A spirometer is an apparatus for measuring the volume

Volume

Volume is the quantity of three-dimensional space enclosed by some closed boundary, for example, the space that a substance or shape occupies or contains....

 of air inspired and expired by the lung

Lung

The lung is the essential respiration organ in many air-breathing animals, including most tetrapods, a few fish and a few snails. In mammals and the more complex life forms, the two lungs are located near the backbone on either side of the heart...

s. It is a precision differential pressure

Pressure

Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...

 transducer

Transducer

A transducer is a device that converts one type of energy to another. Energy types include electrical, mechanical, electromagnetic , chemical, acoustic or thermal energy. While the term transducer commonly implies the use of a sensor/detector, any device which converts energy can be considered a...

 for the measurements of respiration flow rates. The spirometer records the amount of air and the rate of air that is breathed in and out over a specified period of time.
A Tank-type spirometer works on the same principle as the gasometer

Gasometer

A gas holder is a large container where natural gas or town gas is stored near atmospheric pressure at ambient temperatures. The volume of the container follows the quantity of stored gas, with pressure coming from the weight of a movable cap...

. A canister of soda is usually attached to absorb carbon dioxide and a kymograph trace is produced to record changes in total volume gas.