The refractive index n
D
changes with the process solution concentration and temperature.
For most solutions the refractive index increases when the concentration increases. At
higher temperatures the refractive index is smaller than at lower temperatures. From this
follows that the optical image changes with the process solution concentration as shown in
the figure below. The color of the solution, gas bubbles or undissolved particles do not
aect the position of the shadow edge (C).
B B
C
C
A A
Low concentration High concentration
Figure 142 Optical images
The position of the shadow edge is measured digitally using a CCD element and is converted
to a refractive index value n
D
by a processor inside the instrument. This value is used
together with the measured process temperature to calculate the concentration.
a. Optical image
b. CCD element
c. CCD output
V
Figure 143 Optical image detection
Appendix B – Principle of measurement
259
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