What is a Hydrometer, and why use one?
A hydrometer is actually a simple tool, and is very easy to use. It simply measures the weight or gravity of a liquid in relation to the weight of water (which has a specific gravity of 1). The hydrometer is a device that you just float in the liquid. If the liquid is heavier than water (more dense) then it won't sink as far, and if it is lighter than water it will sink further.
A mix of water and sugar will be heavier than water, and a mix of alcohol and water will be lighter than water. Measuring the difference between the two will allow you to calculate the alcohol content, and also help you ensure that all the sugar has fermented. It therefore is useful to:
- Determine how much sugar is present at the start of the fermentation.
- Determine how much sugar to add to the brew to achieve the required strength.
- Keep a check on the progress of the ferment, and test to ensure fermentation has finished; and
- calculate the strength of the finished wine
When making wine or beer from kits the hydrometer's most useful function is testing that the fermentation has finished, however when making country wines it may be important to establish how much sugar is in the original must (wine before it has finished fermenting), so that you can both calculate how much to add, and it will also give you a guide to the alcohol content of the finished wine.
How to use a Hydrometer
When you buy a hydrometer, you should also consider buying a trial jar. This makes it far easier to read the hydrometer than trying to use it in the fermentation vessel.
To take the measurement look at the level of the liquid at the point it touches the hydrometer, with your eye at the same level as the liquid, as seen in the attached image.
The first reading you should take is right at the start of the process before you have added the yeast. This will allow you to check that you have the correct amount of sugar in the liquid before you start. Too much and it may not all ferment, too little and you will end up with a low alcohol 'watery' brew.
You can continue to check the specific gravity throughout the fermentation, and you should find that the liquiid gets 'lighter' as the sugar gets converted to alcohol.
Calculating the Alcohol
Although this is meant as a guide only, provided you have a hydrometer reading at the start of the fermentation, and one at the end, calculating the alcohol content is relatively easy.
Simply take the initial reading, minus the final reading, and divide this figure by 7.45. So for example:
- Start Reading: 1.085
- End Reading: 1.005
- Start minus End: 80
- Alcohol is 80 divided by 7.45
Checking the fermentation has finished
This is the most useful function for makers of wine from kits. It will allow you to check that your fermentation has finished before moving on to the bottling stage.
Simply take 2 or 3 readings on consecutive days, and if all three are the same then the fermentation has finished. Most cider kits will tell you in the instuctions what the final reading should be. If it is wildly different then you may have a problem.