An in depth look at hydrometers and how they work!

At some early stage, for most brewers, a hydrometer becomes part of their brewing equipment. To most it’s a matter of “drop it in and read” but having encountered some unlucky readings, I decided to delve further in the mysteries of the hydrometer.

In theory a hydrometer is basically an instrument for measuring the density of liquids relative to the density of water, measured on a scale called specific gravity, at a given temperature.

In a practical aspect, when substances such as sugar, malt etc are dissolved into our wort, the increase in specific gravity (S.G.) which means there is actually an increase in weight. When our hard working yeast converts these sugars into carbon dioxide and alcohol our density decreases and we encounter a weight reduction or decreasing S.G.

Im unsure who actually devised the system, but it was given that pure water had an S.G. of 1.0000, (remember specific gravity being the ratio of a fluids density in relation to the density of an identical volume of water) Therefore our wort with and S.G. of greater then 1.000  is proportionally heavier than the same volume of water. Put into practice, 20litre wort with an initial S.G. of 1.050. will weigh 1.050 times heavier than the same volume of water.
Given that 1 litre of water =1kg
20 litres x 1.050 (S.G.) = 21 Kilograms

It is worth noting that before hydrometer existed, brewers of old used this method of weighting and comparing the wort against the weight of the same volume of water. Thank god for hydrometer!!!!

Most of this is seemingly quite straight forward, but there is one aspect that we do have to monitor whilst taking readings. Hydrometers are calibrated at a given temperature. The majority that we use in brewing are 20c. (the calibration temp can be found in the paper insert in the upper thin tube on your hydrometer) If we measure our a higher or lower temperature, as we sometimes do, an inaccurate reading will result.

This is because the absolute density of a fluid changes with the temperature, but the specific gravity doesn’t. Utilising the chart below, we can make adjustments and correct our readings.
Correction table for 20C hydrometers

Temperature (C) Correction
4-10 -2
11-17 -1
18-22 0
23-26 +1
27-29 +2
30-32 +3
33-35 +4
36-38 +5
39-41 +6

Hopefully now, when you drop your hydrometer into your test, you’ll be somewhat more informed and have a batter understanding of what is actually happening. Listed below are some other helpful hints to ensure correct readings.
Handle with care – I’m yet to hear of one that has bounced.
Check the calibration of your hydrometer – Floating in water it should read 1.000.
When testing and sample, spin it in the test flask to dislodge and clinging oxygen bubbles, and ensure the hydrometer isn’t clinging to the flask sidewalls.
Take readings at eye level to prevent distortion, and never return samples to your fermenter, as the risk of infection to your beer will be extremely high!
After taking S.G. reading, remove the hydrometer and do a taste test, it should taste like hot flat beer, if it taste unpleasant or fruity you may have an infection!

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