1.3 Hydrometry and specific gravity

In order to measure the strength of the homebrew that you've produced, you're going to need to use your hydrometer again. If you recall at the start of your homebrew experiment, you took a reading of the specific gravity of your starting fermentation mixture. And we referred to this reading as the original gravity. In fact, you can use your hydrometer, as well, to determine when your fermentation has actually finished. If you take specific gravity readings of your final fermentation when you think it's finished, if the readings are constant over the period of two days, then that's a pretty good indicator that all of the sugars from your starting mixture have been used and your fermentation is complete. Now, in order to measure how strong your beer actually is that you've produced, you're going to need a sample of it. And you're going to use the hydrometer again to take another specific gravity reading that we refer to as the final gravity. So you're going to take your hydrometer and put it carefully back into a sample of your finished beer. Again, exactly the same as before, you just let the hydrometer settle, and then you take your reading. So you can see now it's just about settled down, and we have a reading of around 1.015. And this is fairly typical of a homebrew. This reading is referred to as the final gravity reading, and it's lower, if you note, than your starting gravity reading. The reason for that is that all of the sugars in your starting fermentation mixture have been used by the yeast in the fermentation reaction to produce carbon dioxide and alcohol. So the mass of this volume of liquid is now lower, and therefore our density and specific gravity readings are now lower. So now you're ready to take your original gravity and your newly measured final gravity reading, and we can work out just how strong a beer you've actually produced.