Every year, divers enter the water with their tank valves off, resulting in interrupted dives — and worse.

Speak to dive guides and instructors the world over, and you’ll realize it’s a more common scenario than you’d expect: a diver descends to begin a dive, only to suddenly realize he’s got no air. Those who remember their training dump their weights and head for the surface, performing a controlled emergency ascent. Some, tragically, panic and rush to the surface, risking decompression illness, or drown.

Out-of-air (or more correctly, out-of-gas) situations are among the most common causes of dive fatalities, and a surprising number of these occurs right at the beginning of a dive, such as in the example above. The problem isn’t usually the fault of the diver having depleted the tank in the first few minutes of the dive, but that there was a closed tank valve to begin with, or it was only partially opened.

A good equipment assembly and check process should of course include both turning on the tank valve and checking that it is indeed opened, often by breathing in one of the regulators. This is then double-checked by looking at the manometer to confirm that the operating pressure of the tank is sufficient for the dive. This is then reconfirmed during the buddy check. Ideally, these multiple checks will catch any situation where a diver has a closed tank valve, but the world is far from ideal.

The Divers Alert Network (DAN) has consulted two experts, Peter Buzzacott and Gareth Lock, on the issue in their recent edition of Alert Diver, the leading DAN publication.

The Reasons

The out-of-air situation often occurs due to one of the following reasons:

1. Divers have failed to open their tank valves all the way. This can happen either due to an interruption when the diver was assembling his gear or through a simple error. The tank valve may be open far enough that it allows sufficient air to pass through to supply the diver with air at the surface, but as the diver descends, they’ll consume increasing amounts of air, maxing out the capacity of the valve in its partially open state.

2. Divers accidentally partially close their valve after opening. This is usually due to the practice of opening the valve all the way and then turning it back a quarter or half turn. If a diver exaggerates this final part, they may find themselves closing a valve partially, which means it won’t be able to supply enough air at depth.

3. Divers turn their tank valves off, but don’t reopen them. If some time passes between assembling the scuba unit and the dive, many divers will turn off their valves and then forget to reopen them before the dive. The remaining gas in the system will give them a reading on the manometer, and even a few breaths from the regulator before the air is gone, making it harder to spot the error.

4. Another diver accidentally turned the valve off. Sometimes other divers, in an attempt to be helpful, may turn the valve of a seemingly unattended scuba unit off, or may accidentally turn the valve off thinking they are in fact turning it on. As with the example above, the remaining pressure in the system may make this hard to catch.

Avoid The Closed Tank Valve Scenario

DAN recommends the following to avoid out-of-gas situations early in the dive:

1.  Always do a complete gear assembly and check, possibly aided by a checklist to ensure all steps are covered.

2. If a gear assembly and check is interrupted, go through all the steps again to avoid accidentally skipping one.

3. Open valves all the way, avoiding the classical practice of opening them and turning them back a quarter or half turn. This practice stems from when valves were made of brass, and very susceptible to getting stuck if the brass cooled, but modern valves aren’t. And a partially opened valve can lead to confusion over whether the valve is indeed opened or not. For this very reason, many technical diving organizations teach their students to always open valves all the way.

4. Do a buddy check before entering the water, and monitor your manometer as you take a few breaths from your regulator. If the needle drops with every breath, your valve is most likely closed, either fully or partially.

Should you ever enter the water with a closed tank valve, it’s important to keep calm, signal your buddy that you’re out of air and switch to your buddy’s octopus. Your buddy can then check your regulator and valve to see if the problem can be fixed or if you must abort the dive.

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