A hydrometer is instrument that helps you determine the ABV in your homebrew or wine. How does it do this? Sugar is denser than alcohol, so the reading taken before pitching your yeast (OG or original gravity) and the difference in the reading once your wort/mash has been fermented (*FG or final gravity) is used to calculate the ABV. As the yeast turns the sugar into alcohol the reading on your hydrometer will show a lower number as the hydrometer will sink deeper into your liquid. This is due to the fact that once your liquid is fermented the alcohol is not as dense as the sugar so the hydrometer does not float as high. The temperature of your wort will also factor into this reading. Most hydrometers are set to a standard of 60F for use, but as always please read and follow the instructions that came with your hydrometer.
Another measuring device many homebrewers use are refractometers (most have automatic tempurature control). A brewing refractometer is an optical device that uses only a drop or two of liquid to give you a reading. If you use this instrument after fermentation has started the alcohol will skew the reading. So.. if you are using a refractometer for before and after readings you should also use a refractometer calculator (many of which may be found online) to obtain an approximate ABV reading. Once again due to the different manufacturers and types of refractometers you should always read and follow the directions that came with your product. Many refractometers need to be calibrated before each use in order to obtain the correct readings.
When buying a hydrometer you should make sure that it is NOT a Proof & Tralle if you are planning on using it for your homebrew. A Proof & Tralle hyrometer/alcoholmeter is for measuring spirits in distilled liquors, not for measuring the sugar/alcohol content in beer and wines.
Which of these two instruments is right for you? Once again I have scanned over many forums and read the comments, it seems to be a matter of personal preference. Many homebrewers use both, the refractormeter for the OG reading since the liquid is hot and you only need a few drops. They then use the hydrometer for the FG reading as the fermented liquid is at room or lager temperature so obtaining a sample to test is much easier.
Next week - I will take a look at some of the calculations used for determining your ABV once your have your OG and FG readings.
*Just a reminder - when you are taking your FG readings you should do so a few days apart to make sure that fermentation has completed. The FG should be the same for two readings in a row.
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