Around the time I took the plunge to switch from kit brewing to all grain brewing, I bought some new toys. One of those was a refractometer, which is a device used to measure how much sugar is in your beer – essential if you’re going to know how strong it is, and also a handy indicator for when the primary fermentation has finished.

With a traditional hydrometer, each time you want to measure the specific gravity of your beer you have to get a reasonably big sample – it’s essentially a weighted float that sinks deeper as the density of a liquid reduces. One of the big advantages of a refractometer is that it can measure a single drop. Of course, nothing in this life is free – the price in this case is that a refractometer doesn’t actually measure specific gravity directly.

Specific gravity is a measure of density, but a refractometer is measuring the refractive index. This is all very interesting if you’re of a scientific bent but if you’re not, here is the important bit – hydrometers, and all your beery calculations, use specific gravity while a refractometer gives you a reading in units called ‘brix’. So, to turn this into a useable figure you have to do some maths.

I am lazy in most things I do, so when I bought my shiny new toy I paid an extra 90p and got one which was already marked in both brix and specific gravity. Brilliant, I thought. I don’t need to look up all the boring and tedious sums I’m supposed to do, I can just use the reading I get and all will be well with the world.

SG = 1.001843 – ( 0.002318474 x B_o ) – ( 0.000007775 x B_o² ) – ( 0.000000034 x B_o³ ) + ( 0.00574 x B_f ) + ( 0.00003344 x B_f² ) + ( 0.000000086 x B_f³ )

This is the point at which I came unstuck. You see, unlike a hydrometer, the reading from a refractometer is badly affected by alcohol in the liquid you’re measuring. The sort of alcohol you might find in a freshly fermented beer, for example. You have to correct for the alcohol using another boring and tedious sum which obviously is shown right next to the boring and tedious sum I had so cleverly avoided having to read.

I only worked this out when my third brew (my delicious Spring Ale, which will be reviewed shortly) yet again appeared to stop fermenting half-way through. I went furiously googling again trying to work out what I was doing wrong when I stumbled across an article about these corrections that I hadn’t been applying to my refractometer readings and everything became rather unpleasantly clear – my beer had been fine, and I’d just been measuring it wrong!

All of which is a very long and rambling way of saying; if you buy a refractometer (and I really recommend them – best value bit of brewing kit I’ve bought) then you must do the sums, even if it gives you a specific gravity reading. And being a helpful chap, I’ve even made a calculator to do the sums for you – my fantastic Refractometer ABV Calculator.