ADM-870 05/23/0838
13.0 CORRECTION FACTORS AND CONVERSION FORMULAS
13.1 BAROMETRIC PRESSURE DENSITY CORRECTION - LOCAL DENSITY
The ADM-870 AirData Multimeter automatically corrects air flow and velocity readings to represent local or
standard density cfm or fpm as affected by barometric pressure.
13.2 TEMPERATURE DENSITY CORRECTION
Air flow and velocity readings are density corrected for the effect of temperature if the TemProbe temperature
sensor is used during flow or velocity measurements. If the temperature probe is not used, the meter
calculates the air density using an assumed temperature of 70
/ F. If the TemProbe sensor was not used during
an air flow measurement, the displayed reading must be corrected for the density effect of temperature as
follows:
Where:
/ F = measured temperature of air stream.
13.4 RELATIVE HUMIDITY CORRECTION
The ADM-870 AirData Multimeter does not correct for the density effects of relative humidity on air flow and
velocity readings. The effect of minor variations in relative humidity on local air density (and therefore on flow
and velocity readings) is relatively small under normal room temperature conditions. The density effect of
relative humidity at higher temperatures and higher relative humidities may be more significant.
Local air density velocity and flow readings may be corrected for the density effects of relative humidity using
the formula shown below. This calculation requires the use of the vapor pressure, which must be looked up
on a Dew Point/Vapor Pressure conversion chart. The dew point may be measured directly using a very
accurate dewpointer. Psychrometric charts are also available for determining the dew point and may be used
if the ambient temperature and relative humidity are known. Note: If the vapor pressure is in metric units, the
barometric pressure must also be in metric units. Many Dew Point/Vapor Pressure conversion charts are in
metric units.
Local air density velocity readings may be corrected for relative humidity using the following formula.
Where: P
b
= local barometric pressure
V
dry
= velocity corrected for local density (temperature and barometric pressure)
P
vapor
= vapor pressure
Local air density air flow readings may be corrected for relative humidity using the following formula.
Where: P
b
= local barometric pressure
F
dry
= air flow corrected for local density (temperature and barometric pressure)
P
vapor
= vapor pressure
13.5 HOT WIRE ANEMOMETER VERSUS AIRDATA MULTIMETER
The ADM-870 AirData Multimeter measures local density true air velocity. 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 (0.075 lbs/cu ft). Hot wire,
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