Refractometer

One of the most important measurements in brewing is measuring how dense the wort or beer is. Density gives you an idea of the quantity of sugars in the wort/beer – more sugars means a more dense liquid.

Many brewers use a hydrometer to measure specific gravity, ie the density of the liquid relative to that of water. Water is deemed to have a specific gravity of 1.000. A liquid with a specific gravity larger than that is more dense that water. So for example a wort before fermentation might have an SG of 1.065 – this is referred to as the “Original Gravity” or OG. Oldies will remember when OG used to be quoted on beer bottles and pump clips, because it gives you a guide as to how strong the beer is. This isn’t hugely helpful though because alcohol levels depend on the final gravity as well, ie how sweet the finished beer is (therefore how much unfermented sugar is left which did not become alcohol).

A hydrometer works by floating in the liquid. The higher it floats the denser the liquid. A scale up the side allows the SG to be read off.

There’s another way of measuring the density of a liquid, and that is by seeing how much it refracts light. This is where the refractometer comes in.

Andy bought me one of these devices for Christmas, and we’ve been using it for the last couple of brews. To use it you put a few drops (literally) of the wort/beer on a glass plate and lower a plastic cover over it. You then look through an eyepiece and you see a scale with a coloured line. The line moves according to the amount the light has been refracted (bent) by the liquid. The density is then read off the scale.

 

 

 

Refractometers are also used in vineyards to allow the grower to check the sugar levels of the ripening grapes without having to crush a whole tube full of grape juice.

We’ve found our refractometer to be better than a hydrometer for a couple of reasons. Firstly it avoids the need to draw off wort/beer and put it in a cylinder to float the hydrometer in it. This is messy and brings a risk of infection (unless you discard the liquid afterwards, which is pretty wasteful). Secondly, hydrometers are massively affected by the temperature of the liquid. Liquids expand as they get warmer, which changes the density. So all measurements are supposed to be made at 20C. You can correct for this, and we’ve got a huge table that we use to correct for temperature when using a hydrometer. With a refractometer you only use about three drops, which fall to the same temperature as the refractometer glass very quickly.

But there is a downside. Refractometers measure in a different unit: “Brix” as opposed to specific gravity. 1 Brix is about and SG of 1.004. More accurately, °Bx = (((182.4601*S -775.6821)*S +1262.7794)*S -669.5622) where S is the specific gravity (according to Wikipedia anyway). I put that into a spreadsheet, and as far as I’m concerned each 1°Bx   is about 1.004, or as near as dammit. But it’s nice to have checked. :-)