K-Patents PR-33-AC - Manual (Page 49)

10 Principle of measurement 43
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10 Principle of measurement
The K-Patents inline refractometer determines the refractive index n
D
of the process
solution. It measures the critical angle of refraction using a yellow LED light source
with the same wavelength (589 nm) as the sodium D line (hence n
D
). Light from the
light source (L) in Figure 10.1 is directed to the interface between the prism (P) and the
process medium (S). Two of the prism surfaces (M) act as mirrors bending the light rays
so that they meet the interface at different angles.
L
P
M M
S
A C B
Figure 10.1 Refractometer principle
The reflected rays of light form an image (ACB), where (C) is the position of the critical
angle ray. The rays at (A) are totally internally reflected at the process interface, the rays
at (B) are partially reflected and partially refracted into the process solution. In this way
the optical image is divided into a light area (A) and a dark area (B). The position of the
shadow edge (C) indicates the value of the critical angle. The refractive index n
D
can
then be determined from this position.
The refractive index n
D
changes with the process solution concentration and temper-
ature. For most solutions the refractive index increases when the concentration in-
creases. At higher temperatures the refractive index is smaller than at lower tempera-
tures. From this follows that the optical image changes with the process solution con-
centration as shown in Figure 10.2. The color of the solution, gas bubbles or undis-
solved particles do not affect the position of the shadow edge (C).