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The Magnehelic Gage readings taken during the calibration need to be corrected to
the current meteorological conditions using the following equation:
FLOW (corrected) = Sqrt((magn)(Pa/760)(298/Ta))
where:
FLOW (corrected) = Magnehelic Gage readings corrected to current Ta and Pa
magn = Magnehelic Gage readings during calibration
Pa = ambient barometric pressure during calibration, mm Hg
760 = standard barometric pressure, a constant, mm Hg
Ta = ambient temperature during calibration, K ( K = 273 +Co)
298 = standard temperature, a constant, K
After each of the Magnehelic Gage readings have been corrected, they are recorded
in the column titled FLOW (corrected).
Using Qstd and FLOW (corrected) as the x and y axis respectively, a slope, intercept,
and correlation coefficient can be calculated using the least squares regression
method. The correlation coefficient should never be less than 0.990 after a five point
calibration. A coefficient below .990 indicates a calibration that is not linear and the
calibration should be performed again. If this occurs, it is most likely the result of an
air leak during the calibration.
The equations for determining the slope (m) and intercept (b) are as follows:
m =
 
( x)( y)
xy - n
x b = y - mx
x - n
2
2
 
;
where: n = number of observations
y = y/n; x = x/n
= sum of
The equation for the coefficient of correlation (r) is as follows:
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