How to read your hydrometer & how to make temperature adjustments
A hydrometer is an instrument that is responsible for reading the difference in gravity between water that is pure and water containing dissolved sugar while it floats.
It is used as a measure of the progress of the fermentation, where you start with the starting gravity at once you gave brewed your beer and then the final gravity after fermentation. The ABV is calculated from these two variables thereafter.
To give you an idea, water has a specific gravity of 1.000 and beers do generally have a final gravity between 1.015 to about 1.005, depending on the beer style or how the yeast has converted the fermentable sugars into alcohol.
Note that hydrometer readings are set to 15°C and liquid densities are temperature dependent.
Most beer recipes typically have the starting and final gravities to give an indication to the brewer of what the outcome of their brew should be – it acts as a guideline. It is, however, an absolute must for the beer brewing yeast to be about a quarter of a fifth of the starting gravity.
Please take note that if your final gravity of your brew is not the same as that of the recipe, it is not the be all and end all to making beer. The point of beer-making is making a bloody good beer. The hydrometer is just one of the many tools used to brewing and, in this case, is just as measure of fermentation. The brewer need only be concerned if the final gravity is far too high after primary fermentation has ended. (being ½ instead of ¼ of the starting gravity). If the yeast has been prepared properly, this problem should be prevented in this way.
A tip to new, inexperienced brewers, make sure that you do not open the fermenter and check the gravity too often as this allows bacteria to manifest in your beer, leaving a better chance of giving an off-beer. To prevent this, check the gravity once you have pitched the yeast, then let it be until the bubbling in the airlock has ceased. (this gives an indication that fermentation has more or less ended). It is also advised to take a sample of the wort in a trial jar both at the beginning and at the end of fermentation in order to prevent potential contamination. Be sure to not stick the hydrometer in the whole batch. Rather tap into trial jar and place the hydrometer in it to float about. You can also taste the fermentation progress – it should have a beer-like, yeasty taste.
Hydrometers do typically have a temperature correction table as liquid gravity is temperature dependent. A good tip when discussing gravities with other fellow brewers is to also state the standardized value of the hydrometer. Measure the specific gravity of your wort and take the temperature reading, then you can add the correction value on the temperature correction table. This correction value is the one that you add to the specific gravity value.