CFM-88L 07/20/0924
12.0 CORRECTION FACTORS AND CONVERSION FORMULAS
12.1 BAROMETRIC PRESSURE DENSITY CORRECTION - LOCAL DENSITY
The CFM-88L AirData FlowMeter automatically corrects air flow readings to represent local density cfm as affected by
barometric pressure.
12.2 BAROMETRIC PRESSURE DENSITY CONVERSION - STANDARD DENSITY
Standard density air flow is calculated as if the same mass flow (lb/min) existed at standard conditions (29.92 in Hg & 70
/F).
Standard density (mass flow equivalent) results may be obtained by using one of the following conversion formulas:
12.2.1 STANDARD DENSITY - TEMPROBE USED
If the TemProbe was used during the measurement, the conversion formula for standard density is:
Where: P
b
= local barometric pressure (in Hg)
/F = measured temperature of the airstream
12.2.2 STANDARD DENSITY - TEMPROBE NOT USED
If the TemProbe was not used during the measurement, the conversion formula is:
Where: P
b
= local barometric pressure (in Hg)
/F = measured temperature of the airstream
12.3 TEMPERATURE DENSITY CORRECTION
Air flow readings are density corrected for the effect of temperature if the TemProbe temperature sensor is used during flow
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 airstream.
12.4 RELATIVE HUMIDITY CORRECTION
The CFM-88L AirData FlowMeter does not correct for the density effects of relative humidity on air flow readings. The effect
of minor variations in relative humidity on local air density (and therefore on flow 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 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 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
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