Standards adopted

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.

Posted in Engineering Fifth Edition