Refractometers are a class of instrument that measures the refractive index of a sample. Whilst there are many types of refractometers and even more applications for them, we are going to focus exclusively on the use of refractometers in brewing.
When brewers use refractometers, it is to give them a reading of the sugar levels in their sample. Similar to using the more familiar and common hydrometer, unfortunately the hydrometer has several downsides:-
- A relatively large sample is required, 100ml is about the minimum needed in most cases
- This sample needs to be cooled to around 20C to get an accurate reading
- The hydrometer is generally very fragile
Depending on facilities and required sample sizes, it can take over 30 minutes to cool a sample down enough to measure the gravity with a hydrometer, and this is where refractometers really come into their own. A refractometer only needs a few drops of a sample and the most common models feature a system for Automatic Temperature Correction (ATC). This means you can take a couple of drops from the boil kettle and straight away use the refractometer to get a reading of your sugar levels. When it takes just seconds to get a reading, it is much easier to monitor how a sparge is progressing etc.
So how does this voodoo and black magic actually work? If you have ever looked from the surface of a pool or stream and tried to pick up something from under the water, you will have noticed that the object is not where it seems to be, because the light you can see is being refracted by the water. This is what makes the refractometer work, when you calibrate it, you set the 0 point using distilled water. The light passing through the sample is refracted in a specific way, and the angle the light is refracted by will changed based on the amount of sugar or alcohol in the sample.
To take a reading you open the cover plate, simply add a few drops of your sample to the sample window which is the blue area in this image. Next close the cover plate over the window, and look through the eye piece.
What you will see then will vary depending on your particular refractometer, All of them look more or less the same, but brewing refractometers come in a variety of different scales or combination of scales. Some only feature Brix, Plato or Specific Gravity and others feature combinations of these scales. This reading shows results in both Brix and SG. Click the picture to see a larger image.
There is one very important thing you need to remember with refractometers. The reading is affected by the amount of alcohol in your sample. Alcohol refracts light differently to water so once your wort has started fermenting and there is any alcohol in your sample, you will need to adjust the reading to account for the alcohol. If you do not do this then You will forever be thinking your beer has not reached its final gravity. I once had a batch of beer that was stuck at 1.018 for weeks, I pitched fresh yeast and still nothing. I had forgotten to correct for the alcohol and once I used a refractometer calculator, put in the starting and current readings, found out that the beer had indeed finished and it was under 1.010, which I was able to confirm with a hydrometer.
There are several refractometer calculators around, most brewing tools like Beersmith include one, however if you do not have access to them, then I suggest one by Northern Brewer if your refractometer measures in Brix, or Stderr also included a handy SG / Brix converter on the same page if your meter measures in SG
One of the side effects of having to correct your reading to account for the alcohol in your sample is that even if you forgot to take a starting gravity, you can use a final hydrometer and refractometer reading to calculate what your starting gravity was and the amount of alcohol you have.
There are many places you can get brewing refractometers in the UK, and whilst the UKCBN is not endorsing any particular supplier, you can order them from Amazon BrewUK Hop and Grape or many suppliers on ebay
Do you use a refractometer? What do you find good or bad about them? let us know in the comments