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Calibration and Auditing Procedures
Calibration Calculations
Gather together all the calibration data, including the transfer standard calibration information and the MiniVol sampler calibration data sheet. The following calibration calculation procedures are recommended.
Note: These calculations should be done at the time of the calibration, rather than later. This approach will allow additional calibration points to be taken if questions arise about the data that has already been obtained.
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Calculate Qact for each calibration point as determined by the transfer
standard calibration equation (Eq.A from MiniFlo calibration
certificate).
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Calculate and record the standard corrections (Q@std)
for each calibration point as:
where:
Q@std = standard correction flow rate, L/min
Qact = flow rate at ambient conditions (transfer standard),
L/min
Tact = ambient temperature, °K
Pact = ambient barometric pressure, mm Hg
Tstd = standard temperature, 298 °K
Pstd = standard pressure, 760 mm Hg.
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On a sheet of graph paper or computer spreadsheet plot the calculated
standard correction flow rates, Q@std (y-axis) versus
the corresponding rotameter indicated flow rate, Qind
(x-axis). Using a programmable calculator or a spreadsheet and the
following model, calculate the linear regression slope (mvol),
intercept (bvol), and correlation coefficient (r) to
obtain the MiniVol™ sampler flow rate calibration relationship.
For the regression model y=mx+b
So that the model is given by:
A six-point calibration should yield a regression equation with a
correlation coefficient of r > 0.990, with no point deviating more
than 2% from the value predicted by the regression equation.
Note: For actual sample periods, the sampler's average actual
operational flow rate Qact is calculated from the calibration slope
and intercept using
where
Qact = sampler flow rate at ambient conditions, actual
L/min
Qind = rotameter response, L/min
Tact = ambient temperature, °K
Pact = ambient barometric pressure, mm Hg
Tstd = standard temperature, 298 °K
Pstd = standard pressure, 760 mm Hg
mvol = slope of the MiniVol™ flow rate calibration
relationship
bvol = intercept of the MiniVol™ flow rate calibration
relationship.
Tact and Pact readings may be measured
on-site or obtained from a nearby U.S. National Weather Service or
airport weather station. Barometric pressure readings obtained from
remote stations must be at station pressure (not corrected to sea
level), and they may have to be corrected for differences between
the elevation of the monitoring site and that of the remote station.
If ambient temperature and pressure readings are not available, a
seasonal average temperature and barometric pressure can also be used.
Care must be taken, however, that the actual conditions at the site
can be reasonably represented by such averages. It is therefore recommended
that seasonal values represent actual values within 20 EC and 40 mm
Hg.
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