To be honest, a hydrometer is not really a requirement to have (and use) for homedistilling purposes, but....
A hydrometer is an inexpensive piece of test equipment used by brewers, winemakers and distillers. It is generally made of blown glass, with a weighted, bulbous bottom and a long narrow stem (see picture below). The hydrometer is designed to float in liquid with the bulbous end down. A reading is taken by looking at a scale (contained in the stem) where the surface of the liquid being measured.
The Potential Alcohol (PA) level can be calculated by measuring the specific gravity (SG) of the wash, assuming all sugar will be fermented and no sugar additions are made after inoculation. PA values are calculated based on SG/Brix values, not on "Sugar" values. This all sound technical but in fact it is very easy to do.
So, if we measure the sugar content of our wash, we can predict the final outcome of alcohol once the fermentation has stopped.
This will allow us to:
- Avoid wasting to much sugar on a wash which will never be fermented (yeast can only survive a certain % of alcohol before the yeast cells die.
- Understand during the fermentation wether the fermentation stopped as a result of absence of sugar for the yeast to survive on or because of another unwanted reason.
How to use a hydrometer
The first thing you need to do is to calibrate your hydrometer. It is a simple process which will determine wether your readings will give you any usefull information. This is what you need to do:
Keep a trial jar with water near your wash. The idea is that this water have the same temperature as your wash.
Before using your hydrometer (every time!): Check the reading of the water. It should be around 1000 s.g. (0 on some hydrometers) but it will vary depending on temperature and scale errors.
If your hydrometer shows, say 1002 (+2) you obviously need to take your reading minus 2 to get it right. Then simply take all your wash readings minus 2 and thats it!
Similarly of course, if your hydrometer shows 997 (-3) on the water - just add 3 to all your readings.
Remember that you have to check the water reading every time because temperature might have changed and/or your hydrometer scale might have moved inside the hydrometer.
Basic rules you need keep in mind
If your wine contains much CO2 (i.e. still is fermenting or only just stopped) you need to be careful. The CO2 bubbles tend to stick to the hydrometer after a few seconds and then lift it up. Best trick is to give the meter a short spin, then stop it and take a reading immediately.
Remember to calibrate the hydrometer before you use it to get the most accurate reading.
Hydrometers are designed for use at a particular temperature. If the liquid is hotter (or colder) it will give a false reading. There is usually a conversion table supplied with them to help correct readings by. For a general correction table, have a look at: Correction tables
The range of readings (highest to lowest), to make sure it will suit your purpose. A standard range for home brewers is 0.990 to 1.120. For example, in order to achieve a 12% wash, you'll want to start your wash at a SG of 1.090.
What the hydrometer measures. Some hydrometers only measure specific gravity, but most of them measure three things: SG, potential alcohol (P.A.), and sugar content.
The calibration temperature of the hydrometer. The most common
calibration temp is 60° F.
Don't throw back the wash sample you have used during measurement - it might ruin the batch!!!
Be sure to take good records of your readings!
Where to get it
Check the web if there is no homebrew shop in your area.