Air-sharing drills are usually rather civilized affairs. One diver signals that they're out of gas, the other presents a regulator, both exchange a reassuring OK, and they begin a nice, controlled ascent. Everyone knows their part, nobody is particularly surprised, and the instructor watches approvingly from a few feet away.
That's a perfectly reasonable way to introduce the skill. Unfortunately, it isn't necessarily how an actual out-of-gas emergency will unfold.
Imagine reaching for your next breath and discovering there's nothing there. You try again, perhaps thinking the regulator has shifted or something is obstructing it. Still nothing. Your buddy is nearby, but they haven't noticed anything is wrong, and suddenly the distance between you feels considerably greater than it did a few seconds ago.
At that moment, you're probably not thinking about which hand signal you should use or whether your buddy prefers primary donation. You want something to breathe from, and you want it immediately. Your buddy, meanwhile, is about to discover that the diver approaching them may not be particularly interested in following the sequence they practiced in class.
This is what makes gas sharing more complicated than a regulator exchange. There are two divers involved, one of whom is already in trouble, and both need to get safely to the surface.
When You're the Diver Who Needs Gas
The first thing to understand is that an out-of-gas emergency may leave very little room for thoughtful decision-making. You might recognize the problem immediately, or you might spend a precious moment trying to understand why the regulator isn't delivering gas.
If your primary regulator has failed but gas is still available from the cylinder, your alternate second stage may solve the immediate problem. But if the cylinder is empty or the gas supply has otherwise been lost, switching to another regulator connected to that same supply accomplishes nothing. You need your buddy.
Get their attention as quickly as you can. Use the out-of-gas signal if you're able, but don't assume you need to complete the signal before moving toward an available breathing supply. A buddy who is watching something interesting on the bottom may have no idea you're having an emergency until you are directly in front of them.
Ideally, your buddy recognizes the situation and presents a working regulator. You take it, clear it, and begin breathing. In practice, the instinct to breathe may be considerably stronger than the desire to follow procedure. Divers have been known to reach for the regulator already in someone else's mouth, particularly when they are frightened or struggling.
That possibility is worth acknowledging rather than pretending everyone will behave perfectly. It is also a good argument for staying close enough to your buddy that getting help doesn't become an exhausting underwater pursuit.
Once you have a breathing supply, resist the temptation to treat the emergency as finished. Your buddy needs to be breathing too, and you still have an ascent to make together. Get control of your breathing, establish contact, and make sure you're both ready to move. The last thing either of you needs is to lose the donated regulator or become separated on the way up.
When You're the Diver Providing Gas
Being the donor sounds like the easier assignment, at least until someone who cannot breathe comes toward you with considerable urgency.
You may have only a moment to recognize what is happening. Perhaps your buddy gives the expected signal and waits for your response. Perhaps they simply appear in front of you and reach for a regulator. Either way, your first job is to get them breathing without losing your own breathing supply in the process.
How you accomplish that depends on your equipment. With primary donation, you give your buddy the regulator you've been breathing from and switch to your backup. With a conventional octopus arrangement, you provide the alternate while continuing to use your primary. There are good reasons to prefer one arrangement over another, which I've covered in Long Hose vs. Standard Octopus.
What matters here is that you know your equipment well enough to respond when the encounter doesn't go according to plan.
This is especially important if your buddy grabs the regulator from your mouth. Trying to retrieve it or insist that they take a different one is unlikely to improve the situation. Find your backup, establish your own breathing supply, and deal with the equipment arrangement once both of you can breathe.
The donor also needs to avoid becoming so absorbed in the other diver's emergency that they stop paying attention to their own surroundings. You still have buoyancy to manage, a depth to monitor, and potentially a frightened diver holding tightly to you. If the donated regulator is on a short hose, your range of movement may be particularly limited.
Once the immediate panic has passed, check that your buddy is actually breathing comfortably. Make sure the hose isn't trapped or stretched and establish a position that allows you to remain together without interfering with either diver's equipment. What you need now is enough control to make a safe ascent, not a perfectly executed classroom demonstration.
Getting Back to the Surface
One of the more dangerous assumptions about air sharing is that the difficult part ends when the recipient takes the first breath. In reality, that is when the second part of the emergency begins.
You now have two divers breathing from one gas supply, and at least one of them may be consuming considerably more gas than usual. You may also be dealing with awkward hose routing, unfamiliar equipment, poor visibility, or a buddy who is still frightened. None of these circumstances makes controlling your ascent any easier.
Stay together and begin ascending without unnecessary delay. Both divers should participate in controlling buoyancy rather than expecting the donor to manage everything, although the donor may initially be in a better position to monitor depth and ascent rate. Maintain contact without preventing either person from venting gas or reaching their equipment.
The temptation to rush upward is understandable, particularly for the diver who has just experienced the fear of being unable to breathe. But an uncontrolled ascent creates another emergency. Keep breathing normally from the available supply, vent expanding gas, and ascend at a controlled rate.
For a recreational dive without mandatory decompression obligations, a discretionary safety stop should not become another obstacle to getting home safely. If the remaining gas cannot support the stop, continue to the surface. A gas emergency is not the time to sacrifice an adequate breathing supply for an otherwise sensible precaution.
Once on the surface, establish positive buoyancy and stay together until both divers are stable. Keep your regulators available if conditions warrant them, attract attention from the boat or shore as necessary, and make sure neither diver is left struggling while the other begins removing equipment.
Whatever caused the original problem can be investigated later. The dive is finished.
The Dive You Planned Before Getting Wet
A surprising amount of the success or failure of gas sharing is determined before anyone enters the water.
If your buddy uses primary donation and you use a conventional octopus, you should each know what to expect. Better yet, show one another exactly where the regulators are and how they will be deployed. What seems perfectly obvious when you're familiar with your own equipment may be anything but obvious to someone encountering it for the first time.
The same applies to gas planning. Having a regulator available for donation is of limited value if you don't have enough gas remaining to bring two divers to the surface. A realistic minimum gas reserve accounts for the possibility that someone other than you will need to breathe from your cylinder, potentially while both of you are under considerable stress.
And then there's proximity. I regularly see buddy pairs separated by distances that seem perfectly acceptable until you imagine one of them suddenly running out of gas. A buddy who is too far away to help promptly is not providing much of an emergency gas supply, regardless of how well either diver performs the donation drill.
Practice matters too, but it should resemble actual diving rather than an exercise performed while kneeling motionless on the bottom. Sharing gas while maintaining buoyancy, adjusting position, managing a hose, and ascending together introduces complications that are largely absent from the introductory drill. Those are the circumstances in which familiarity with your equipment and your buddy begins to pay off.
None of this requires assuming that an out-of-gas diver will panic or behave irrationally. Plenty of divers recognize the problem, communicate clearly, and handle the emergency with admirable composure. We simply shouldn't build our entire response around the expectation that everyone will.
When someone cannot breathe, their immediate concern is getting the next breath. When you're the person providing it, your responsibility is to help them without becoming another casualty. The exchange of regulators is only the beginning. What matters is that you can both breathe, regain control, and get out of the water together.