TESTO 645 Instruction Manual page 32

Determining absolute pressure
Example:
If you are at 800 m above mean sea level, you have an annual
mean air pressure of 920 mbar.
The annual mean air pressure has to be reduced by 10 (910 mbar
air pressure) to correspond to the barometer display
(1003 mbar) and to the barometer's height offset (to 1013 hPa).
Add the difference between your ambient air pressure and the
static process air pressure (e.g. positive pressure in an air duct
which is to be measured - can be measured using the differential
pressure probe at up to 100 mbar) to this (air) pressure value.
Example: We have measured an annual mean air pressure of
910 mbar. We will now add 90 mbar based on the static process
pressure. We therefore assume an absolute air pressure in the
measurement medium of 1000 mbar.
The table on page 31 is also saved in the Testo Comfort software.
Absolute pressure can be calculated automatically using this
software if the height above mean sea level, barometric pressure
and differential pressure are entered. The calculated absolute
pressure is then taken over by the instrument.
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