Calibrating concentration measurement - PR-21-S

PR-21 Series User Guide

Document code
IM-EN-PR21S
Revision
C
Language
English
Product
PR-21-S
Document type
User guide

The concentration calibration is organized in six layers.

Figure 1. Concentration calibration layers
1
The information from the CCD element and the Pt-1000 temperature element. The position of the shadow edge, see Figure 3, is described by a number called CCD and scaled 0 … 100 %.
2
Sensor calibration: The actual refractive index nD is calculated from the CCD value. The process temperature is calculated from the Pt-1000 resistance. The sensor output is nD and temperature TEMP in degrees Celsius. The calibrations of all sensors are identical, which makes sensors interchangeable. The calibration of each sensor can be verified using standard refractive index liquids.
3
Chemical curve: The indicating transmitter DTR receives nD and TEMP and calculates the concentration value according to chemical curves. The result is a temperature-compensated calculated concentration value CALC.
4
Field calibration: Adjustment of the calculated concentration value CALC may be required to compensate for some process conditions or to fit the measurement to the laboratory results. The field calibration procedure, see Field calibration, determines the appropriate adjustment to CALC. The adjusted concentration is called CONC. If there is no adjustment, CALC and CONC are equal. Therefore the chemical curve is kept intact as a firm base for the calculation, the adjustment is merely additional terms.
5
Damping: see Configuring output signal damping.
6
Output signal: The range of the 4 … 20 mA signal is defined by its two endpoints on the CONC scale, see Configuring mA outputs.