In-line process refractometers

for liquid concentration measurement

K-Patents measurement products

Vaisala K-PATENTS® Process Refractometer

Vaisala K-PATENTS Process Refractometers are used for in-line liquid measurements for the purpose of process control and industrial automation. We invest heavily in research and development, maintaining a worldwide reputation for innovation, reliability and high quality. 

Customer reviews:
“The measurement is reliable and stable despite of heavy membrane vibration.”
– Whey protein concentrate plant, New Zealand.

“Our annual waste at filling line was million liters. Replacing volumetric flow and turbidity meters with in-line refractometers, helped us to cut the changeover time to a legal minimum of 5 min, reducing waste by two thirds.”
– Dairy, Europe.

“Real-time process refractometer is a great alternative to titrators saving time and materials”. 
– Pharmaceutical plant, Austria               

Industrial process refractometer in pharmaceutical manufacturing

Vaisala K-PATENTS Process Refractometers are commonly used to determine the liquid concentration or density. This measurement has been an important element in the processing industries for more than a century.

The performance of all types of refining, manufacturing and quality control operations can be dramatically improved with our digital refractometer technology.

For instance, the in-line concentration measurement can be used as a real-time predictive tool for the final concentration. A quick and accurate response is needed to optimize production. Cost reduction is possible by reducing the variation of mean average of the product concentration. The cost saving is related to the value of the component being measured.

Watch the video

Refractive Index measurement principle, critical angle measurement

Completely digital measurement principle

The measurement principle is critical angle measurement. There are three main components in the refractometer; a light source, a prism, and an image detector.  

The light source sends light rays in different angles to the prism and process interface. Rays with a steep angle are partly reflected to the image detector and partly refracted to the process. Rays with a low angle are totally reflected to the detector. The angle from which the total reflection starts is called the critical angle. 

The CCD camera detects a bright field and a dark field corresponding to partly reflected light and totally reflected light. The position of the borderline between the bright and the dark area correlates with the critical angle which is a function of the refractive index and therefore correlates with the concentration of the solution.

A built-in temperature sensor measures the temperature T on the interface of the process liquid. The sensor converts the refractive index nD and temperature T into concentration units. Vaisala K-PATENTS refractometer can indicate different scales, for example, Brix, liquid density or concentration by weight. The diagnostics program ensures that the measurement is reliable.

Learn more about the Refractive Index Measurement Principle (PDF).

Installed process refractometer

Three generations of refractometers

Currently, we have 3 refractometer technology platforms in production. Most recent and advanced is the PR-43 platform where up to 4 refractometers can be connected to one user interface via Ethernet or 4-20 mA connection. 

The range of refractometer applications is wide as most liquid processes require continuous monitoring of concentration to help maintain quality and reduce costs. Vaisala serves the target industries according to their different nature and technology requirements to ensure a high standard of products and services.

Click on the industry of your interest to learn about the refractometer models and applications.

Chemical >>
Food and beverage >>
Oil refining and petrochemical >>
Pharmaceutical and biotechnology >>
Pulp and paper >>
Semiconductor >>
Sugar >>
Sweeteners >>

MI with user

Refractometer system

Vaisala K-PATENTS Refractometer stand-alone models are smart devices that measure and indicate concentration units and diagnostic information and provide 4-20 mA or Ethernet output signal to the PLC.

All refractometers can also be connected with a physical user interface or indicating transmitter. The user interface options range from a rugged, multichannel, industrial computer, to an indicating transmitter and a compact, lightweight interface, allowing the user to select their preferred way to access and use the refractometer measurement and diagnostics data.

The refractometer has a built-in web server with a homepage where the instrument can be configured, monitored, verified, and diagnosed via an Ethernet connection. 

Because of the identical calibration for refractive index measurement, each refractometer can be freely interchanged without optical recalibration.

Key benefits

Full measurement range

The refractometer works over the full measurement range of Refractive Index (nD) 1.3200 – 1.5300, which corresponds to 0-100 Brix. Optional measurement ranges available.


Built-in CORE-Optics element eliminates the risk of leakages and drifts.

Completely digital system

Color, bubbles, particles, flow, temperature or pressure changes do not affect operation or accuracy.

Withstands low and high process temperatures

Compact model -40°C...130°C (-40°F...266°F), probe model: 40°C...150°C (-40°F...302°F).

Temperature compensated measurement output

The refractometer has a built-in Pt1000 temperature sensor for fast process temperature measurement and automatic temperature compensation.

Compatible with CIP and SIP cleaning

The refractometer withstands CIP (clean-in-place) and SIP (sterilization-in-place) processes and cleaning and rinsing of facilities.

Highly accurate

The accuracy of the in-line measurement is comparable to the accuracy of the production laboratory.

Read more from our accuracy statement here 

Easy on-site instrument verification

The refractometers are delivered calibrated with NIST traceable standard Refractive Index liquids. This allows easy instrument verification within user’s own quality assurance system using standard liquids and a built-in verification procedure.

Easy installation

Installation in the main processing line or vessel directly or via variety of flow cells; no by-pass arrangements are required. Also in vessels with mixers or scrapers is possible. Engineered custom solutions available.


The refractometer requires no regular maintenance as there are no wear and tear components that would have to be changed regularly. This means a low cost of ownership.

Product approvals and certifications

All instruments are developed considering the industry specific regulations and safety aspects. Product approvals and certificates allow manufacturing and supply of products in compliance with ATEX, CSA, FM, IEC, CRN, KEMA, DEKRA AND SANITARY 3-A, EHEDG.


Fast and efficient viral vaccines purification by sucrose density gradient

WEBINAR: Fast and efficient viral vaccines purification by sucrose density gradient

K-Patents Webenar - The Benefits of Refractive Index (RI) in Development and Production of Active Pharmaceutical Ingredients (APIs)

WEBINAR: The Benefits of Refractive Index (RI) in Development and Production of Active Pharmaceutical Ingredients (APIs)

Blogs and customer cases

K-Patents measurement products

BLOG: A deep-dive into the technology of liquid concentration measurements with refractive index

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To ensure continuous support before and after purchase of our products, we offer local application consultation, training, maintenance and support expertise locally via our authorized sales distributors as well as by the Headquarters and local service centers.