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45
The orifice manometer readings need to be converted to the actual airflows they represent using the
following equation:
Qa = 1/m[Sqrt((H20)(Ta/Pa))-b]
where: Qa = actual flow rate as indicated by the calibrator orifice, m3/min
“H20 = orifice manometer reading during calibration, (inches) “H20
Ta = ambient temperature during calibration, K ( K = 273 + C)
Pa = ambient barometric pressure during calibration, mm Hg
m = Qactual slope of orifice calibration relationship
b = Qactual intercept of orifice calibration relationship.
Once these actual flow rates have been determined for each of the five run points, they are recorded in
the column titled Qa, and are represented in cubic meters per minute.
The continuous flow recorder readings taken during the calibration need to be corrected to the current
meteorological conditions using the following equation:
IC = I[Sqrt(Ta/Pa)]
where: IC = continuous flow recorder readings corrected to current Ta and Pa
I = continuous flow recorder readings during calibration
Pa = ambient barometric pressure during calibration, mm Hg.
Ta = ambient temperature during calibration, K ( K = 273 + C)
After each of the continuous flow recorder readings have been corrected, they are recorded in the
column titled IC (corrected).
Using Qa and IC 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 =
 
n -
x
xm - y = b ;
x
n -xy
y)x)((
2
2
_ _
where: n = number of observations y = y/n; x = x/n = sum of
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