How to calibrate and read a refractometer?

FJDynamics Digital Brix Refractometer

Refractometer is an instrument used to determine concentration. It uses light to test the concentration of liquids as the light will pass from one medium to another. When light passes through the liquid, it is partly diverted by the dissolved solids in the liquid. The angle of refraction is the angle between the refracted ray and the normal at the point of refraction.

Refractometers can measure the refractive index and optical properties. And the refractive index is one of the most important physical constants of substances. Many types of pure substances have a certain index of refraction. If the substance contains impurities, the refractive index will change. And there will be deviations. More impurities, the greater degree to which the light will deviate. Here we offer a guide on how to use a refractometer, including how to calibrate refractometer and how to read a refractometer.

Generally speaking, refractometers include:

There are many types of refractometers. And all types of refractometers have their own applicable scenarios. Abbe refractometer measures the refractive index of transparent liquids or solids. It can also measure the percent sucrose. Besides, gemstone refractometer mainly measures the refractive index of gemstones. Sugar refractometer determines the solubility of sugar-containing solutions. And it also measures the density of fruit wines. Other non-sugar solubility or refractive indices can also be mainly measured by conversion. It is a must for sugar testing in scientific research and industries like food, beverage, and machinery. Take a coffee refractometer as an example. It can measure the total dissolved solids of a cup of coffee. So, the coffee refractometer is suitable for owners of coffee shops to make coffee recipes.

Different scales for refractometers are as below:

Brix: Brix refractometer measures percent sucrose. Refractometers are widely used in the food and beverage industry. It helps quality control by this unit of measure.

Coolant: A refractometer determines the freezing point. And it ensures the effectiveness of ethylene glycol and propylene glycol coolants.

Clinical: Medical refractometer measures serum albumen and urine-specific gravity. Refractometers have high clinical significance.

Salinity Measures: The refractometer measures regular salt (NaCl) solutions. To get accurate salinity measures, you need to use a salinity refractometer.

Specific Gravity: A refractometer measures the density of a liquid against the density of water. And the specific gravity is 1. If the value is greater than 1, it means that the density is greater than that of the reference. If the value equals 1, it means that the density is the same as that of the reference.

Brix Test

With people’s pursuit of fruit taste, brix test is becoming more and more important to us. Brix scale is according to the index of refraction and specific gravity measures. When solids are partly dissolved in water, the specific gravity increases above zero. The higher the concentration of the solids dissolved in water, the higher brix reading will be. Brix meter is widely used in a variety of industries. In agriculture, it applies to measure the ripeness of fruit and vegetables. Also, the brix scale increases as the fruit and vegetables reach the ripening stage. Water-soluble solids such as sugar and acid also peak at this stage. The basic trace elements and minerals of soil are both related to the degree brix of fruit, which is from brix test.

Brix measurement:

In recent years, crop juices and fruit juices have dominated the beverage industry. And the brix measurement allows us to distinguish the quality of crop juices and fruit juices more effectively. Brix meter refractometer can be very effective in measuring the degree brix in these crops and fruit. Thus, brix measurement is to classify the brix level of different fruits to predict fruit quality and taste. The low or high brix level shows the amount of sugar in the fruit. For example, the apple’s brix level includes 4 values. An apple with a brix value of 6 means it has a low amount of sugar. A brix value of 10 means it has medium sugar content. A brix value of 14 means it has a high amount of sugar. And a brix value of 18 means it has the greatest sugar content. As a result, we can consider an apple with a brix value of 18 to be a high brix fruit.

How to Calibrate Refractometer

Follow the 3 steps below to learn traditional handheld refractometer calibration:

  1. Open the cover of the refractometer. And then drop 2-3 drops of distilled water or standard solution on the prism surface. Close the lid so that the sample solution covers the entire prism surface. There should be no air bubbles. The sample solution should be temporarily left on the cover for about 30s. This operation will make the sample temperature same as the room temperature.
  2. Place the refractometer toward a light source. And then focus on observing through the eyepiece. You will see a circular area marked with a scale. If the image is not clear enough, adjust the focusing barrel to make the read line clearer. Then, the upper part of the area will be blue and the lower part be white.
  3. Use distilled water or standard solutions as samples. Then, look into the eyepiece and adjust the refractometer calibration screw. Wait until the read line between the blue and white areas coincides with the 0 scale line. To this point, how to calibrate the refractometer is clear.


The temperature around should be under 20°C or 68°F. It aims to ensure that the accuracy of refractometer readings will not lower. If the temperature fluctuation is above 5 degrees, re-adjust the meter for the accuracy of the measurement results. If the refractometers have an automatic temperature compensation system, recalibrate the instrument whenever there is temperature fluctuation. Ensure that the temperature is 20°C or 68°F when you calibrate the refractometer. After the refractometer’s adjustment, the temperature of the testing environment can fluctuate within a certain range (10℃~30℃). Refractometer calibration needs to be at the temperature of the testing environment. If this is difficult, check the calibration chart. However, FJD digital refractometers have automatic temperature correction ATC. So, it allows the instrument to automatically perform temperature compensation. This kind of refractometer is also called ATC refractometer.

How to Use a Refractometer

As we know, there are many types of refractometers on the market. Because of that, it can be very complicated to grasp how to use a refractometer. When we use a traditional handheld refractometer, we have to consider the testing environment. Thus, ensure that the temperature of the testing site is under 20 degrees Celsius.

Follow the 9 simplified steps below to learn how to get a refractometer reading:

  1. Using drops of distilled water. Transfer drops of calibration fluid to the prism (clean glass) of the refractometer. Then close the cover plate and drop liquid on the prism. Once drop the liquid on the prism, the cover plate will be closed. The drops of the liquid sample must cover the whole prism surface.
  2. Focus on the eyepiece and read the scale of where the boundary line intersects with. Then, adjust the angle of refraction. Make small mirrors and mirrors until the light passes through the prism. Next, turn the Dispersion Compensator knob. The dark and light portions will appear.
  3. Use the adjusting screw for calibration to set the refractometer reading of the boundary line as 0.
  4. Wipe the prism with a soft cloth.
  5. Use a dropper pipette to transfer a few drops of the system coolant sump to the prism (clean glass). And then close the cover plate. Do not put the refractometer under a coolant stream. Failing to follow these directions could damage the refractometer prism and calibration.
  6. Look at the scale through the eyepiece. Focus on the eyepiece and read the scale where the boundary line intersects with.
  7. Record the temperature measured by the refractometer.
  8. When testing darker sample solutions, the reflection method should apply to reduce the error. Remove the protective cover as the light passes the surface. And then make observations after the light passes indirectly.
  9. Wipe the prism with a soft cloth and water.


Refractive index measurements of traditional handheld refractometers need to be at 20 degrees Celsius. If the temperature is not 20 degrees Celsius, you should check the table to make corrections to the measurement result. When the temperature increases, the refractive index of the solution decreases, and vice versa. Thus, when the measurement temperature is higher than 20 degrees Celsius, the correction number should be additionally added. When it is lower than 20 degrees Celsius, the correction number should be additionally subtracted. In a word, refractometer readings are greatly affected by temperature changes.

How to Read a Refractometer

Obtaining accurate readings from the traditional handheld refractometer is very important. So, here are some suggestions on how to read a refractometer:
  • Place the refractometer toward a light source, but avoid direct sunlight exposure.
  • Adjust the zero position of the refractometer. For accurate readings, adjust the zero position before using the refractometer. After refractometer calibration, use paper to absorb the water traces on the cover and prism surface. After adjusting the zero position, keep the refractometer from vibration. Do not use the method of falling to remove the residual water. Otherwise, the adjusted zero position would move.
  • Make sure to cool it down to room temperature for the greatest accuracy. Next, rinse the cover plate and prism surface with clean tap water after the test. The sample solution remaining on the mirror surface must be totally removed. Then, wipe off the residual water with paper. If the liquid sample is still remained, the measured concentration will be high next time. Thus, the refractometer reading will be inaccurate.


To conclude, refractometers are of great significance to our life. It is important to know how to use a refractometer, how to calibrate refractometer and how to read a refractometer. With the development of technology, digital refractometers have occupied the mainstream market. The digital refractometer will be easier to operate with standby time longer and water resistance better. Also, high-precision automatic temperature correction ATC will make measurement more trouble-free. With the advancement of agriculture and technology, the demand for fruit quality and taste testing will increase. As a result, digital refractometers will have a wider range of applications in the future.
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