) conditions, using appropriate gas equations.