Trick I learned working at a brewery:
1) Fill up your sample tube farther than you would
2) Place sample tube in sink or other place you don't mind getting beer
3) Slowly lower hydrometer into tube, allowing beer to spill over the top of the tube (there, no bubbles!!!)
4) Give the hydrometer a healthy spin. This will knock out any bubbles that might be clinging to the hydrometer (and
possibly throwing off the reading)
5) Note where the
meniscus is on the hydrometer. The meniscus is the curve a liquid makes in a cylinder due to surface tension.
6) How to read the meniscus properly:
When your sample is in the tube the meniscus will look like it does in A. However, once you add your hydrometer, the meniscus will creep up the side of your hydrometer. The correct reading will be as such: