breathing rate, due to exertion, stress or panic. Common configurations of diving cylinders and diving regulators used as a backup or reserve for emergencies include: An independent set carried by the diver; a complete, backup scuba set such as a "pony" (or bailout cylinder), or the cylinder not currently in use of an independent twin set, or a cylinder not currently in use of a group of side-mounted cylinders, or a separate rescue set brought down by the rescuer, or a "spare air" - a small independent set with integrated regulator and mouthpiece A potentially independent pair of twins connected by an isolation manifold and carried by the diver. If the diver can isolate them in time, the remaining gas in the uncompromised side is available. Sharing gas with another diver via an "octopus regulator" - a second, backup regulator or second stage on a scuba set that is in use by the buddy or rescue diver There are two main ways of delivering breathing gas to the out-of-air scuba diver; provide the casualty with a separate demand valve, preferably on a long hose or , so that rescuer and casualty can breathe simultaneously, and concentrate their efforts on getting to a safer place, or share a single demand valve with the casualty by "buddy breathing", which is a more risky procedure, and requires constant attention to breathing and management of the demand valve by both divers. The additional task loading of buddy breathing may reduce the capacity of the divers for other activities required for safe resolution of the emergency. The gas capacity of the cylinder is important. Divers breathing at depth consume more gas because the gas must be delivered to them at ambient pressure, and volumetric breathing gas consumption is driven by partial pressure of CO2. At the end of a deep dive they will need more gas to breathe during the longer ascent to the surface and during any decompression stops. The mixture of the breathing gas is important. Hyperoxic gases cannot be breathed safely below their maximum operating depth because of the risk of oxygen toxicity and hypoxic gas cannot be breathed safely in shallow water because the partial pressure of oxygen falls below that needed to sustain consciousness. A surface supplied diver can provide breathing gas from their pneumofathometer hose, which can be inserted under the neck seal of the helmet and turned on