Gas volume units, STPD, BTPS and ATPS

Since the mass of gas in a unit volume is dependent on pressure and temperature (see Gas laws) they have to be specified with their pressure and temperature.

Three quantities are used.

STPD: Standard Temperature and Pressure Dry, so at 1 atm (760 mm Hg), 0^o C and p_HO2 = 0;

BTPS: Body Temperature Pressure Saturated, defined at 37^o C, ambient pressure and saturated with water vapor (p_H2O = 6.3 kPa or 47 mm Hg);

ATPS: Ambient Temperature Pressure Saturation, so at ambient temperature and pressure, with p_HO2 saturated.

Respiratory volumes are usually reported at BTPS. On the other hand, moles of gas (i.e. O₂, CO₂ production) are commonly reported at STPD.

Often volume measurements are performed neither at STPD nor at BTPS, but at ATPS.

Conversion among these conditions can be easily performed by applying the Gas laws for ideal gases, with volume proportional to absolute temperature T, and inversely proportional to pressure P.

As an example, if a spirometer is used to measure the tidal volume (VT) of a subject in a mountain region where barometric pressure P=450 mm Hg, and the spirometer is at 23^o C (knowing that the pressure of water vapor at saturation at 23^o C is 21 mm Hg, and that at the body temperature of 37^o C is 47 mm Hg) ATPS can be converted to BTPS. For convenience pressures are all in mm Hg, so recalculation in bar or Pascal is not required.

V_BTPS/V_ATPS = ((T_BTPS / T_ATPS )) · ((P_ATPS) / P_BTPS))

= ((273+37)/ (273+23)) · ((450-21)/(450-47))

hence,

VT_BTPS = 1.1149 VT_ATPS .

Similarly, a BTPS volume can be converted to a STPD volume:

V_STPD = ((P_BTPS -47)/760) · (273/310) · V_BTPS = 0.00116 · V_BTPS

When P_BTPS -= 760 mm Hg, then:

V_STPD = ((760-47)/760) · (273/310) · V_BTPS = 0.826 V_BTPS