I S A
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14 Parameter Accuracy
Parameters, calculated with ISA spectral data can achieve the accuracy of 5% (typical 5 % - 10 %). In detail the
accuracy can vary by the change of the water matrix. If the water matrix has higher variability, for example
day/night or seasonal changes, this has to be analyzed and a special calibration must be performed. In gen-
eral, calibrations adapted to specific conditions can provide reliable measurements even under difficult con-
ditions. Changes in the water matrix can be detected by other parameters (conductivity, pH value, tempera-
ture etc).
1.
The accuracy of spectral data calculated parameters always is influenced by the quality of calibration.
A higher number of calibration points will have the result of a more accurate calibration!
2.
The range of the reference value pairs
∗
must cover the entire measuring range evenly distributed as possi-
ble. The minimum number of reference value pairs is 25, a smaller number reduces the quality of the cali-
bration and may subsequently lead to erroneous measurements.
3.
The right choice of the analytical method and the care in the procedure are the most important require-
ments for the accuracy of the calculation. The accuracy of the ISA parameter calibration is depending upon
the specific accuracy of the chemical method for the parameter.
4.
After the calibration, the calibration has to be checked and (when appropriate) adjusted over a longer pe-
riod (for example, one week). This increases the stability of the measurement.
5.
Measurements with high accuracy over a longer period are only possible with correspondingly adapted
maintenance, a maintenance schedule is highly recommended here.
In general periodic cleaning and recalibration increases the measurement quality. The interval of the mainte-
nance is determined by the measurement conditions and can last from several weeks to several months.
see
Manual ISA and Process Spectrometer Commissioning – Maintenance – Service
∗
For the calculation and calibration to the desired parameter of a specific application, it is necessary that for each parameter
reference values from chemical laboratory analysis and the corresponding spectral values are provided.
In case of a one-parameter calibration a spectrum is assigned to one reference value, in a multi-parameter calibration there is
more than one reference value assigned. The spectral data of a spectrum plus one or more corresponding measurement val-
ues are called
reference value pair
.
