Refractometers are widely used in the wine and beer industry by to track fermentation, but less commonly used by home brewers. However, if used properly a refractometer can be a great tool to track specific gravity in place of or to supplement your hydrometer. This week, I take a look at refractometers, how they work and how an average home brewer can use one. I’m going to use BeerSmith as the refractometer conversion tool, as the hand calculations are fairly complex and could occupy another entire article.
A refractometer is an optical device that, like a hydrometer, measures the specific gravity of your beer or wort. It does so by sampling a small amount of liquid, and looking at its optically. The main advantage over a hydrometer is the small sample size needed – typically only a few drops.
If you start with a glass of clear water, you will notice that the water and glass bend the light passing through it in a certain way. The bending of the light by the water is called refraction. Light bends to different degrees as it passes through different substances. This is the same effect that glasses lenses in eyeglasses are based upon – the lenses bend the light allowing glasses to adjust the focus of an image and make it clearer to your eyes.
If you add sugar to your glass of water, the light will bend more. The refractometer takes advantage of this effect to measure the amount of bending (refraction) which indicates the amount of sugar in the sample. Most refractometers use a prism and a light source to illuminate the sample. On inexpensive refractometers, you hold the instrument up to a natural light source. More expensive models have internal light sources.
Most brewing refractometers measure samples in Brix, which is a scale used to measure specific gravity primarily by wine makers. Some also use a Refractive Index (RI) scale. Both the Brix and RI indexes need to be converted to standard specific gravity or Plato scales using a formula, as wort does not have the same reflective properties as plain sugar water.
Calibrating Your Refractometer
Before you use a refractometer, it needs to be calibrated. Most refractometers are calibrated by using a sample of distilled water. You lift up the daylight (sample plate), and add a few drops of distilled water. Close the daylight plate and allow the water to spread across the sample plate. Make sure there are no bubbles. Refractometers are temperature sensitive, so allow the sample to reach room temperature unless you have a model that automatically compensates for temperature.
Hold the refractometer up to natural light and take a reading. Most refractometers have a calibration knob or screw that will let you adjust the zero setting. What you want to do now is adjust the refractometer so it reads zero with distilled water in it. This may take a few tries. If you can’t zero it out or it is not adjustable, you can handle the adjustment using BeerSmith (see the calibration items under the refractometer tool).
If you want an accurate reading, you should also calibrate the refractometer using a wort sample that has a known specific gravity. You can do this calibration by mixing up a small amount of dry malt extract with water, then take an accurate hydrometer reading and also refractometer reading and enter both readings into the BeerSmith refractometer tool (use the “calibrate refractometer settings” button).
Using Your Refractometer when Beer Brewing
Using the refractometer is very similar to what you just did when calibrating it. Open the sample plate, make sure it is clean and dry, then add a few drops of your wort. Again, if the wort is hot allow it to cool to room temperature first (ideally 68F). Close the sample plate, check for bubbles, and then hold the refractometer up to a natural light source.
Reading the refractometer is easy – just take the reading directly from the sight scale. The reading you take will most likely be in percent/degrees Brix or RI.
Refractometer Limitations when Brewing Beer
Here’s where some people get disappointed when using a refractometer. Refractometers are calibrated to measure the amount of sugar (sucrose) in a clear sample of water. The sugar in barley beer (maltose) is a different animal. Therefore some adjustment needs to be made to take into account the fact that we’re measuring colored maltose and not clear sucrose. You can’t just use the measurement you made with the refractometer.
Converting the Brix measurement to a specific gravity or Plato measurement made on a sample of unfermented wort is a moderately complex calculation which requires a spreadsheet or a tool like BeerSmith. However, there is yet another complication: once the wort starts fermenting alcohol is produced, and the alcohol changes the overall equation yet again.
In practice, this means that in order to calculate the true gravity of a fermenting or fully fermented beer, you need not only the current refractometer reading, but also the starting gravity. So if you are using a refractometer, it is critically important you record the starting gravity of the wort before fermentation if you want to calculate a mid-fermentation or final gravity for your beer.
Converting Brix to Specific Gravity or Plato with BeerSmith
Now that we’ve calibrated your refractometer, and understand the limitation, open up the Refractometer tool in BeerSmith. Assuming this is your original gravity reading for unfermented wort, select the “Unfermented Wort Gravity” calculation at the top and enter your Brix (or RI) reading from the refractometer. The “corrected gravity” will show your original gravity for the beer.
Once you have your original gravity and the wort is fermenting, you can take additional readings. In this case use the “Fermenting Wort Gravity” calculation in the tool and enter both your Brix refractometer reading and the original gravity. The corrected gravity will show your current adjusted reading.
There is a third calculation in BeerSmith, called “Finished Beer ABV/OG” which lets you back out the original gravity of the beer if you forgot to measure it in the first place. In this case you need to take a final gravity measurement with both the refractometer and an accurate hydrometer, and enter those readings to get the original gravity.
Don't make another bad batch of beer! Give BeerSmith a try - you'll brew your best beer ever.
Download a free 21 day trial of BeerSmith now