How To Use a Refractometer

A refractometer can be a great addition to your toolbox, but there are a few quirks to using it.

I recently decided to pick one up from the Northern Brewer table at NHC since they were offering a nice discount.

If you’re not familiar with the refractometer, it’s basically a device that measures the degree to which light bends passing through a liquid.  Refractivity is the same principle that makes a stick appear bent when you put it into water.  And in our case we’re measuring the refractivity (that’s a word, right?) of sugar solution, or wort.

Using the device is pretty easy.  The sample goes on the prism at one end and you look through the viewfinder on the other end to read the scale. Since the sample size is only a few drops, this is much easier and faster than having to collect and cool a whole hydrometer jar worth of wort/beer.  All you need to do is place a few drops on the prism and close the lid.  Make sure there are no air bubbles or dry spots and take a reading.

Homebrewing/Winemaking Refractometers usually measure on the Brix scale. You’ll need to do a conversion to figure out the gravity points, but it’s roughly SG = Brix x 4.  So 10 Brix is about 1.040 SG.  If you use Beersmith (some other programs might have this feature as well), you can do a calibration against wort measured with a hydrometer to get a more precise conversion, but the 4x number should get you within a point or 2.

Of course the first thing I did upon arriving home was run out to the garage and grab a sample of wort out of a carboy to try out my new toy.  And the readings didn’t make any sense.

The conversion factor only applies to unfermented wort, sugar in water solution.  Once alcohol is present you need a much more complicated formula to convert Brix to SG.  This is because alcohol has a different specific gravity than water.

Fortunately there are a couple of easy options.  MoreBeer has a great spreadsheet, and of course Beersmith will do the calculation as well.  In both cases you’ll need to know the original starting gravity of the wort, but that shouldn’t be a problem because you wrote it down, right?

We’ve jumped ahead a bit, but you’ll also want to calibrate your refractometer.  Just like with a hydrometer, you’ll need some distilled water at 60ºF (or whatever the calibration temperature of your refractometer).  It should read 0 Brix.  If it doesn’t, adjust the calibration knob or make a note of the difference.

It’s also worth noting that some models feature Automatic Temperature Correction (ATC), but it usually has a finite range – not anywhere between boiling and freezing.  Check the manual for the specifics on your model.

I’m still trying to get the hang of using it, but overall it’s a welcome addition.  If only for the fact that I don’t have to throw away a whole hydrometer jar of beer to check the gravity during fermentation.  Being able to take more gravity readings pre-boil and during the boil will help to make sure I hit those starting gravity numbers as well.

I’ll probably keep doing hydrometer readings just as a sanity check for now.  But once I get my revised sample taking technique down (I’m good at making a mess), then the hydrometer will probably go n the shelf for good.

Tagged as: how-to, refractometer, use