17
the density of the air is 0.075 lb/ft. The velocity values displayed by
the AXD530 are the actual velocities
only
if the density where the
Pitot probe measurements are being taken is 0.075 lb/ft
3
. Otherwise, a
correction step must be performed to obtain a correct value. Listed
below in
decreasing
order of accuracy are methods for determining a
correction factor.
Note
: In the United States, the barometric pressure reported by the
National Weather Service is corrected to sea level and,
therefore, cannot be used unless your measurements are taken
at sea level. The preferred method is to use the atmospheric
pressure at the location of the measurement. If you do not
have access to that value, then Method 3 in this appendix can
be used. This method is most accurate when the pressure
reported by the national weather service is 29.92 in Hg due to
the assumption of standard density.
Method 1
If you have the absolute pressure, temperature and relative humidity,
record the differential pressure from the AXD530, the absolute
pressure, the temperature, and the relative humidity at the
measurement location. Compute the density using the techniques used
in the 1993 ASHRAE fundamentals handbook. With the density and
the differential pressure, use the formula for Pitot probes also found
in that handbook to compute the actual velocity.
Method 2
If you have the absolute pressure and the temperature, you have two
options.
a) Record the displayed velocity measurement along with the
absolute pressure and temperature. Convert to units of °F and in
Hg if the temperature and pressure were not taken in those units.
To obtain actual velocity, use Equation 1 in this appendix to
calculate the correction factor and multiply the velocity reading
by that factor.
b) If using the equation is not convenient, the Air Pressure vs. Air
Temperature chart in this appendix can be used to determine the
correction factor. However, inaccuracies can result from using the
graphical technique instead of a direct calculation to obtain actual
velocity.
