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Example Calculations
The following example problems use data from the attached calibration worksheet.
After all the sampling site information, calibrator information, and meteorological
information have been recorded on the worksheet, standard air flows need to be
determined from the orifice manometer readings taken during the calibration using
the following equation:
1. Qstd = 1/m[Sqrt((H20)(Pa/760)(298/Ta))-b]
where:
Qstd = actual flow rate as indicated by the calibrator orifice, m3/min
H2O = orifice manometer reading during calibration, (2O
Ta = ambient temperature during calibration, K (K = 273 + °C)
298 = standard temperature, a constant that never changes, K
Pa = ambient barometric pressure during calibration, mm Hg
760 = standard barometric pressure, a constant that never changes, mm Hg
m = Qstandard slope of orifice calibration relationship
b = Qstandard intercept of orifice calibration relationship.
Note that the ambient temperature is needed in degrees Kelvin to satisfy the Qstd
equation. Also, the barometric pressure needs to be reported in millimeters of
mercury. In our case the two following conversions may be needed:
2. degrees Kelvin = [5/9 (degrees Fahrenheit - 32)] + 273
3. millimeters of mercury = 25.4(inches of H2O/13.6)
Inserting the numbers from the calibration worksheet run point number one we get:
4. Qstd = 1/1.47574[Sqrt((7.25)(749/760)(298/293)) - (-.00613)]
5. Qstd = .6776261[Sqrt((7.25)(..9855263)(1.0170648)) + .0.00613]
6. Qstd = .6776261[Sqrt(7.2669947) + .00613]
7. Qstd = .6776261[2.6957363 + .00613]
8. Qstd = .6776261[2.7018663]
9. Qstd = 1.831
Throughout these examples you may find that your results may vary some from those
arrived here. This may be due to different calculators carrying numbers to different
decimal points. The variations are usually slight and should not be a point of concern.
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