ADM-880C 07/20/09
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13.0 CORRECTION FACTORS
13.1 BAROMETRIC PRESSURE DENSITY CORRECTION
The AirData Multimeter automatically corrects air flow and velocity readings to represent local density cfm or fpm as effected
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.
13.3 RELATIVE HUMIDITY CORRECTION
The ADM-880C AirData Multimeter does not correct for the density effects of relative humidity on air flow and velocity
readings. The effect of 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
13.4 HOT WIRE ANEMOMETER VERSUS AIRDATA MULTIMETER
The ADM-880C AirData Multimeter measures local density
true
air velocity or standard density sea level equivalent air
velocity. This is in contrast to thermal anemometers or "hot wire" instruments which only measure mass flow standard density
velocity. 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 29.921 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
Summary of Contents for AIRDATA ADM-880C
Page 42: ...ADM 880C 07 20 09 37 FIGURE 6 3 VELGRID ASSEMBLY...
Page 50: ...ADM 880C 07 20 09 45 FIGURE 10 1 FRAME STORAGE FIGURE 10 2 FLOWHOOD IN CASE...
Page 51: ...ADM 880C 07 20 09 46 FIGURE 10 3 FLOWHOOD ASSEMBLY...
Page 53: ...ADM 880C 07 20 09 48 FIGURE 10 7 1X5 FRAME ASSEMBLY FIGURE 10 8 3X3 FRAME ASSEMBLY...
