Buddy breathing

Buddy breathing is a rescue technique used scuba diving "out of gas" emergencies, when two divers share one demand valve, alternately breathing from it.

Running out of breathing gas generally happens as a result of staying underwater too long or breathing equipment failure. Although simple lack of judgement is a common reason for staying underwater too long, there are others: the need to complete decompression stops before surfacing and the need to leave an overhead environment such as a shipwreck or a cave before surfacing. Equipment failure resulting in the loss of all gas could be caused by failure of a pressure retaining component such as an o ring or hose in the regulator or, in cold conditions, a freezing of water in the regulator resulting of a free flow from the demand valve.

Buddy breathing is discouraged by many training agencies because other more reliable techniques and equipment exist. The technique needs training and regular practice by both divers if it is to be used successfully in a crisis; panic and task loading being the main reasons for it failing.

Many divers fit a second demand valve, often called an "octopus", to their diving regulators, which is for emergency use by another diver. Divers doing deep diving or decompression stops routinely carry a complete, independent backup scuba set for their own or their buddy's emergency use. See diving cylinder for more details of breathing equipment configurations.