What does it do?
A hydrometer should be found in any wine or beer making situation. It will measure the Specific Gravity (SG) of the liquid you are about to ferment and this will then in turn give you a guide to the Alcohol by Volume (ABV) you will be able to produce.
The hydrometer will then be used throughout the fermentation to ensure sugar is being converted into alcohol. We will be able to tell this by the daily drop in the gravity. As more sugar is converted to alcohol then the gravity will fall.
For example the start gravity in a typical wine will be 1.075 to 1.090 (a lot of instructions will leave out the point and just say 1090 for example). This will change on a daily basis by about 10 points but this does depend on temperature and nutrition. After a few days the gravity will have typically dropped to 1.040 and will finish in the region of 1.000 to 0.990.
How does it work?
It will measure the amount of sugar in the liquid. The more sugar in the liquid, the higher the reading. The more alcohol there is in the liquid the lower the reading (that is after fermentation as the sugar has now been converted into alcohol). For example, if the hydrometer is placed in water at 20°C, it will read 1.000. This is always useful to know as you can test your hydrometer. In essence the denser the liquid (the more sugars in it) the higher gravity reading and as alcohol is thinner than water the lower the reading the more alcohol.
How do I use the Hydrometer?
A lot of people prefer to use a hydrometer with a trial jar. The trial jar is 200mm long and has a diameter approximately 35mm. It is made of clear plastic and allows you to fill this with the liquid you are about to test. Fill the jar to about 35mm from the top and simply lower the hydrometer into the liquid.
As illustrated to the right, you should take the reading from the lower of the two levels you see when looking at the side of the test jar. This reading is quite simply the Specific Gravity (SG). Be careful that the hydrometer does not stick to the side of the trial jar, its best to give it a gentle spin to prevent this. Always stand the jar on a flat surface and expect that some liquid might overflow. If the wine is still bubbling then the reading will only be a guide (as there will be bubbles in the liquid) and if froth (in the case of beer) is present then gently blow this away.
Most people only use the hydrometer as a guide but if you want to be really accurate then this should be done with a liquid temperature of 20°C. If the liquid is 5°C higher then add 0.001 and similarly if its 5°C lower then take off 0.001.
How do I use this to calculate ABV?
The ABV can we worked out really simply by taking the start gravity from the finish gravity and dividing this figure by 7.362. For example the starting SG is 1.080 and this ferments down to 0.990. The drop is 90 points. This divided by 7.362 is 12.23% ABV.
When making wine
Start Gravity should be 1.070 (normal finished ABV will be 10.5%) to 1.090(normal finished ABV will be 13%). Finish Gravity should be 0.990 (for dry wines) to 1.005 (for sweet wines).
We always recommend that wine should be fermented down to dryness and should you like a sweeter wine then add sugar or grape juice at the end to reach your preference in sweetness. It is a dangerous business stopping the fermentation early.
Finally we would say that the hydrometer is probably the most essential piece of equipment to make wine and beer consistently and with success. It’s also the best value for money as well.
When making beer
This is very difficult to guide you on as there are so many variations. However a typical beer will start at 1.045 (this is what you will see on the pump label in your local pub) and finish at 1.012 giving a 32 point drop (divided by 7.362) making it 4.5%ABV