LumaSense Technologies INNOVA 1412i - Manual (Page 165)

Chapter 12
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BE6025-15 1412i Photoacoustic Gas Monitor LumaSense Technologies A/S
Page 165 of 202
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.12.14 , Fig.12.15a and Fig.12.15b ).
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.12.15b ). 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.
Gas Concentrations – Two-point Calibration
During a two-point span calibration, the total signal in the cell (V
low
)
is measured first with a low concentration (C
low
) of span gas, and
then the total signal in the cell (V
high
) is measured with a high
concentration (C
high
) of span gas. This enables the Monitor to
compensate for any un-linearity in the relationship between the
signal measured in the cell and the concentration of the gas in the
cell (this is illustrated, for a gas A1, in Fig.12.15a ).
Here are some guidelines about the concentration of gas you should
use during a two-point span calibration:
· The Low Concentration should be 10 to 30% of the highest
concentration you expect to measure.