In underwater diving, an alternative air source, or more generally alternative breathing gas source, is a secondary supply of air or other breathing gas used by the diver in an emergency. Examples include an auxiliary demand valve, a pony bottle and bailout bottle.
An alternative air source may be fully redundant (completely independent of any part of the main air supply system) or non-redundant, if it can be compromised by any failure of the main air supply. From the diver's point of view, air supplied by a buddy or rescue diver is fully redundant, as it is unaffected by the diver's own air supply in any way, but a second regulator on a double cylinder valve or a secondary demand valve (octopus) is not redundant to the diver carrying it, as it is attached to his or her main air supply. Decompression gas can be considered a redundant gas supply only when the risk of breathing it at the current depth is acceptable.
Read more about Alternative Air Source: Fully Redundant Alternative Air Sources, Non-redundant Alternative Gas Sources
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