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I know how to use my hydrometer, and I know what numbers to look at and write down on my brewing sheets, but I'm not sure what those numbers actually represent, or even what units I should use to label those readings.

What's the science behind the hydrometer? How are they calibrated? What else should I know about the theory of using one to make better beer?

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3 Answers 3

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Good Answer by Fishtoaster. The science is ancient, discovered by Archimedes.

1: Any object, wholly or partially immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object.

In other words, if you put a ball, with volume 1 litre completely under water, there is an upwards force on the ball (buoyancy or flotation) equal to the weight of 1 litre of water. (i.e. 1 kilogram-force or 9.81 Newtons)

2: (Corollary) Any floating object displaces its own weight of fluid.

If we place a floating object of mass 1 kilo, it will displace exactly 1 kilo of water, or 1 litre of water. If the volume of our object is greater than 1 litre (i.e. the object is less dense than water), it will float.

So, with a Hydrometer, it is weighted such that the weight of the submerged volume at the 1.000 reading is exactly equal to the weight of the hydrometer. If we dissolve solids into the water (sugar) that volume of water is heavier, and less of it needs to be displaced in order for the hydrometer's weight to be matched, and the Hydrometer floats higher.

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The general principle is based on density. That is, less dense things will float on top of denser things.

By analogy, imagine a human floating in a pool of water. Now imagine a human floating in a pool of saltwater- the human will float higher in the saltwater, since the density of the saltwater is higher.

A hydrometer works similarly- it's just a marked glass tube that's weighted such that in water, it'll sit at 1.00. If you put it in something denser than water (like wort, which is water with stuff in it), it'll sit a little higher out of the liquid, so it's at something like 1.050. That number is the density* of the liquid.

One thing to note is that hydrometers are calibrated to a specific temperature: 15˚C / 59˚F. If your liquid is not that temperature, you want to run your measurement through a conversion chart to account for the change in density of liquids based on temperature.

*Or rather, it's measuring the specific gravity of the substance, which is a dimensionless unit equal to the ratio of the density of the sample divided by the ratio of a reference liquid, both at a given temperature and pressure. The reference liquid here is pure water, and the sample is water with stuff in it (ie, wort).

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Heavy ships can float because of the principles of buoyancy. Hydrometers measure the specific gravity of liquids. Specific gravity is a measurement of the density of a substance compared to that of water. Density describes how densely the molecules of a substance are packed. Objects float more easily in liquids that are more dense because the more closely packed molecules support more weight. Buoyancy is determined by the amount of the force pushing an object upward in relationship to the downward force of air pressure. Liquids with higher density enhance the buoyancy of an object. Read a hydrometer where the surface of the liquid meets the stem of the hydrometer.

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