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The following example problems use data from the attached VFC sampler calibration worksheet.
After all the sampling site information, calibrator information, and meteorological information have
been recorded on the worksheet, actual air flows need to be determined from the orifice manometer
readings taken during the calibration using the following equation:
1. Qa = 1/m[Sqrt((H2O)(Ta/Pa))-b]
where:
2. Qa = actual flow rate as indicated by the calibrator orifice, m3/min
3. H2O = orifice manometer reading during calibration, in. H2O
4. Ta = ambient temperature during calibration, K ( K = 273 + oC)
5. Pa = ambient barometric pressure during calibration, mm Hg
6. m = slope of Q actual orifice calibration relationship
7. b = intercept of Q actual orifice calibration relationship.
Note that the ambient temperature is needed in degrees Kelvin to satisfy the Qa equation. Also, the
barometric pressure needs to be reported in millimeters of mercury (if sea level barometric pressure is
used it must be corrected to the site elevation). In our case the two following conversions may be
needed:
8. degrees Kelvin = [5/9 (degrees Fahrenheit - 32)] + 273
9. millimeters of mercury = 25.4(inches of H2O/13.6)
Inserting the numbers from the calibration worksheet test number one we get:
10. Qa = 1/.92408[Sqrt((3.8)(293/749.3))- (-0.00383)]
11. Qa = 1.0821573[Sqrt((3.8)(.3910316)) + 0.00383]
12. Qa = 1.0821573[Sqrt(1.48592) + 0.00383]
13. Qa = 1.0821573[1.2189831 + 0.00383]
14. Qa = 1.0821573[1.2228131]
15. Qa = 1.323
It is possible that your answers to the above calculations may vary. This is most likely due to different
calculators carrying numbers to different decimal points. This should not be an area of concern as
generally these variations are slight.
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