All diving depends on a supply of breathing gas. Gas supply systems can be broadly classified into self-contained or SCUBA systems (properly known as self-contained breathing apparatus) and remotely supplied systems, where gas is piped to the diver through an umbilical tube.
SCUBA
The majority of scientific diving is undertaken using SCUBA systems. This system allows the diver greater freedom of movement than most remotely supplied systems. It is also the system taught by all recreational diving training agencies, the route by which most scientists initially learn to dive. SCUBA equipment is generally relatively lightweight, and low cost. Crucially for many scientific studies, the diver does not have to overcome the drag exerted by currents on a breathing gas umbilical (which can be considerable when the diver is operating a significant distance away from the umbilical tender). It is therefore much easier for the free-swimming diver to hover just above the seabed without being swept rapidly down-tide, or to maintain position on the seabed without being heavily weighted. These are crucial requirements for many scientific studies. SCUBA has the disadvantage that the dive profile is limited by the quantity of breathing gas a diver can carry.
Remotely supplied systems
Remotely supplied systems, where the diver's breathing gas is supplied via an umbilical either directly from a low-pressure compressor or from storage cylinders, is the most commonly used system for most civil engineering and offshore oilindustry divers. The remote supply is usually surface-supply, e.g. a compressor or air bank located on a boat or quayside, but it may also be a supply from a diving bell or a habitat to a tethered diver. Remote supply systems normally use positive pressure full-face band-masks or lightweight diving helmets fitted with hard-wire communication systems (see 'Voice communications' section). Such systems are 112 Methods for the Study of Marine Benthos, Third Edition