In general, those standards which have been used by the Chartered Institution of Building Services Engineers in their Guide to Current Practice, are used here.
The more important values are:
Density of Air 1.293 kg nT3 for dry air at 101 325 Pa and 0°C.
Density of Water 1000 kg nT3 at 4°C and 998.23 kg m"3 at 20°C.
Barometric Pressure 101 325 Pa (1013.25 mbar).
Standard Temperature and Pressure (STP) is the same as Normal Temperature and Pressure (NTP) and is 0°C and 101 325 Pa, and the specific force due to gravity is taken as 9.807 N kg“1 (9.806 65 N kg-1 according to ASHRAE (1997)). Meteorologists commonly express pressure in mbar, where 1 mbar =100 Pa.
Both temperature and pressure fall with increasing altitude up to about 10 000 m. ASHRAE (1997) gives the following equation for the calculation of atmospheric pressure up to a height of 10 000 m:
P = 101.325 (1 — 2.255 77 x 10“5Z)5’2559 (2.1)
T = 15 — 0.0065Z (2.2)
For the calculation of temperature up to a height of 11 000 m, where p is pressure in kPa, t is temperature in K and Z is altitude in m above sea level.
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