If you have bought a complete starter kit then it may be that it included a hydrometer. Or maybe you have been brewing your own beer for a while and you are either considering buying a hydrometer or have just bought one. Whatever the case, you may well be wondering how on earth to use it.
What you want to know is the strength of the alcohol you make and whether the liquid you have fermenting at the moment is ready to bottle.
So first of all, let’s consider what a Hydrometer does. The technical answer to this is that it measures the Specific Gravity (SG) of a liquid. What this means is that it will measure the amount of sugar in a liquid. That’s it. That’s what it does. Easy eh!
Think about it. When you make alcohol you add a load of sugar (or something containing sugar such as fruit or malt extract) and then some yeast. The yeast “eats” the sugar turning it into alcohol. Therefore if there is less sugar left, the alcohol is nearer to being ready. So as your alcohol is fermenting, you can take a reading and see how it is getting on. The more sugar there is left in the liquid, the higher the hydrometer reading will be. The more alcohol there is in the liquid, therefore the less sugar, the lower the reading.
Incidentally, if a hydrometer is dropped into water at 20°C, it will read 1.000. You can always try this just to test your hydrometer.
There are many variations when making beer so it is best to look at the instructions on the kit, however a typical beer will start at a specific gravity of 1.045. This is the reading you should get if you were to drop your hydrometer into the fermenting bin just after adding the yeast and before fermentation has begun. When fermentation has finished it will probably read about 1.012.
How do I use this to calculate ABV?
The ABV (or Alcohol By Volume) is easily calculated. You need the start and finish number, take the difference between them and then divide this figure by 7.362.
For example the starting point for our beer is 1.045 and this ferments down to 1.012. The drop is 33 points. This divided by 7.362 is 4.5% ABV. Don’t worry about doing the maths, I have put a handy calculator on this site. Just feed in your numbers and it will tell you all you need to know.
The best way to use the Hydrometer?
It is probably easiest to use the hydrometer with a trial jar. This is a slim container into which you can add a small sample of your beer. It is made of clear plastic and you simply fill the jar about three quarters full and then drop in your hydrometer. If you overfill it you may spill some liquid when the hydrometer goes in so be a little careful.
Make sure you work on a flat level surface for an accurate reading. The liquid may well not have a dead flat surface level so take the reading from the lowest point. Also make sure that the hydrometer is not sticking to the side of the trial jar. If it does, just spin it slightly to release it. If there is any froth on the beer, just gently blow it away.
To be really accurate you should take your measurements at 20°C. If the liquid is 5°C higher then add 0.001, similarly if its 5°C lower then take off 0.001.
There you are then. Not hard is it? If you don’t yet have a hydrometer, you can buy one quite cheaply at Home Brew Online.
If you can’t take away and divide or at least can’t be bothered, use my handy ABV calculator here. (Coming soon – 11th Nov. 2016)