Food and beverage, chemical, papermaking – all of these industries share something in common. In order to ensure safety and a high-quality end product, measuring the concentration of key substances in their liquid processes is extremely important.  

A Vaisala process refractometer gives you high-quality optical measurements of your liquid processes. With the elbow flow cell design, the flow of the fluid usually keeps the prism clean – nine times out of 10, nothing extra is needed. But in some demanding applications, a prism washing system is necessary, which we’ll look at more closely in this article. 

How do I know if my application needs a prism washing system?

The chances are, it doesn’t. Vaisala process refractometers use optical measurements to measure the concentration of various solutions such as acids, alcohols, solvents, hydrocarbons, and sucrose. This can be done in almost any liquid  – basically, if you can pump it, a Vaisala refractometer can measure it. 

But in order for the measurement to be accurate, the prism in the refractometer needs to be free of residue. This is usually not an issue, but in some applications, the solution being measured can cause build-up on the prism. This can happen for example in the pulp and paper industry with black liquor measurement, or in the food and beverage industry with blanching of starchy foods like potatoes. As the build-up on the prism grows, it can cause the measurement values to drift, leading to inaccurate measurements. In the worst cases, measurement becomes impossible until the prism is cleaned. 

What kind of prism washing systems are available?

This is where prism washing systems come into play. Vaisala offers different kinds of systems depending on your application and need. The most common is steam washing. This is exactly what it sounds like – at regular intervals (for example every 20 minutes) the prism is blasted with steam for a few seconds. The steam helps keep the prism crystal clear even in the most challenging environments. 

If your application is sensitive to heat and steam can’t be used – which can happen in the chemicals, mining, or food and beverage industries for example – there is also the option of a high-pressure water system.   Much like the first option, the prism is cleaned with short blasts of water at regular intervals. The wash can also be made with a suitable solvent depending on the process fluid.

Is a prism washing system expensive or difficult to install?

A prism-washing system is cost-effective and can be added to Vaisala refractometers as a retrofit. The initial investment for such a system is low, as is the operating cost as only small amounts of steam, water, or solvent are needed for washing. 

While choosing the right system for your application may seem complicated, in practice it isn’t. Vaisala has decades of experience in working with liquid process measurements across a wide range of industries and our prism washing solutions are field-proven and easy to install. Our experts will help you to determine if you need a prism washing system in the first place – and if you do, will advise you on the best system for your application.

In this way, in the uncommon case where your application is too challenging to use a Vaisala refractometer as is, there is an off-the-shelf solution available to make sure your liquid process measurement is as accurate, stable and repeatable as possible. The end result is that your process – and your business – runs smoothly. 
 

Liquid chemical processes that benefit from refractive index measurements are varied from a wide range of salts within the chemical industry, as in both water- and petroleum-based chemicals. Across industries, RI is relevant for water-based chemicals like glycols, resins, and polymers; petroleum chemicals such as plastics and solvents; And in petroleum industrial additives, lubricants, waxes and the like. 

In this blog, we answer the questions we did not have time to address during our webinar "Liquid Measurements in demanding chemical applications: Density Vs. Concentration". If you haven't yet watched the webinar, you can view the recording at any time.  

Question: In the future will it be possible to measure very small amounts of water (ppm level) in organic solvents? This would be very useful for organic chemistry applications such as distillation or evaporation of organic solvents to remove as much water as possible.

Answer: Yes. The Vaisala refractometer can measure small amounts of water in solvents if they are dissolved in the solution. Currently, the sensing limit is around 100 ppm, but it depends on process parameters like temperature the medium itself.

Question: We blend two products on the fly via static mixers and pressure. Can an inline refractometer help us determine the percentage of each material present during the entire blending process? We currently take samples at the beginning, middle, and end of the blending process. We currently use Gas Chromatography to determine the concentration of each material at these sampling points, but that can take 1-2 hours per sample. 

Answer: Yes. We can calculate concentration ratio if we know the concentration of feed products. Please send us more information on your specific chemicals and we can verify suitability in detail.

Question: Is there a correlation between concentration and pH on spent Sulfuric acid?

Answer: Yes. Please contact us to speak to an application engineer and learn more. 

Question: Can this sensor send a 4-20 mA signal to a device?

Answer: Vaisala Polaris refractometers have built-in 4 … 20 mA and Modbus RTU outputs. 

Question: I see a lot of flow-through installations. Can Vaisala’s refractometers be installed in a submersion application into a tank or other vessel?

Answer: Yes. You can install the Vaisala Polaris inline refractometer directly to a tank or vessel. We have standard sensor flange connections. See more on the PR53 GP product page for details. 

Question: I often get questions about the compared accuracies between Coriolis and Refractometers. Can you address this? 

Answer: Yes, we have a comprehensive table in our eBook that compares key features of common liquid measurement methods: Coriolis, microwave, ultrasonic, and nuclear. Below is part of the table. To view the entire table, including comparisons of installation options, maximum operating temperature, operating pressure, typical accuracy, long-term stability, verification and more, please download the PDF. 

Question: Do Refractive Index measurements work for dark fluids?

Answer: Yes. Because the refractometer measures reflected light the process medium can be completely dark, clear or anything in between. This is one of the unique properties of inline refractometers and refractive index measurements, making them flexible in liquid chemical measurement applications. 

Learn more about Vaisala refractometers in liquid chemical processes.