Measurement of volume and flow in gases

The ability to measure gas flows and volumes accurately is essential to the anaesthetist, both for precise delivery of gas mixtures and for monitoring respiratory volumes in the ventilated or spontaneously breathing patient. Gases are fluid, compressible and usually invisible, which makes them difficult to measure. Gas volumes (and associated flows) can be measured directly using a system with a calibrated chamber, but in clinical practice, measurement is usually made indirectly, using a property of the gas that changes in parallel to flow or volume and which can be more easily determined.