In modern scuba sets, a buoyancy compensator (BC) or buoyancy control device (BCD), such as a back-mounted wing or stabiliser jacket (otherwise known as a `stab jacket`), is built into the scuba set harness. Although strictly speaking this is not a part of the breathing apparatus, it is usually connected to the diver`s air supply, in order to provide easy inflation of the device. This can usually also be done manually via a mouthpiece, in order to save tank air while on the surface. The bladders inside the BCD inflate with air from the ‘direct feed’ to increase the volume of the SCUBA equipment and cause the diver to float. Another button deflates the BCD and decreases the volume of the equipment and causes the diver to sink. Certain BCD`s allow for integrated weight, meaning that the BCD has special pockets for the weights that can be dumped easily in case of an emergency. The aim of using the BCD, whilst underwater, is to keep the diver neutrally buoyant, i.e. neither floating up or sinking. The BCD is used to compensate for the compression of a wet suit, and to compensate for the decrease of the diver`s mass as the air from the cylinder is breathed away.
Diving weighting systems, ranging from 2 to 15 kilograms, increase density of the scuba diver to compensate for the buoyancy of diving equipment, allowing the diver to fully submerge underwater with ease by obtaining neutral or slightly negative buoyancy. While weighting systems originally consisted of solid lead blocks attached to a belt around the diver`s waist, some modern diving weighting systems are now incorporated into the BCD. These systems use small nylon bags of lead shot pellets which are distributed throughout the BCD, allowing a diver to gain a better overall weight distribution leading to a more horizontal position in the water. There are cases of lead weights being threaded on the straps holding the cylinder into the BCD.
Many modern rebreathers use advanced electronics to monitor and regulate the composition of the breathing gas.
Some scuba sets incorporate attached extra stage cylinders, as bailout in case the main breathing gas supply is used up or malfunctions, or containing another gas mixture. If these extra cylinders are small, they are sometimes called "pony cylinders". They often have their own demand regulators and mouthpieces, and if so, they are technically distinct extra scuba sets.
The diver may carry two or more sets of breathing equipment to provide redundant alternative gas systems in the event that the other fails or is exhausted. Modern recreational rigs most often have two regulators connected to a single tank, in case the primary regulator fails or another diver runs out of air. Some divers instead connect their backup regulator to a smaller "pony cylinder" for extra safety, and there are also emergency systems which mount a simple regulator directly to the top of a small cylinder. Rebreather divers often carry a side-slung open-circuit "bail out" to be used in the event the rebreather fails.
In technical diving, the diver may carry different equipment for different phases of the dive; some breathing gas mixes may only be used at depth, such as trimix and others, such as pure oxygen, which only may be used during decompression stops in shallow water. The heaviest cylinders are generally carried on the back supported from a backplate while others are side slung from strong points on the backplate.
When the diver carries many diving cylinders, especially those made of steel, lack of buoyancy becomes a problem. High capacity buoyancy compensators are used to allow the diver to control his or her depth.
An excess of tubes and connections passing through the water tend to decrease diving performance by causing hydrodynamic drag in swimming.
Some diver training organizations and groups of divers teach techniques, such as DIR diving for configuring diving equipment.