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Hydrometer 101

Or how to measure the alcohol content of a liquid without gunpowder – the definitely more fun yet not to be recommended method of yore, described in our previous post.

Enter the hydrometer, a glass tube, weighted on one end and with graduated markings on it. It works like a fishing float that indicates the density, or specific gravity of a liquid, compared to water. As alcohol is thinner than water, the higher the alcohol content, the deeper the float sinks. Pure water has a specific gravity of 1.000 on the hydrometer scale.

Temperature is a key factor when measuring the gravity of alcohol: if it’s too low, it will be “heavier”, if it’s too high, it will be “lighter” and the hydrometer will sink lower in it, making you think your distillate has got more alcohol in it – which, alas, is wrong.

The hydrometer should indicate the temperature it’s calibrated to, and also include an adjustment table. A standard measuring temperature is 70 °F or 20°C.

Here’s how to use it:

- Fill your hydrometer jar about 2/3” from the top with the liquid you want to test. Insert the hydrometer slowly not allowing it to drop.

- Give the hydrometer a spin using your thumb and index finger. This will remove the air bubbles that may have formed. Also, makes sure it’s not sticking to the sides of the jar.

- Read where the surface of the liquid cuts the scale of the hydrometer.

- Read again. A small raised collar of liquid will adhere to the hydrometer, above the surface. To get the accurate, true surface level reading, you must look across the top of the liquid.

- Test the spirits before adding any flavorings or other additives, as they alter the specific gravity of the liquid and will distort the hydrometer readings.

Now, you surely can play your whiskey still by ear – and taste. But knowing how a hydrometer works will give you a couple of solid conversations starters, and using it will add a touch of science to your distilling endeavors.

It will not only tell you the strength of your spirits, but also help you control the fermentation process by indicating how much of the material has been turned into alcohol, and also measuring the potential alcohol content of your mash. Which is a subject for an advanced hydrometrology post.

Posted by Jason Stone on


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