Process Refractometer: Frequently asked questions

Former K-Patents is now part of Vaisala

The products are named, e.g. Vaisala K-PATENTS® Process Refractometer PR-43-G. What does K-PATENTS mean?
K-Patents Oy, an industry leader and supplier of K-PATENTS Process Refractometers, was acquired by Vaisala Oyj in 2018. Following the acquisition and merger, K-Patents is part of Vaisala as Industrial liquid concentration measurements business area. The renowned K-Patents name remains in the products naming, yet also reflecting Vaisala. 

Where can I buy Vaisala K-PATENTS products?
You can continue ordering Vaisala K-PATENTS process refractometer products and services via your current local sales distributor in your country, as before.

Where can I get support for my former K-Patents products?
Our Customer Service will be happy to answer your questions via Help Desk or you can contact your local sales distributors.

What does a process refractometer do?

Process refractometer measures concentration in-line. It is used in industrial scale manufacturing for process control and product quality. The measurement itself is based on refractive index (or RI) of a liquid, and is very accurate and reliable.

What is a "refractive index"?
The refractive index or RI is a physical property of a liquid, which is proportional to the concentration. The RI can be described as the bending power of light at the surface of a liquid.
Read more: Refractive Index as a Concentration Measurement Principle (pdf).

Why the refractometer is the best tool for liquid concentration measurement?
There are clear benefits for using process refractometers such as:

  • bubbles, impurities, particles or color of the liquid do not interfere with the measurement
  • the measurement itself is very accurate and reliable
  • the process refractometer is a robust instrument designed for harsh conditions


What makes Vaisala K-PATENTS process refractometer unique?
The design of Vaisala K-PATENTS process refractometer makes full use of the advantages of the measurement principle, namely:

  • completely digital system: no signal drift, no need for regular recalibration, no influence by bubbles or suspended solids.
  • robust and sanitary design to meet the needs of various industries: food industry, heavy duty industry, harsh and corrosive industries, and ultra-pure fine non-metallic industries.


What is my economical gain if I start to use a Vaisala K-PATENTS process refractometer in my process?
The use of Vaisala K-PATENTS process refractometer brings substantial benefits such as cost saving through efficient use of raw material, sustainable use of resources, reduction of energy consumption, eliminated product recalls, etc.

What exactly can Vaisala K-PATENTS refractometer measure?

Does Vaisala refractometer measure same materials that can be measured by hand-held and lab refractometer?
Yes, this is correct. Although Vaisala K-PATENTS refractometer instrument is digital, the measurement principle is the same.

Can the accuracy of measurement of a process refractometer be predicted?
Accuracy is application specific. For dissolved solids, the measurement accuracy is Refractive index nD ±0.0002 corresponding typically to ±0.1 Brix or % by weight. For other solutions the accuracy depends on the Refractive Index.

Can a process refractometer measure wood pulp fibers in water ("consistency")?
No, it cannot, because that is a suspension and not a solution and thus there's no liquid concentration to measure.

Can a process refractometer measure kaolin ("china clay") in water?
No, it cannot, because kaolin clay is chemically inert and doesn't dissolve in water, thus there's no concentration, just particles in water.

Can a process refractometer measure Titanium Dioxide (TiO2) in water?
No, it cannot. Distinct from dyestuffs which operate on a molecular level, pigments (TiO2 among them) tend to be particulate and insoluble and thus there's no liquid concentration for the refractometer to measure.

Can the refractometer measure at lower temperature, e.g. at 0ºC?
Yes, it can. The refractometer is able to measure at sub-zero temperatures. Local transmitters and user interfaces have to be placed in a warmer environment.

Calibration of Vaisala K-PATENTS Refractometer

Can the refractometer output be calibrated in density units (specific gravity)?
Yes, it can.

Can the refractometer output be calibrated in Baumé? Balling? Brix? ºBx? ºPlato? ºOeschle? Gram per liter?
Yes, any unit is fine as long as it is a (liquid) concentration unit.

Why is Vaisala K-PATENTS refractometer factory calibration made in room temperature with R.I. standard liquids? Wouldn't actual process liquid at actual process temperature be better?
Calibration using standard Refractive Index liquids is easily reproducible elsewhere at any time. 

Is a field calibration required after start-up?
In some cases it might be needed. Field calibration of the refractometer is made using customer’s actual process conditions and with the reference to customer’s preferred concentration determination method.

Durability of the process refractometer materials

What is the lowest temperature the refractometer can operate in?
The refractometer can operate at ambient temperatures of min. -40°C (-40°F), max. 45°C (113°F);

Can you guarantee that your material stands our process conditions?
It is the buyer’s responsibility to select the material and to ensure that the material is satisfactory for the intended service conditions. Vaisala, as a manufacturer, ensures conformity of the material to the specification.

Installing and using the process refractometer

It is very important to try to arrange a good velocity of the process liquid at the prism. The recommended flow velocity is at least 1.5 m/s (5 ft/s) to keep the prism clean.

The velocity in my process is 1.5 m/s but the flow is not continuous, i.e. it stops at some intervals. Does this cause problems with the refractometer?
Yes, but the problems can be avoided by mounting the refractometer in a recirculation loop with continuous flow.

What if the flow is only 0.6 m/s?
In some cases, higher velocity can be achieved using a smaller diameter process line.

What about the pressure drop?
For a venturi type restriction the remaining pressure drop is small. This kind of restrictions are often used for magnetic flow meters.

Anything else I could do instead of using a smaller process line?
If flow velocity is not sufficient or the process liquid is sticky, we recommend to install a wash nozzle for cleaning the prism at pre-set intervals with high pressure water or steam.

When is a regular automatic prism wash recommended?
We recommend automatic prism wash if the process velocity is low or the process medium is so sticky that you get coating problems despite the good velocity.

Our refractometer output differs from our laboratory values. What should we do?
You can get new calibration constants (see instructions for field calibration in the product manual) or contact Vaisala or your local authorized Vaisala distributor.

Liquid concentration measurement

Vaisala offers a wide range of products tailored to liquid measuring needs of different industries. Learn more about the refractometer technology and its applications, and visit the library of product related assets.