Frequently Asked Questions:

General Questions

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What does a process refractometer do?

A process refractometer is an instrument for in-line (production line or vessel) measurement of concentration based on the refractive index of a liquid.

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 use the refractometer principle for concentration measurement?
This principle makes it possible to design an instrument with

  • no influence by bubbles in the medium (something that gets all other types of measurement to show too low concentration)
  • no vibrating or moving parts, no thin-walled pipes that burst
  • no radioactivity, i.e. no special permits, no paperwork, no hassle with used radioactive sources.

But why just a K-Patents process refractometer?
K-Patents design makes full use of the advantages of the measurement principle:

  • Completely digital design: No signal drift, no need for regular recalibration, no influence by bubbles or suspended solids.
  • Robust design: Probe tip machined out of one piece of solid steel, light source a light emitting diode (LED)
  • Wide variety of uses: there are refractometer models for food industry, heavy duty industry, corrosive industry and even for ultra-pure fine non-metallic processes.

Whether you want to measure orange juice, black liquor in a pulp process or the process of diluting HCl, K-Patents has a refractometer to suit your needs.

What is my economical gain if I start to use a K-Patents Process Refractometer in my process?
If you get a continuous measurement (or control) of the concentration in your process, how would you estimate your economical gain?

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What exactly can a K-Patents refractometer measure?

Is it correct that a K-Patents Process Refractometer can measure any materials that can be measured by an Abbe refractometer?
Yes, this is correct. Although the K-Patents instrument is digital, the principle of measurement is the same.

Can the accuracy of measurement of a process refractometer be predicted?
For dissolved solids we can as a rule promise ±0.1 % 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, 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. 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.

No lower temperature is mentioned in the specs, can we measure at 0ºC?
We may even measure at sub-zero temperature, but the Indicating Transmitter has to be placed in a warmer environment because of the Liquid Crystal Display (LCD) operating range 0-50ºC. In low temperatures there may also be problems with condensation in and ice outside of the probe.

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Calibration of a 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?
Any unit is fine as long as it is a (liquid) concentration unit.

Why is the K-Patents Process Refractometer factory calibration made in room temperature with R.I. standard liquids? Wouldn't actual process solutions at actual process temperature be better?The advantage with the standard liquid calibration is that it is easily reproduced anywhere later on.

...but then you may have to make a field calibration after start-up?
Yes, but when we make a field calibration we are sure that we have the actual process conditions, and also that we calibrate according to the customer's normal concentration determination method, e.g. use laboratory method as a reference.

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Durability of the process refractometer materials

Please note that it is the buyer’s responsibility to select the product material and to ensure that the material is satisfactory for the intended service conditions. It is the manufacturer’s responsibility to ensure that a material conforms to the specification.

What is the minimum temperature of sensor and electronics?
The components are specified to -40ºC except the Liquid Crystal Display (LCD) in the indicating transmitter that is limited to operating temperature 0ºC and storage temperature -20ºC.

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. It is the manufacturer’s responsibility to ensure that a material conforms to the specification.

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Installing and using a process refractometer

It is very important to try to arrange a good velocity of the process liquid at the prism. We recommend 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?
Do you think you could use a smaller diameter process line to achieve higher velocity?

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?
It is probably possible to install an automatic prism wash for your refractometer to clean the prism when the velocity isn't high enough.

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

What can we do, when the output signal doesn't agree with our laboratory values?
Fill in a Calibration Data Report (see the last page of your Process Refractometer Manual), e.g. 6 to 10 points would be OK. Then send your report to K-Patents or your local representative and they will calculate your new Calibration Constants for you.

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