How do I properly use a hydrometer? How much unfermented wort do I put in the vial to get a good original gravity, possible alcohol by volume reading?
The hydrometer must be able to float freely with a little free space under the bottom of the bulb.This is often done using a "test jar" (vial) which is a tall thin vessel that just takes the hydrometer with a small (eg. 1cm) gap each side. The side clearance must not be too small or surface tension effects may come into play. The jar is filled from the brew vessel and the sample usually discarded after use. The test jar is usually glass or plastic and so the float level can be easily observed through the sides of the test jar. Remember to take the reading from the normal level of the liquid and not from the top of the "meniscus" of liquid that "curves" up the side of the hydrometer stem. An example here
Many brewers just steralise their hydrometer with very hot water, cool with tap water and put the hydrometer directly in the fermentation vessel. It can be more difficult to read accurately but as a general method it does work. It also saves beer if many readings are taken.
I use a 100ml Pyrex graduated cylinder. Usually only needs 80ml or so. The plastic tube they come in works good too and uses only 40ml or so. Hydrometer only needs enough volume to float.
Get a good one! One with built in thermometer is very useful. Make sure yours reads 1.000 with distilled water. If not, note and adjust the offset to all readings taken when used.
Let the wort cool to the optimal temp for your hydrometer, usually 70°F to avoid having to do temp calibrations correct readings. All hydrometers are different follow the data sheet included with your hydrometer. Some are even as low as 50-60°F.
Decarb finished beer for reading. Let it stand so there's no bubbles attached to hydrometer and foam on top falls in.
Read from the lowest level of the solution surface not the top of the solution that creeps up on walls and hydrometer. From meniscus.
Spin the hydrometer to free bubbles and center.
Do not add test sample back to the beer! Many will say you can sanitize the test vessel and hydrometer and return it. IMO it's not worth the risk and hassle, just keep your tools rinse clean and discard samples. (pre boil wort you can toss back into the boil with no problem)
Baseline. Compare brew water readings with distilled and adjust further readings accordingly.
Upgrade. There's a better tool for prefermentation gravity readings and only uses a few drops of wort. Buy a refractometer, you won't regret the purchase.