Ion Science SF6 P1:p - Manual (Page 31)

SF6 P1:p MANUAL Ion Science Ltd
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Known problems
Calibration in the ppm mode
Commercially available calibration gases for ppm operation are produced using nitrogen or so-called
synthetic air which considerably differs as to its composition in particular with respect to humidity from the
ambient air. The P1:p leak detectors respond besides to the SF
6
share in the mixture also to these
differences so that the results of the calibration may be incorrect. Especially at low concentrations
considerable deviations are unavoidable.
For this reason we recommend to perform the calibration exclusively with a mixture of pure SF
6
in a defined
quantity of natural air. Here it needs to be observed that the mixing chamber used for mixing the gases is
thoroughly purged with ambient air before embarking on the calibration. Otherwise again differences in
humidity could result. Sufficient mixing of the gases in the mixing chamber must be ensured through fans, for
example.
Moreover, it needs to be noted that the gas mixture in the mixing chamber must be considered as not valid
as soon as the composition (especially degree of humidity, background contamination) of the ambient air
changes. For this reason, the gas mixture should not be used over several days. Instead a new mixture with
ambient air should be produced each time it is required. In particular it is not permissible to move the mixing
chamber to a different environment (out into the open, for example).
Calculating the concentration
Concentration [ppm] =
1,000,000 x ( SF
6
volume[cc] / mixing chamber volume [cc])
Example:
Mixing chamber volume = 100 litres = 100,000 cc
SF
6
volume = 5 cc
Concentration = 1,000,000 x (5 / 100,000) = 50 ppm
If no mixing chamber having a suitable volume is available, then the required mixture can also be produced
by using two smaller mixing chambers having a volume of 1 litres each, for example. The calculation is run
according to the equation stated above. For the second mixing process the SF
6
volume needs to be divided
in addition by the volume of the first mixing chamber. Example:
1
st
mixing chamber:
5 cc SF
6
in 1 litre of air = 5000 ppm (calculation, as above)
2
nd
mixing chamber:
1 cc mixture from the first mixing chamber in 1 litre of air = 5 ppm
10 cc mixture from the first mixing chamber in 1 litre of air = 50 ppm.
Alternatively the calibration for the ppm mode can also be performed with the aid of a leak calibrator like the
CAL-CHECK (Part No.: A-21520) (see also 0,
Coupling calibration factors (“Cross Mode Calibration”). Here the
resulting gas concentration is calculated from the leak rate of the leak calibrator and the intake air flow of the
leak detector.