Homebrewing is a science as much as an art. A lot of right brain AND left brain stuff going on. Creating something that is subjective and personal taste driven, but at the same time, has processes that are fairly exact and deviating from them can be detrimental, or downright disastrous.
I have frequently described to fronds that ask me about it, that it is kind of like a cross between cooking and a mad scientist experiment…that you get to drink when its done.
One of the parts of the process is measuring the amount of dissolved sugars in the solution just before and after fermentation is done. Mostly this is so that you can then calculate the amount of alcohol present in the finished product. This is referred to the “specific gravity” of the solution. With 1.00 being equivalent to water, 1.100 would be quite a lot of dissolved sugars and 1.010 would be the low range, of a finished beer, after the yeast ate up all they could handle, and left you with alcohol and carbon dioxide.
You can measure this is two basic ways. One is by filling a tall narrow container of the solution and floating a hydrometer in it. The higher this device floats in the solution the higher the specific gravity is of the solution you are testing. Basically the solution “pushes” up the hydrometer and there are graduation marks on the side that you can eyeball and take a measurement.
There are a couple factors that complicate this measurement. One is temperature. If you are measuring something hot, the reading will be a bit lower than that same sample would be at room temperature. So you have to adjust for that. Also you have to waste several ounces of your precious beer to take a measurement like this since you have to pull out enough to float the hydrometer.
The other way to measure specific gravity with a refractometer. A small device that looks 
a little like a mini telescope. You open a little flap at the end of the deice and put a single drop of the solution on it, close the flap and look through the scope and as the incoming light is “refracted” you can read it against a graduated scale as you are looking through it. This way is NOT temperature affected and only requires a drop or two of what you are measuring. Two pretty good advantages. They cost $30-$50 though.
Homebrewfinds is a good site that shows sales and bargains for hombrewing ingredients and equipment. You can follow them on twitter @homebrewfinds Recently they posted a refractometer for only $20 or so. I might have to pull the trigger on that one.
