Most people are naturally neutrally buoyant. With a normal relaxed lung volume, they neither float nor sink. Take a deep breath and you will float. Empty your lungs and you will sink. However, as soon a diver puts on a wetsuit, more water is displaced and the diver will float. It is necessary to strap on some ballast in some form to counteract the buoyancy of the suit and this is most convenient and commonly supplied in the form of lead weights.
Most divers simply thread their block-style lead weights on to a length of 5cm webbing and use it as a belt. If you need more comfort than a weight belt, a harness can help. Another way to carry lead is in the form of lead-shot and that's where a shot belt or one that takes shot in pouches comes in useful. If you use a lightweight aluminium diving cylinder you may prefer to strap some V-shaped lead to that. There are other systems that allow you to strap part of the weight to your tank. Otherwise all ballast should be possible to ditch in an emergency and that's why all systems have a quick-release buckle of some type designed in conjunction with them.
The question always asked is: How much lead should I use? The answer is not easily given because it depends on the size and thickness of the suit, the buoyancy or lack of it of any other equipment (aluminium tanks can be near neutral whereas steel tanks are always very heavy even when immersed), and the density of the water being dived in.
Sea-water displaces more weight than fresh water so you will need more weight when diving in the sea than in a lake. A thick neoprene suit with bulky jacket over displaces more water than a thin 3mm suit alone. Don't forget that the air in your tank has weight too. A full 12 litre cylinder is nearly three kilos heavier than when it is empty.
With new equipment at a new location every diver will need to a weight check. Whatever system you use and however much weight you use, you should check that you are neutrally buoyant with a nearly empty tank and full equipment by checking that you float vertically with your eyes just above the surface with a full lung of air and that you sink under the water when you exhale fully.
Alas, many divers learn to dive with plenty of lead simply to keep them on the bottom of the pool at a time when they might be nervous and breathing with large lung volumes but carry on using inordinate amounts of ballast during routine diving. They never get to experience the joy of diving completely because they area always having to fin forwards in order to stay at one depth. This means they use a lot more air than they should too. A relaxed diver has just the right amount of weight to complete the dive with a tank that is nearly empty.
Some BCs have an integrated weight-system. This is normally composed of two side pockets that can contain pouches, which themselves contain the weights. These must be retained securely at all times until the moment when you want to release them. At this time it should be easy to rip them away in order to either pass them up into a dive-boat or jettison them in an emergency. Modern integrated-weight pouches are retained by a mixture of Velcro covered flaps and patented quick-release buckle systems. Some BCs also have additional weight pockets for 'trim-weights'. These are pockets that cannot be accessed to drop the weight during a dive and are usually positioned high up at the back to neutralise the effect of a buoyant aluminium tank.
Happy Diving - John Bantin