Water Works

Is it possible and safe for two Scuba Divers to share a tank?

like one person carrie the tank and all and the other just breath off of another regulator? or is it possible to just buy a 9 ft octo line and hook up another regulator and have two people dive like this?

Possible–yes. In normal recreational ‘buddy-system’ diving, if one diver suffers an out-of-air (OOA) emergency (either through inattention or gear failure), their buddy can share air in this fashion, allowing both divers to make a safe(r), slow, controlled ascent while continuing to breathe normally.

Safe–well, that really depends on the actual parameters of the planned dive (environment, max. depth, planned bottom time, intended task), but I certainly wouldn’t recommend it as a general practice. Reasons being (among others):

(1) Sharing a single cylinder+reg system between two divers means that there is no redundant supply of air available in the event of an OOA emergency. Both divers will therefore be forced to make rapid, (un)controlled emergency ascents, either by ditching their weights or swimming up, exhaling all the way to the surface to release expanding air, putting both divers at high risk of lung overexpansion injuries, decompression sickness, and/or drowning. (This is why new divers are trained to use octopus regs only for emergency ascents, and not to prolong a dive–and certain of my former colleagues would also do well to remember that!)

(2) The diver on the long octo line is not phsyically attached to the gear, so an inattentive or careless movement by his/her buddy may pull the octo-reg out of that diver’s mouth unexpectedly, causing him/her to inhale water and increasing the risk of choking/drowning. (To avoid this situation, the non-gear-wearing diver must either hold tight to the octopus mouthpiece, or to their buddy, or both, throughout the dive, thus restricting the mobility of both divers.)

(3) Sharing a single cylinder between two divers for an entire dive means that cylinder-duration for any given depth will be approximately halved compared to that for a single diver, increasing the risk of running out of air unexpectedly at depth. (Even if air supply is carefully monitored, the divers may have insufficient ‘air-time’ to accomplish their intended purpose).

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