S Q I
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Appendix D – SQI (Spectral Quality Index)
Precondition for the calculation of the SQI is a corresponding calculation, see attached
Manual ISA and Process
Spectrometer Commissioning – Maintenance –Service.
In Situ spectrometer are used for decisions in the fields of food processing, chemical process cycles, drinking
water monitoring, as well as in the field of wastewater treatment plant control and flow control.
Functional principle:
The measurement method of in situ spectrometer, like the ISA, is based on absorbance
spectra. Multiple calibrations can be created with these spectra and so different chemical ingredients can be
measured online by calculation. No chemicals are needed for this measurement method – it is not predicated
on any laboratory test or an existing DIN standard.
Based on the time behaviour of the spectral data in combination with associated analytical laboratory tests,
an algorithm is determined with a chemometrical process, to calculate the different parameters. This
chemometrical process is a statistical method which can develop specific prediction algorithms for the par-
ticular parameters.
A statistical method specially developed for the ISA allows the progressive adaptation of the calibrations,
quality monitoring in the calculation of the algorithm
has been given special consideration. Thus an im-
provement of the algorithm by statistical parameters is already possible during the model development.
The aim is to expand a general algorithm by
on site specific
data and so to generate
user-specific
algorithms.
Every determination which is not DIN standard brings the risk of incorrect determinations. These failures can-
not be eliminated completely by using statistically developed algorithms. When there are temporary situa-
tions which are not included in the statistical data set, the measurement reliability cannot be furthermore
approved.
For this reason
online quality detection
and thus
quality documentation
of these calculations are essential
for the use of spectral sensors especially.
For this purpose the Spectral Quality Index (SQI) is determined and saved for each calculated parameter by
the ISA Spectrometer. The scaling of the SQI is as follows – under a value of 3.5 a measurement reliability of
95% can be assumed. Up to a value of 4.0 a certainty of 90 % is given. However, when there is a permanent SQI
above 4 determined over a longer time, it must be assumed that the spectral data set present in the range of
the used wavelengths is no longer sufficiently statistically proven.
In this case a warning is issued, it is not recommended to use the calculated results for the process control.
The system can be shifted to “emergency control signals” automatically. This process is similar to the failure
of the lambda probe signal in the control of internal combustion engines.
A deviation of the SQI to values higher than 4 can also be caused by defects in the measurement system or by
contamination or blocking of the optical measuring path.