This is essential for measuring your alcohol strength. First, to confirm you have run the still correctly and have achieved the highest percentage or purity your model can make, and secondly, to safely water your alcohol down to a strength that is safe and drinks well. This has a partial scale measuring 30-70% alcohol by volume so is really only suitable for Pot Still models, Air Still and Vodkamaker stills. Measurements are in 1% markings. Hydrometers are usually calibrated at 20C and are best used in a Hydrometer Testing Jar -under "accessories". To work out how much water to add use this basic calculation each time: Alcohol collected (in litres) multiplied by the Current strength (%) then divided by the Final strength required (%). This will work out your Final volume when actually diluted. For example if you had 5 Litres of spirit from your still at 50% a/v and wished to reduce it all to 35% for drinking 5 x 50 / 35 = 7.14 Litres when diluted Then simply substract the Intial volume of alcohol (in litres) from this answer to tell you how much water needs to be added 7.14 - 5 = 2.14 Litres of water to add to dilute. If you need help simply phone the shop.