The Hydrometer is best described as a floating weight. It is the most important instrument used by the wine maker or brewer.
WHAT CAN AN HYDROMETER DO:
The hydrometer is used to determine the ‘Specific Gravity’ of the beer or wine either before fermentation begins or at the end of fermentation. It does this by reading the fermentable sugar content in the wort (brewing recipe) or grape juice. The more sugar there is in the liquid, the thicker or denser it will become and the higher the ‘Specific Gravity’.
It can Check sugar content at start of fermentation, following up to see when fermentation has ceased.
Works out incremental sugar dosing of strong wine (if too much is added at one time fermentation will be inhibited.
Calculates how much sugar is needed to give a required alcohol strength
It is used to determine when the wine or beer is fully fermented.
Find out how much sugar to add to impart the correct alcohol content
It can be use to calculate how much 'potential' alcohol can developed in the beer or wine.
Determines when wine or beer should be drawn off or racked.
Checks that wine is not too dry, which can ruin flavour, fullness and keeping quality.
In short, it checks the entire progress of fermentation for either brewing or wine making.
WHAT CAN'T AN HYDROMETER DO:
An hydrometer cannot read actual alcohol content of a wine or beer that has finished fermenting. it can only be used to calculate 'potential alcohol" by reading the sugar content.
HOW TO USE THE HYDROMETER
The hydrometer has long glass section containing the calibrated scale and it is weighted at the bottom, (with a bulbous lower end) so that it will float upright in a liquid. The scale of figures on the hydrometer appear to be ‘upside down’ the smallest being at the top and the largest at the bottom.
To read the hydrometer you can either float it directly in the fermeter or decant some of the liquid into a tall test jar.
Always spin the hydrometer after placing it in the liquid to remove any air or gas bubbles clinging to the glass which will give an inaccurate reading.
Steady the hydrometer and take your reading immediately it stops bobbing as all hydrometers will slowly rise up.
Starting SG reading......................1045
Final SG reading.................... - 1000
Subtracting one from the other leaves a figure of 45 . To determine potential alcohol you then have to divide this figure by 7.46 and you have the approximate alcohol in your beer.. In this case 6.03 percent.
The accuracy of the hydrometer can be tested in water, where it should float at S.G. 1000. at 60 F.
IMAGE below shows BettABrew Imperial Stout at SG1050 before fermentation.