I S A
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13.2 ISA Formula Examples
Here are a few examples of how to calculate spectral data in virtual sensors.
In this formula the absorbance values at frequencies 284nm, 332nm and 628nm are used to generate a NO3
value. A lower limit is also implemented. The exact coefficients are derived from ISA Soft software calcula-
tions.
ISA NO
3
example
# ISA NO3 (example)
Value = -0.06347;
Value += ISA(284) * 28.547863;
Value += ISA(332) * - 51.927711;
Value += ISA(628) * 30.110743;
if (Value < 0) Value = 0;
Value;
The formula calculates the Spectral Absorbance Coefficient (SAC) at 254 nm.
ISA SAK254
# ISA SAC254
SAC = ISA(254)*(1000 / ISA.Pathlength);
if (SAC < 0) SAC = 0;
SAC;
ISA SAK254 with turbidity compensation
# ISA SAC254 with turbidity compensation
offset = 0;
for(i=600; i<700; i+=2) = ISA(i);
offset /= 50;
# calculates the average of the absorbance spectrum drift
# in the range 600nm to 700nm
SAC = (ISA(254)-offset)*( 1000.0 / ISA.Pathlength );
if ( SAC < 0 ) SAC = 0;
SAC;
This formula makes spectral data retrievable for a Modbus master device.
ISA Modbus export
# ISA Spectrum Modbus-Export
for (i=0; i<=255; i++) MODR(i) = ISA0(i);
