The volume of expired gas measured is refered to as VE_{ATPS} as it is measured at *ambient* *temperature* and *pressure*, and the gas is *saturated* with water vapour. However the volume of a gas changes as a result of each of these factors, so in order to compare measurements and make further calculations you need to convert to VE_{STPD} — the volume expired at *standard* *temperature* (273°K) and *pressure* (760 mm Hg) under *dry* conditions. Do this for the data below.

VE_{STPD} = VE_{ATPS} x (BP–SWVP)/760 x 273/(273+t)

where:

- VE
_{STPD}and VE_{ATPS}are the expired air volumes, as defined in the text, expressed as litres/min - SWVP is saturated water vapour pressure at ambient temperature = (1.1001 x t) – 4.19
- BP is barometric pressure (mm Hg)
- t is ambient temperature (°C)

Barometric pressure: 761 mm Hg

Temperature of expired air: 25°C

Volume of air in the Douglas bag: 47 litres

Collection time: 5 min

VE_{STPD} = VE_{ATPS} x (BP–SWVP)/760 x 273/(273+t)

VE_{ATPS} = 47/5 = 9.4 litres/min

SWVP = (1.001 x t) – 4.19 = (1.001 x 25) – 4.19 = 20.835 mm Hg

Hence V_{STPD} = 9.4 x (761–20.835)/760 x 273/(273+25)

i.e. V_{STPD} = 9.4 x 740/760 x 273/298

i.e. V_{STPD} = 8.38 litres/min

VE

or