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Exercise Science 1

## Section 1

The volume of expired gas measured is refered to as VEATPS 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 VESTPD — the volume expired at standard temperature (273°K) and pressure (760 mm Hg) under dry conditions. Do this for the data below.

## Formula for calculating VESTPD

VESTPD = VEATPS x (BP–SWVP)/760 x 273/(273+t)

where:

• VESTPD and VEATPS 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)

## Worked Example

Barometric pressure: 761 mm Hg
Temperature of expired air: 25°C
Volume of air in the Douglas bag: 47 litres
Collection time: 5 min

VESTPD = VEATPS x (BP–SWVP)/760 x 273/(273+t)

VEATPS = 47/5 = 9.4 litres/min
SWVP = (1.001 x t) – 4.19 = (1.001 x 25) – 4.19 = 20.835 mm Hg
Hence VSTPD = 9.4 x (761–20.835)/760 x 273/(273+25)
i.e. VSTPD = 9.4 x 740/760 x 273/298
i.e. VSTPD = 8.38 litres/min

VESTPD: litres/min (2 DP)