Aim: To check that you are properly weighted in the water.
Reason to Learn: One of the main reasons for divers using too much air and bumping into coral and bottom features is not being properly weighted. By checking for proper weighting, or performing a buoyancy check, you can ensure that you have the correct amount of weight based on your body, exposure suit, and equipment. You should do this check whenever you change diving locations, exposure suits or equipment, or haven't dived for a while.
Step One: Make sure you do this check in water that is too deep to stand in and is the same as the water you'll be diving in - ie. a freshwater swimming pool won't help to check weighting for diving in the ocean (which is salt water). If you have a full cylinder you should add approximately 2 pounds (1 kilogram) to compensate for the fact that your tank will become more buoyant throughout the dive.
You should start when you are relaxed and positively buoyant in the water.