LumaSense Technologies 3434i SF6 Leak Detector - Manual (Page 162)

Chapter 12
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BE6028-14 3434i SF6 Leak Detector LumaSense Technologies A/S
Page 162 of 191
Fig.12.14b Graph showing a non- linear measurement curve
The question about whether to perform a single- or a two-point span
calibration is difficult to answer, as it is dependent on how linear the
relationship is between a gas’s concentration and the signal it
produces in the cell (see Fig.12.13 , Fig.12.14a and Fig.12.14b ).
However, we can give you some guidelines on whether you have to
perform a single- or a two-point span-calibration.
· Single-point calibration should be used when you wish to measure
in the linear range. We define the linear range as the dynamic
concentration range in which you can measure with an error of
5% of the real concentration (this is illustrated in Fig.12.14b ). For
the most common gases, the linear range is from its detection
limit and up to 10 000 times its detection limit.
· Two-point calibration is recommended to be used when you wish
to measure in the non-linear range and when you wish to
measure over a dynamic range of 100 000.
Gas Concentrations – Single-point Calibration
If a single-point span-calibration is performed then the span gas
should have a concentration which is at least 100 times its detection
limit and preferable not more than the highest concentration you
expect to measure.
For certain gases it can be inexpedient to use a concentration of at
least 100 times its detection limit, e.g. if you wish to measure a
poisonous gas with a high detection limit, it would be natural to use
a lower concentration. However, this means that the noise and
vibration will have a larger influence on the calibration. This must,
therefore, be taken into account when considering using a lower
concentration.