Breathing tube (breathing apparatus)
A breathing tube is a flexible tube for breathing through, as part of a scuba set or other breathing apparatus or a medical oxygen apparatus or anaesthetic apparatus (Here they are distinguished from the medium-pressure hoses which are often found as parts of modern breathing apparatus.)
They are wide, and usually corrugated to let the user's head move about without the tube pinching at kinks.
Each end usually has a screw connection. They may contain a one-way valve to keep the air or gas flowing the right way.
According to the expected usage, they may be pure rubber, or rubber reinforced with canvas (outside or embedded) or similar. If the canvas layer is outside (as in the Siebe Gorman Salvus and the IDA71 and some old gasmasks, it protects the rubber from damage from scrapes but makes it harder to wash all the salt off after a saltwater dive.
Breathing tubes have to be long enough to connect the apparatus to the diver's head in all attitudes of his head, but should not loop about excessively to cause hydrodynamic drag or risk snagging snag on things, or contain excess dead space. In the IDA71 the looping over-the-shoulders breathing tubes can be tethered down to the diver's shoulders. See also in this image of a modern military rebreather.
Some early rebreathers had one breathing tube, which the breathed gas went through both ways: this is called the pendulum system; others have two breathing tubes, with one-way valves keeping the gas flowing the right way.
In some rebreathers, parts of the apparatus are connected to each other by lengths of breathing tube, resulting in more lengths of tube in the apparatus: