Requirements for refractive index measurements

Modern refractive index measuring systems meet specific requirements that enable long-term and care-free real-time substance identification and determination of critical factors, such as the concentration and purity of solutions, over the whole production process. They fulfill the five most important requirements for process control and monitoring, as well as latest maintenance and hygienic demands.

The refractive index of a transparent optical medium, also called the index of refraction, is the factor by which the phase velocity is decreased, in relation to the velocity of light in vacuum, when the optical medium has interacted with the electromagnetic radiation. For transparent materials, the refractive index is a dimensionless quantity higher than 1.

The refractive index is measured for various reasons. It is a relevant parameter when you want information on materials’ clarity, for example glasses. In addition, the purity of chemicals and pharmaceutical ingredients can be determined.

In fluids such as drinks or foods, the refractive index is a measure of dissolved or submicronic particles. The Brix scale relates the refractive index to sugar concentration. And that’s how the sugar concentration can be controlled and monitored throughout the process, by continuously measuring the refractive index.

Common industrial applications include fruit and vegetable juices, concentrates and purees, jams, syrups, compotes and other pasty fluids as well as micro-emulsions, biotechnical liquids and pharmacological materials.

There are several methods for measuring the refractive index, for example:

• interferometry,
• the deviation method,
• the Brewster Angle method.

However, for process refractometers the critical angle method is most commonly used. Here, the sample is in contact with the face of the refractometer prism which has a known refractive index. The light source illuminates the prism–sample interface from within and the output beam is projected onto the photo receiver array inside the refractometer. A sharp demarcation between a bright and feebly illuminated portion of the photo receiver array is used to determine the refractive index and calculate the concentration.

Inline refractometers can be used for continuous, extremely accurate, real-time substance identification and determination of critical factors such as the concentration and purity of solutions. Continuous monitoring enables uniform product quality while reducing out-of-spec production. They must be rugged and reliable enough to operate over extremes of temperature, pressure and vibration. In addition, many applications call for instrumentation that can operate reliably in harsh chemical environments. Keeping this in mind, users want modern inline refractometers to meet the following requirements.

They should be maintenance-free.

A soldered optics, in contrast to fluid seals or gaskets, is used to seal the sensor head from the measured fluid. No checking and replacing of gaskets and inevitably readjusting the refractometer in regular intervals. No possible leaks which could allow process media to enter and damage the device. Soldered optics dramatically extends the instrument’s service life, increases reliability and safety, and reduces costs.

They should never require adjustment.

The ideal sensor is once adjusted in the factory, and then operates with the stored adjustment values for its entire lifetime.

They should be highly accurate.

The perfect inline refractometer delivers refractive index and concentration results comparable to those of laboratory refractometers and efficiently handles weight concentrations from 0 to 100.

They should be hygienic and CIP/SIP ready.

The sensor has to comply to the current hygienic guidelines like EHEDG and should be suitable for hygienic applications, such as for measuring pharmaceuticals, milk, sugar solutions, syrups, fruit juices, foods and beverages containing pulp and other pasty liquids.

Additionally the sensor should be suitable for cleaning in place and sterilization in place at temperatures up to 145 °C, and should be back to work within specifications only a few minutes after cleaning.

They should be intuitive.

No special training should be required to operate the inline refractometer. The unit itself should provide user-friendly diagnostics based on standard status categories according to the NAMUR NE107 standard, and it should be easily adapted to support any number of changing communications protocols and display types.

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