K-Patents PR-03 - Manual (Page 8)

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2 PR-03 instruction manual
1.2 Principle of measurement
The K-Patents inline refractometer sensor determines the refractive index (R.I.) of the process solution by
measuring the critical angle of refraction. Light from a light source ((L) in Figure 1.2) in the sensor is
directed to this interface. Two of the prism surfaces (M) are total-reflecting mirrors bending the light rays
that thus meet the interface at different angles.
L
P
M M
S
A C B
Figure 1.2 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 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 borderline (C) between the areas shows the value of the critical angle and thus
of the refractive index (R.I.) of the process solution.
The R.I. changes with the process solution temperature and concentration. In higher temperatures the R.I. is
smaller than in room temperature (standard R.I. 25
◦ C). When the concentration changes, the R.I. normally
increases when the concentration increases. From this follows that the optical image changes with the
process solution concentration as shown in Figure 1.3. The color of the solution, gas bubbles or undissolved
particles do not affect the result.
B B
C
C
A
A
Low concentration High concentration
Figure 1.3 Optical images
The optical image thus achieved is converted to an electric signal by a digitizer inside the sensor. This
electric signal is then sent via an interconnecting cable to the Indicating transmitter’s microprocessor for
further processing, displaying and transmitting.