The hydrometer is a simple device that is floated in the beer or the beer within a test jar. The depth of which the hydrometer sinks gives the measure of the sugar level in the liquid.
The hydrometer measures the specific gravity (density or weight) before fermentation starts and the reading is noted.
A hydrometer is the only sure way of checking if the fermentation has finished by measuring how much sugar is left in the beer. A trial-jar is useful, but not essential.
When the reading is below 1.004, it is safe to bottle. The timing of bottling the beer is critical. If it is bottled before 1.004, then
there will be too much gas produced by the secondary fermentation and the bottles could burst (this is also true if too much sugar is added to the bottles).
If using a pressure barrel fitted with a safety release valve, then the beer can be barreled when the reading is 1.006 .
Summarising, the hydrometer allows you to find out:-
- The amount of sugar naturally occurring initially in the malt.
- The amount of sugar to be added to reach a desired final alcohol content.
- The progress of the beer during fermentation.
- A final check when the beer is ready to be barreled or bottled.