Why do I need a Hydrometer anyway?

Using a Hydrometer for Beer Brewing

A hydrometer is one of the simplest tools a home brewers has at their disposal, but also an important one so I thought I would spend a few moments this week discussing how to properly use a hydrometer and also how to adjust your hydrometer readings for temperature.  Most brewers rely on a hydrometer to determine their original and final gravity, and more advanced users will also track mash gravity and end of fermentation gravity.

What is a Hydrometer

A hydrometer is a very simple device that looks like a large thermometer.  When you immerse it in wort or finished beer it sinks to a varying degree depending on how dense the wort is and provides a reading of the specific gravity.  Most hydrometers used by home brewers are scaled for specific gravity readings, which is technically a unitless measure that generally ranges from 1.000 for water to 1.100 or higher for high gravity barley wines.  An average beer might have a starting gravity between 1.040 and 1.050 and a final gravity around 1.010.

The reason specific gravity is unitless is that is is simply a measure of the density of the liquid relative to water – so 1.000 would be the density of distilled water, and most wort or beer has a gravity slightly above that of water (1-10% higher actually).  To calculate the specific gravity of a liquid sample with known density, we just divide its density by the density of water – that is the specific gravity value.

Many professional brewers use hydrometers that measure in degrees Plato, which is another density system developed by Bohemian scientist Karl Balling in 1843 and later improved by Fritz Plato.  This scale is a measure of density relative relative to the percent sucrose in the water, so a reading of 11 degrees plato would be equivalent in density to water with 11% sucrose dissolved in it.

Converting from plato to specific gravity is not strictly linear, but most brewers use the approximation of 1 degree plato = 4 points specific gravity, so 12 degrees plato would correspond to 48 points of specific gravity, or a measure of 1.048 approximately.  For significantly larger values the approximation starts to drift off, so its best to use a calculator at that point (such as the one in BeerSmith).

Actually Using a Hydrometer

Use of a hydrometer is a pretty simple affair.  You typically remove a small amount of sample wort, place it in a clear sample cylinder and then immerse the hydrometer in the liquid.  Read the gravity reading from the scale on the hydrometer where it crosses the water-air boundary.  There will be a slight curve along the water-air line (called the meniscus), so if you want to be really accurate you should take the reading at the lowest point in that air-water curve (the bottom of the meniscus).

One final cautionary note – many beginners tend to take the sample in the tube that the hydrometer was sold in.  You need to be a bit cautious when doing so as the tube is quite small and the hydrometer will sometimes stick to the side a bit which could give you an inaccurate reading.  Ideally you want it floating freely in the wort, which is why more advanced brewers will purchase a small sample vessel or use another vessel to hold the sample.

Adjusting for Temperature

Hydrometers are all calibrated to be accurate at a standard temperature.  For most home brewing hydrometers, the calibration temperature is 60F (20C), though a few laboratory hydrometers are calibrated to a different temperature (usually 68F/20C).   The calibration temperature is usually printed on the scale of your hydrometer in really small letters.

Manufacturers calibrate the hydrometer to be accurate at their calibration temperature, and its often a good idea to validate that by cooling a sample of distilled water to that calibration temperature and verifying that your hydrometer reads 1.000.

If you use your hydrometer at another temperature other than the calibration temperature you should add or subtract a small adjustment to get an accurate reading.  In practice, if you are working near room temperature the adjustment is relatively small (typically one point).  However when you measure hot wort (such as wort coming from the mash tun or boiler) the difference can be significant and you should adjust your hydrometer for the calibration temperature.

The formula I use in BeerSmith is:

sg = sg_measured + sg_measured * (1.628E-5 * (tc – t) – 5.85E-6 * (tc*tc – t*t) + 1.532E-8 (tc*tc*tc – t*t*t))

where sg_measured is the measured value, tc is the calibration temperature and t is the temperature (both in celsuis the sample was measured at.  This gives a pretty accurate measure, but its not much fun to calculate by hand, so there is a hydrometer calculation tool in BeerSmith to do this adjustment for you.

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