Refractometry - The technology to measure inline fluid concentration

There are many different methods for determining the concentration of a liquid inline or online. No wonder you are somewhat confused when choosing a good and reliable system. Very often a detour is taken via a density measurement to calculate the concentration from there. After this you will learn that this is not necessarily the best choice. Refractometry, or measuring the refractive index of light in the liquid, offers so much more.

Different liquid concentration measurement techniques - Which one will you choose?

There are a large number of measurement principles based on density (density) to measure fluid concentration. The most commonly used techniques are U-tube, Coriolis, Acoustics (Sound Velocity), differential pressure (d/p cell) and even nuclear radiation. This variation in techniques already indicates that no single method can be used under all conditions. For measuring refractive index, there is only one technique: refractometry. More than that, refractometry can be used to measure density. But why take that detour?

Why is it best to choose refractometry?

Refractive index is the only measurement technique that measures all dissolved substances in the liquid. And that's what it's all about, isn't it? Other density-based techniques assume some correlation between density and the measured quantity, which in turn is related to concentration. Take coriolis flow meters. These measure mass flow rate which is correlated with density which in turn is correlated with concentration. We all know the phenomenon that when you put a pencil in a cup of water, the pencil is seemingly refracted. That angle of refraction depends on the concentration of solute. A refractometer measures the angle of refraction and thus the concentration.

No influence of air bubbles or solid particles

As you can see in the figure, the angle of refraction is not affected at all by solid particles in the water or air bubbles. The concentration of solute is really the only thing that determines the refraction angle. All density-based measurement techniques are disturbed to a greater or lesser degree by air bubbles or particles.

Did you know that air bubbles interfere with a density measurement? 1% air means 1% error in density which corresponds to an error in concentration measurement of 1.5 - 2.5%. Unlike many other measurement techniques, solid particles are not a problem for a refractometer. Particles in a liquid stream can not only interfere with those measurements, but can deposit on vital components or clog small channels. E.g. with acoustic principles or with techniques that rely on microwaves. The latter, by the way, only measure total solids, i.e. dissolved and undissolved particles in the liquid. And that's not what you want.

Read the full blog post and find out why inline refractometry is the only technique that directly measures the concentration of your medium.

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