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How to work out the alcohol ABV of your home brew beer


How to use a hydrometer correctly to determine the alcohol content of your beer

A trick of the home brewer's craft is to keep a hydrometer handy. This tool will help any beer brewer to make great beer.

What is a hydrometer?

At its most basic scientific purpose, a hydrometer is an instrument that measures the specific gravity of liquids, that is to say it measures the ratio of the density of the liquid to the density of water.

Did you get that?

Why would a home brewer use a hydrometer?

A home brewer uses the hydrometer to monitor the fermentation progress and measure the alcohol content of his produce.

Hydrometers can measure specific gravity, potential alcohol and the approximate sugar per litre content.



So the big question then, how does one use a hydrometer?

If you float the hydrometer in a test tube of water you will find it gives you a gravity reading of 1.000. This makes sense as there is no water displacement occurring.

Not let's assume we are at the point where you have prepared your beer wort. It's time to add the hydrometer to the beer wort in a test tube. Not only is there water in the wort but other mixed in ingredients including sugar, thus meaning some displacement can occur.

Spin the hydrometer around in the tube - this will dislodge any bubbles that are helping to float the hydrometer above what should be the actual reading. 

Take note of the reading which is where the hydrometer crosses the water / air line and write it down as you will need it for your equations later on. It's called the starting or original gravity. 

Let the brew ferment.

When you think fermentation is complete, take a reading. Then wait 24 hours and take a second reading.

If they are the same, you have your final gravity measurement.

A handy rule of thumb to beer in mind is when the final gravity is approximately a quarter of the starting gravity you’re done with fermentation. 

Let your beer 'chill out' in the drum a bit longer. While the bubbles may have stopped, chemical reactions are still occurring and they will help make your beer taste even better.

How to work out the alcohol content of your beer using the hydrometer's specific and final gravity readings

It's a crude or rough measurement but the equation is simple:

(starting reading minus final reading ) x 131 = alcohol by volume (ABV)

Given that hydrometers are calibrated to be used at specific temperatures one needs to use the taken readings a guide rather than a wholly accurate value. For example if your hydrometer is calibrated to be used in an environment of 15 degrees centigrade but it's warmed to 20 degrees, there's a chance your readings will be slightly out.

To be frank, for the average home brewer, it hardly matters if you 5 per cent beer if actually 4. 8 per cent!

And there you have it. Well the simple version anyway. There's quite a bit of science behind how the units are calibrated but provided your readings are semi accurate, you shouldn't need to worry about it too much!

A single caution though. You shouldn't feel the need to take readings all day every day as you wait for fermentation to finish. Exposing your beer to the atmosphere does raise the possibility of a contaminant getting in so beer that in mind.

Image credit to Daniel Spiess via Creative Commons Licence

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