What does a hydrometer measure?

Question

OK, technically it measures fluid density, but what I really want to know is what it's telling me during beer brewing. Something to do with starting off with lots of sugar and ending up with alcohol.

A nice technical description of what's going on including appropriate numbers would be appreciated.

Feel free to give me links to useful sites.

Answer 1

In beer-making terms, a hydrometer tells you how much sugar, and other stuff, is in your beer.  However, all you're interested in is sugar.  As the sugar gets eaten by the yeast and turned into alcohol, the hydrometer drops because sugar is way heavier than water, while alcohol is just a little bit lighter.  When the hydrometer stops dropping for 3-4 days straight, fermentation is complete (as the yeast is no longer turning sugar in to booze) and you can either bottle or rack.

There are no appropriate numbers, as every beer is different.  I've had beers with starting gravities above 1.10 (imperial stout) and lower than 1.45 (bohemian pilsner) with finishing gravities anywhere from 1.025 (very sweet and heavy) to 1.005 (ultra-light).  Just ferment until it finishes.

Answer 2

http://www.defalcos.com/hydrometer.htm

Answer 3

It's a comparative measure of the density of the liquid you are measuring to that of water.  So basically it's telling you how much c**p is in solution vs. that of water.  In the case of brewing that c**p would be sugars of all shapes and sizes.  As wort ferments the sugars are consumed so lets say you start with a reading of 1.080 and wind up with a measure of 1.040 what this would represent is the amount of dissolved sugars the yeast have converted in to alcohol (and CO2).  Beer to my knowledge will never ferment back to 1.000 as that residual number is an indication of sugars which are not fermentable such as long chain starches which weren't converted during the mash.

So you can then take that difference and plug it in to a formula specific to figuring out ABV, that I can't tell you anything about as I'm sure there's probably some mathematical reasoning going in to it, not my forte.  

Hydrometer readings are very sensitive to temperature as well and must be corrected accordingly.

Answer 4

You start off with a lot of sugar in your wort. This makes the original gravity (OG) high. As the yeast ferments the sugar, it gets turned to alcohol, which has a lower specific gravity and you wind up with a final gravity (FG) that is less than the OG.

(OG - FG) x 105 = ABW

ABW x 1.25 = ABV

where ABW = Alcohol by weight and ABV = alcohol by volume.

Answer 5

Specific gravity in fluids.

Answer 6

Look at the word. Hydro-(which means "water") and meter (which means to measure.) So it measures the amount of water in something.