ATPS, ATP, BTPS, STPD
The volume of a number (n) of gas molecules depends on the thermodynamic temperature (T) and the ambient pressure (P). The following relationship holds for dry gas:
V = n·R·T/P
where R = gas constant, and T is
expressed in Kelvin (K = 273.2 + ºC). Air and expired
gas are made up of gas molecules and water vapor. In a gas
mixture saturated with water vapor and in contact with water
(such as occurs in the lung) the number of water molecules
in the gas phase varies with temperature and pressure. As
the number of molecules is not constant, the above gas law
should be applied to dry gas. This also holds outside the
lung when gas saturated with water vapor is compressed or
cools down.
As gas volumes vary with temperature and pressure, the conditions
during which they are measured must be recorded. To that end
volume displacement spirometers need to be equipped with a
thermometer; if meters employ other measuring principles the
manufacturer should state clearly how corrections need be
performed as the composition of the gas and gas viscosity
may then come into play.
BTPS |
in respiratory physiology lung volumes and flows are standardized to barometric pressure at sea level, body temperature, saturated with water vapor: body temperature and pressure, saturated. |
ATPS |
measured at ambient temperature, pressure, saturated with water vapor (e.g. expired gas, which has cooled down): ambient temperature and pressure, saturated. |
ATP |
like ATPS, but not saturated with water vapor (e.g. room air). |
STPD |
oxygen consumption and carbon dioxide delivery are standardized to standard temperature (0 ºC), barometric pressure at sea level (101.3 kPa) and dry gas: standard temperature and pressure, dry. |
The water vapor pressure of a saturated gas is temperature
dependent. The table below can be used to convert from ATPS
to BTPS conditions; e.g. 1 L ATPS at 20 ºC equals
1.102 L BTPS.
| Conversion from ATPS to BTPS conditions | |||||||
| Temp. ºC |
Corr. factor |
Temp. ºC |
Corr. factor |
Temp. ºC |
Corr. factor |
Temp. ºC |
Corr. factor |
| 16 | 1.123 | 21 | 1.097 | 26 | 1.069 | 31 | 1.039 |
| 17 | 1.118 | 22 | 1.091 | 27 | 1.063 | 32 | 1.033 |
| 18 | 1.113 | 23 | 1.086 | 28 | 1.057 | 33 | 1.026 |
| 19 | 1.107 | 24 | 1.080 | 29 | 1.051 | 34 | 1.020 |
| 20 | 1.102 | 25 | 1.074 | 30 | 1.045 | 35 | 1.013 |
It is uncommon to express the oxygen uptake and dioxide delivery as mol per unit of time. The volume is not converted but expressed in STPD conditions: standard temperature and dry gas at standard barometric pressure: 0º C, 101.3 kPa, dry.
Convert Fahrenheit to Celsius temperature, v.v.
One can approximate the water vapour pressure (PH2O, in kPa) of saturated air at temperatures (t, in °C) between 10-45 °C with sufficient accuracy as follows:
Pt,H2O = 0.42013 + 0.07985·t - 0.000751·t² + 0.000078·t³
In obsolete units (mmHg):
Pt,H2O = 3.10594 + 0.59886·t -0.00561·t² + 0.00058·t³