The Vaisala K‑PATENTS® in‑line refractometer determines the refractive index nD 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 nD). Light from the light source (L) in the following figure 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.
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 nD can then be determined from this position.
The refractive index nD 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 following figure. The color of the solution, gas bubbles or undissolved particles do not affect the position of the shadow edge (C).
The position of the shadow edge is measured digitally using a CCD element and is converted to a refractive index value nD by a processor inside the instrument. This value is used together with the measured process temperature to calculate the concentration.