How to use a refractometer

A refractometer is used to measure the amount of sugar in a solution. This is done based on the refraction index of the liquid. More sugar in solution will produce a higher refraction index. The scale of the refractometer is calibrated to read the equivalent concentration of sugar for a given refraction index. Using a refractometer sounds and looks complicated, but it is easy once you get the hang of it.

Making a measurements by using a refractometer is a simple process.

  1. Place about 3 drops of solution on the prism of the refractometer and close the cover
  2. Look through the refractometer at a bright light. Where the separation of colors occurs is the measurement. (If there an inadequate amount of liquid, or there is significant suspended material, the line will appear blurry)

using a refractometer

In similar way that alcohol skews hydrometer measurements, alcohol will skew refractometer measurements. As the concentration of alcohol increases, the refraction index also increases, although at a different rate than with sugar. For every gram of alcohol produced by yeast 2.0665 grams of sugar are consumed.[1] Because the alcohol yield is known equations can be generated to accurately reflect fermentation progress using a refractometer. However, these equations are different than those used with a hydrometer. Further discussion of this can be found in a variety of sources.[2][3] It is important to note that equations developed for a hydrometer cannot be used directly while using a refractometer. The table below summarizes the two sets of equations.

ABV

Apparent Attenuation

Hydrometer

Refractometer

Final Measurements

not accurate

OG – Original Gravity as measured with a hydrometer.
FG – Final Gravity as measured with a hydrometer
OB- Original Brix as measured with a refractometer
FB – Final Brix as measured with a refractometer

When using a refractometer, you can ensure the accuracy of your measurements by calibrating it with water. Tap water works fine. The small amount of dissolved minerals will not significantly change the measurement. For accuracy, it is important that the water use for calibration is within one degree centigrade of the sample that will be measured. Once the offset is measured, the refractometer can be adjusted to zero the measurement. This is normally done by turning a small screw on the top of the refractometer.

3

Temperature will effect measurements as well, even if your refractometer has Automatic Temperature Correction (ATC). In my experience it is better to allow the sample to come to room temperature than relying on the ATC of the refractometer. One easy way to quickly cool a small sample is by using a disposable 3ml plastic pipette and a bowl of water. Pull a sample into the pipette and then place the pipette bulb side down into the bowl of water. After a few minutes it will stabilize to room temperature.

4

To expedite to process you can stir the water with the pipette (Again, bulb side down in the water, holding the tip or trunk of the pipette.). For convince, the water in the bowl can be used to calibrate the refractometer as it will be very close to the sample temperature.

5Unlike density (which is typically measured with a hydrometer) refraction index is quite sensitive to temperature change. It only takes a one degree centigrade change in temperature to create a change of 0.1°B. This can be seen by examining the properties of aqueous sucrose and changes in refraction index over temperature. A change of 0.1°B corresponds to a change of 0.00015 in refraction index. [4] A change of 0.00015 in refraction index corresponds to 1 degree Centigrade.[5]

[5] Shu, Xuewen, et al. "Sampled fiber Bragg grating for simultaneous refractive-index and temperature measurement." Optics letters 26.11 (2001): 774-776.

written by Steven Deeds

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