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It floats! A practical guide to using your hydrometer effectively!

Posted by Info BeerLab on


Of the two scientific instruments most commonly used by home brewers the hydrometer is certainly the least well understood. We all understand thermometers -temperature degrees are a part of everyday life. But what about relative density, specific gravity or degrees Plato? And then there’s that mysterious meniscus!  Using a hydrometer correctly takes much of the guess work out of brewing and is an indispensible measuring tool. This article will explain how -and also when- to use your hydrometer, to really get the most from it!

For brewers the hydrometer achieves three simple tasks. It allows a way of determining the sugar content of the wort; monitor the progress of fermentation, and, with some simple maths, to calculate the alcohol content of the finished brew. For grain brewers, it can also help measure how efficiently the mash extracts sugars.

A hydrometer reading is literally a calibrated measurement of how high (or low) it is floating in our wort or beer. This reading is higher in denser liquids, and lower in thinner ones. -So what makes a liquid denser? In this case it is dissolved maltose sugar (and some dextrin) that float the hydrometer higher, and later, as yeast gradually depletes the sugar, drop it lower.  Pretty simple- but how do we make this work for us?

Calibration, readings and temperature

Firstly let’s take a closer look at how hydrometers are calibrated. Specific Gravity (S.G.) is how home brewers measure the density of their wort or beer. This is a weight measurement, and is relative to pure water, which represents a reading of zero; this is written in Specific Gravity as 1.000. A 10 percent weight of sugar to weight of water gives us a reading of 1.040. The S.G. is often written as 1040 or simply 40.  Brewers simply refer to this as a ‘gravity reading’, and use two common abbreviations according to when the readings are taken during the brewing cycle. They are:  Original Gravity (O.G.) and Final Gravity (F.G.). The O.G. reading is taken before fermentation commences and the F.G. at the very end.

So how do we actually take a reading? A hydrometer is supplied (usually) with a tall measuring flask. This holds enough liquid to float the hydrometer in without it touching the bottom. A sample of wort or beer is taken and the hydrometer floated in it (this is best done over a sink). Place on a counter and wait a few seconds for it to come to rest. Then take a reading from the surface of the water- not where the water meniscus reaches up! You now have a specific gravity reading- remember to write it down on your recipe sheet!

A quick word about the temperature:  Modern hydrometers are typically calibrated to read at 20C and a higher or lower temperature liquid affects the accuracy. In practical terms a few degrees either way is not that noticeable, but larger jumps of 5-10 degrees should be taken into account. A warmer temperature of 32C (typical of warm wort) would give a difference of plus 1.003. Our reading of 1.040 would actually be 1.043.  This is because warm dissolved sugar isn’t as dense, and this affects the reading. Cooler temperatures give the opposite effect- at 10C you would minus 1.002 from 1.040 giving you 1.038. A calibration chart or online calculator is useful to make adjustments when necessary.

What to expect

Brewers take an Original Gravity sample reading before the fermentation has begun and the temperature is close to room temperature (18C-22C). The sample is then discarded or tasted, but never returned to the fermenter. The likelihood of contamination is too great a risk for the few mils of lost beer! To give you an idea what to expect, for a beer of average strength an O.G. reading would typically be between 1.042-1.058.

There is a common habit amongst beginner brewers to take several sample readings towards the end of the fermentation to ascertain progress (and from curiosity). This is well and fine if your fermenter is equipped with a tap, but if it means opening the fermenter each time, rather avoid doing so, as it opens the beer to airborne contaminants like wild yeast and bacteria. The less you putz with your beer the better!

Experienced brewers will usually wait the full 14 days until fermentation is completely finished and only then take the F.G. reading, usually before commencing bottling or kegging. At this stage it’s easy to see ( and taste) whether there has been a problem with the fermentation or not. ( For more information on this see the article”Is my beer ready yet?”).

Confusion can often arise at this stage, as the gravity reading may seem too high, even though the beer has stopped fermenting. The reason for this is un-fermentable complex sugars and dextrin. These add flavour and body to your beer, and, as the yeast cannot break them down, they remain in solution and cause the reading to be higher. Beers typically finish anywhere from 1.005 to 1.016, with drier beers on the low side, and rich malty ones fairly high. As a general rule the F.G will be 20-25% of the O.G. for example a 1.040 beer will read 1.010 at the end of fermentation.

If your gravity reading seems higher than expected (according to recipe or experience), let the beer ferment a little longer. Always be cautious, as unfermented beer can over-carbonate bottles or-even worse- cause them to explode!

Recipes, Target Gravity and Alcohol by Volume

Where our hydrometer really comes into its own is when working with recipes. Any good recipe will supply the expected O.G. and F.G for the beer. And, with experience it becomes possible to hit these numbers exactly (or be only one or two degrees out at most). Brewers refer to these as ‘target gravities’ and each beer style has its own typical range.

For example, from the BJCP Style Guidelines here are the average gravities for a Dry Stout: O.G.1.036 to 1050, and F.G. 1.007 to 1.011. Knowing this, if your O.G. was 1.065 it would mean you were brewing a stronger beer than the style usually is – something more like a Foreign Extra Stout in fact (O.G.  1.056-1.075)!. A higher original gravity can often means a higher final gravity, especially with malty beers.

From your two readings, determining the alcohol content is surprisingly easy. To do so, subtract your F.G. from your O.G. drop the decimal and multiply by 0.129. This will give you the standard Alcohol by Volume percentage.

E.g.: O.G. 1.058 - F.G. 1.016 = 0.042              42 X 0.129=5.41% ABV

And there you have it- a quick tour of the hydrometer! To cap off, it’s interesting to note that on many hydrometers there is a choice of calibrations. Apart from S.G. there is often Degrees Plato and sometimes ‘Potential Alcohol’ which is a fairly rough indicator of what percentage to expect. Degrees Plato expresses density as grams of sucrose per hundred grams of liquid- i.e. 10 degrees Plato would mean 10% sucrose by weight to 100% liquid. It’s a more common calibration amongst professionals and crops up only occasionally in homebrewing.

Prior to the invention of the hydrometer a rather cumbersome method of weighing a barrel of wort and comparing it to the weight of a barrel of water was used. The wort barrel weighed more because of the dissolved sugar in it! Fortunately you will find your hydrometer a much easier option!

Good Brewing!

Copyright: Nick Birkby. Used with exclusive permission to Beerlab.