Calibrating my refractometer

I bought a refactometer quite some time ago. Just cheap one. I thought it might be a good alternative to a hydrometer and result in less wasted beer (150ml every time I took a reading with a hydrometer). I tried to use it – I thought it was straightforward – but I couldn’t produce any decent readings. So I gave up and went back to my trusted hydrometer.

Then I read the instructions!

There’s a fundamental drawback with a refractometer. It only works with unfermented wort. As soon as there is alcohol in the wort (i.e. fermentation has started), this affects the reading. Now this isn’t a showstopper, however, as there are calculations available to take this into account. It is just a little more involved.

But first things first, you need to calibrate your refractometer. There is plenty of guidance around, but I followed the instructions on Brewer’s Friend. You will find them here. I made up some test wort and varied the gravity by adding additional amounts of DME and water to the same sample. This enabled me to generate a number of different sample readings from one lot of test wort. My correction factor averaged out at 0.96

Brix	SG	Correction
14.8	1.061	0.98834
10.1	1.042	0.96414
8.0	1.034	0.93706
10.7	1.045	0.95573
13.0	1.055	0.95803
8.4	1.035	0.95660

To get an SG reading for my wort, I use the online calculator together with my Brix reading and correction factor. The wort for Brew 63 had a reading on 13.0 and this is equivalent to 1.055. This was the target SG for this brew!

As the wort ferments you can use the online calculator with your initial Brix reading (13.0 in the case of brew 63) and then enter the current Brix reading together with the correction factor. This takes into account the alcohol that is present in the beer. I have been checking this daily with brew 63 and the refractometer readings are exactly the same as my trusty hydrometer.

UPDATE: It’s been a couple of months since I wrote this post and I am now pretty comfortable using my refractometer. I normally only use my hydrometer to take the first and last readings during fermentation – arguably the only two readings that really matter! All the daily readings are done using my refractometer. Although it might be slightly less accurate, it is much quicker and uses less beer to take a reading (5ml rather than 100ml).