What is a Refractometer and How to Use It? 5 Essential Things You Need to Know

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A refractometer is an optical instrument that measures the sucrose concentration of a solution based on the refractive index produced by light in that solution.

If we prepared several containers with different solutions of sugar water and placed a pencil in each one, we would see that the pencil appears to bend more in those with a higher sugar concentration.

What the refractometer does is take a sample (in the case of beer, a sample of wort) and project the curve that light produces onto a reticle containing a scale, making it possible to determine the angle formed by the light.

This value is expressed in degrees Brix and tells us the percentage of sucrose present in the sample.

Brix refractometer

Degrees Brix are, in short, a unit of quantity (°Bx) used to determine the total ratio of dry matter (generally sugars) dissolved in a liquid.

Technically, the refractive index (RI) is the ratio between the speed of light in a vacuum and the speed of light in the sample.

This is equal to the sine of the angle of incidence (I), that is, the angle at which light enters the water, divided by the sine of the angle of refraction (R), the degree to which light appears to bend.

A refractometer looks like a small telescope. It has a window that we need to lift to place the sample (1 or 2 drops), and we should wait about 30 seconds to allow the sample to stabilize thermally and spread out evenly.

The result, given in degrees Brix, can be roughly converted to specific gravity using the following equation:

Where:

• DO: The initial or original gravity, read as we normally do.
• DIBrix: The initial gravity reading in degrees Brix obtained with the refractometer.

For example:

Some brewers simply multiply the reading by 4, that is, they take the refractometer reading, multiply it by 4, and the result is the specific gravity.

For example:

Which corresponds to a gravity of 1.044.

A slightly more accurate formula would be the following:

Using this formula, a reading of 11° Brix gives a result of 1.043.

What type of refractometer do you need?

There is a wide variety of refractometers available, but homebrewers generally use the ones designed to measure fructose.

These have a range of 0-30° Brix (1.000-1.120), which is more than enough for our purposes, and they also feature automatic temperature compensation (ATC).

How to use a refractometer?

Before using a refractometer, the first thing we need to make sure of is that it is calibrated. To do this, we place a few drops of distilled water on the glass and close the cover, making sure there are no empty spaces or air bubbles in the sample. We wait 30 seconds, hold the refractometer pointing it toward a light source and look through the lens/eyepiece.

We will see a blue area and a white area. The horizontal line that separates the two fields should read 0° Brix; if it does not, we need to adjust the calibration using the screw provided for this purpose.

Once calibrated, we clean the window and take another measurement to make sure it is properly calibrated.

Temperature correction

If our model has automatic temperature compensation, simply taking the reading will be enough to know the result.

Otherwise, we need to use a chart to find the temperature compensation correction factor (just as with the hydrometer).

Calibration should be done at room temperature, generally at 20°C (68°F); no correction factor should be used.

How to apply the correction factor?

1. Measure the temperature of the wort or beer with a thermometer.
2. Write down the hydrometer reading (specific gravity or density).
3. Look up the correction in the table according to the temperature.

Apply the correction:

• If the temperature is above 20°C, subtract the correction.
• If the temperature is below 20°C, add the correction.

For example,

• Hydrometer reading: 1.050 SG
• Wort temperature: 26°C
• Correction (from the table): -0.003

1.050 (reading) – 0.003 (correction) = 1.047 SG corrected

How to calibrate a refractometer?

If we compare the reading of a wort sample taken with the hydrometer with the reading given by the refractometer, we will see that they are different.

Refractometers measure the percentage of sugar in a sucrose solution. Our wort, however, is not just sugar and water, but we can apply a correction to obtain an accurate figure.

The correction factor is different for each brewer and for each beer. For very dark or high-gravity worts, it may be necessary to use a different correction factor.

To determine this factor, we must follow this procedure: first measure the gravity with the refractometer, then take a sample, cool it and measure it with the hydrometer.

Next, we need to convert the hydrometer value into degrees Brix; this is done using the following equation:

This DO is the original gravity or initial gravity (DI) that we measure with the hydrometer. We then divide the refractometer reading by the hydrometer reading, which should give a result between 1.02 and 1.06.

Correction factor

If we carry out this check over several brew days and take an average, we will get the correction factor we need for our beers.

Now that we have the correction factor, all we need to do is take the value given by the refractometer and divide it by this number.

For example, if the reading is 14.6 Brix, we have:

One thing to keep in mind is that the refractometer has a margin of error of 0.2 to 0.3 Brix, so it is less accurate than a good hydrometer.

What stands out about this method, however, is how quickly we can take wort readings at any point in the process.

To calculate the final gravity, the calculations depend on the initial gravity; in other words, even if the final readings give the same result, the outcome is not the same if the initial gravities are different.

Where

• DF: The calculated final gravity.
• DFBrix: The refractometer reading taken after fermentation.

Frequently asked questions (FAQ)

1. Why doesn’t the refractometer measure gravity accurately after fermentation?

The refractometer is designed to measure sugar in water. During fermentation, the presence of alcohol alters the refractive index of the sample, resulting in a falsely high reading. To get the real final gravity (FG), you must run the Brix reading through an alcohol correction calculator that takes the original gravity (OG) into account.

2. What does ATC stand for on a refractometer and why is it vital?

ATC stands for Automatic Temperature Compensation. This feature is crucial because the refractive index changes with temperature. A unit with ATC internally adjusts the scale to compensate for variations (generally between 10°C and 30°C), allowing accurate readings without having to cool the sample to exactly 20°C.

3. Is it possible to measure the gravity of dark or very cloudy worts?

Yes, although the reading can be more difficult due to the opacity of the sample. In beers such as Stout or Porter, the dividing line between the blue and white fields may look blurry. In these cases, it is advisable to filter the sample through a coffee filter or simply use a stronger light source to better define the line on the scale.

4. How does the presence of hops (dry hopping) affect the refractometer reading?

Suspended particles, such as hop or yeast debris, can refract light erratically. If the sample is taken directly from the fermenter during or after dry hopping, it is essential to let the sample settle or filter it lightly. Any solid that prevents the prism cover from closing perfectly will ruin the accuracy of the measurement.

5. How often should a manual refractometer be calibrated?

It is advisable to check the calibration with distilled water at the start of each brew day. Although they are robust instruments, sudden changes in ambient temperature or small knocks can shift the adjustment screw. A quick calibration to 0° Brix ensures that all the effort you put into your efficiency calculations is accurate.

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