How To Use Your Hydrometer

HBO_StaffHBO_Staff Administrator Posts: 2,038

You can use your hydrometer for a couple of useful reasons. One of its purposes is to see what stage of fermentation your brew is at, to see if its finished its first stage of fermentation (ready for bottling/barrelling) or stopped prematurely. The other purpose for your hydrometer is to calculate the alcoholic ABV% of your brew. To use your hydrometer to see if your brew has finished its first stage of fermentation and is ready for bottling or barrelling is simple. Firstly sterilise your hydrometer to make sure of no contamination. Next, gently place it into your fermenting vessel to take a gravity reading (alternatively you can take a sample of your brew into a trial jar and place your hydrometer in the trial jar so it floats). If using a kit there is sometimes a guideline gravity reading of what your brew should be in your beer, cider or wine kit instructions. After you've taken your first reading you'll have to leave it for 24 or 48 hours before you take your second. If your first and second readings are the same after 48 hours and all signs of small bubbles rising through the liquid to the surface have completely stopped, this usually means fermentation has stopped. If in doubt leave it another day or so, or if the reading is too high and your fermentation has stopped prematurely have a look at the post below: http://homebrewonline.vanillaforums.com/discussion/comment/29#Comment_29 To calculate the alcohol content of your brew: When brewers talk about Specific Gravity they will usually specify either the Original Gravity (OG), or Final Gravity (FG). The OG is a measurement of the beer or wine before it ferments, and the FG is the measurement that is taken after fermentation is complete. The difference between the two tells you how much alcohol is in the brew. As an example, a typical Pale Ale will start off with an OG of around 1.045 and finish off with an FG of around 1.008.

Enter your readings into the calculator with the correct decimal points on this link (eg 1.045 and 1.010); http://www.brewersfriend.com/abv-calculator/

Alternatively calculating the percentage of alcohol is as simple as plugging some numbers into the following equation.

% Alcohol = ((1.05 x (OG – FG)) / FG) / 0.79 x 100

 So, given a few numbers suggested above:

OG = 1045
FG = 1008

The equation would look like this: ((1.05 x (1045 – 1008))/1008) / 0.79 x 100

So, this beer would be about 4.9% alcohol.  There are various tables with the alcohol conversion numbers to work out the alcohol content, any of the proven ways can be used, but if you're not sure then post your questions here and we'll be happy to help. These hydrometers come in their own handy trial jar and are easy to read; 

http://www.home-brew-online.com/equipment-c40/testing-c49/stevenson-hydrometer-with-trial-jar-case-p656

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