ADM-870 05/23/0822
6.5 VELOCITY: LOCAL DENSITY VS STANDARD DENSITY (MASS FLOW)
The AirData Multimeter measures true air velocity past the sensor at a given time, when used in the local
density mode. This is in contrast to thermal anemometers or "hot wire" instruments which measure mass flow
(mass flux/unit time). Mass flow represents the number of molecules of air flowing past a given point during a
given time. Mass flow only represents true velocity when measured at standard sea level conditions of 29.92
in Hg and 70
/ F (.075 lbs/cu ft). Hot wire, mass flow, "velocity" readings at density conditions other than standard
must be corrected for local air density conditions if these results are to represent true velocity.
Air velocity readings taken with the ADM-870 in the standard density mode are comparable to readings taken
with a hot wire anemometer. If velocity readings taken with the AirData Multimeter are to be compared with hot
wire anemometer readings, the actual air velocity should be measured in the local density mode with the
AirData Multimeter, and the hot wire readings must be corrected for local air density conditions.
The precise method for calculating density corrected air velocity measurements taken with a hot wire
anemometer requires the use of the following equation:
Where: P
b
= local barometric pressure (in Hg)
/ F = temperature of air stream
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