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How to Use a Refractometer

Grainger Editorial Staff

A refractometer is a tool that can determine the concentration of a particular substance in a liquid solution. It uses the principle of refraction, which describes how light bends as it crosses the boundary between one medium and another.

A classic way to illustrate refraction is to look at how a pencil that's half-submerged in a glass of water appears bent or discontinuous at the boundary line between one medium (air) and the other (water), because of how the light is refracted.

By measuring how much refraction a solution causes, you can learn about its density, which in turn can give you valuable information about how much of a substance is dissolved in solution. 

The most common refractometers are handheld, analog instruments. The sample is placed on a cover plate and a prism and then held to the light to view the scale inside the meter. The readout scale is usually calibrated for determining the concentration of one particular substance.

Here are some common types of refractometer readouts:

  • Salinity refractometers have a readout that gives the percentage of salt.
  • Brix refractometers have a readout that gives the percentage of sucrose, and are used in the food and beverage industry for quality control.
  • Coolant refractometers have a readout that gives the freezing point and efficacy of ethylene glycol and propylene glycol, coolants used in many air conditioning systems. Coolant refractometers often have a readout that can also be used with car battery fluid to indicate battery condition.
  • Clinical refractometers have a readout that gives serum albumen and urine specific gravity, which can be used to test for urine sample tampering.

Using a Handheld Analog Refractometer

To take a measurement with a handheld analog refractometer, follow these steps and read the manufacturer's instructions:

  1. Calibrate the refractometer with a standard solution before use. Since the reading will be affected by temperature changes, it's best to calibrate at the temperature of the test environment. If this is not possible, correction charts may be used. Some refractometers have automatic temperature correction (ATC), a feature that allows the instrument to automatically correct for temperature differences.
  2. Place a small amount of liquid (usually 2–5 drops) on the prism, and secure the cover plate. This will evenly distribute the liquid on the prism.
  3. Point the prism end of the refractometer toward a light source and focus the eyepiece until the scale is clearly visible.
  4. Read the scale value at the point where the dark and light portions meet.

The refractometer readout shown above is measuring 30 degrees Brix. 


Other Types of Refractometers

Digital refractometers require a drop of the tested solution to be placed in a well. That well is illuminated by a light source, usually an LED light, and the meter interprets the light transmission into refractive index or whatever unit of measure the instrument is programmed to read.

Abbe refractometers, also called laboratory refractometers, are larger bench-top instruments that provide highly precise measurements of refractive index in lab settings.

Commonly Asked Questions

Q: I need to test the concentration of lubricating oil but I have a Brix refractometer. Can I use it?

A: Yes, you can use it if the refractive range is similar. In this case, you need to prepare known samples of the lubricating oil and determine the corresponding Brix values. From this data, a chart can be created to convert from the Brix value to the percent oil value.

Q: How do I maintain a refractometer?

A: Generally, refractometers require very little maintenance. When the measurement is complete, wipe the prism with a soft lens tissue. When the instrument is not in use, keep the cover closed to avoid scratching the prism.

The information contained in this article is intended for general information purposes only and is based on information available as of the initial date of publication. No representation is made that the information or references are complete or remain current. This article is not a substitute for review of current applicable government regulations, industry standards, or other standards specific to your business and/or activities and should not be construed as legal advice or opinion. Readers with specific questions should refer to the applicable standards or consult with an attorney.

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