Why Scuba Diving Regulators Free-Flow at the Surface
To minimize the risk of free-flowing when entering the water, make sure both your primary and alternate regulator is facing down.
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Free-flowing regulators can be an annoyance at the surface. They’re loud, embarrassing and waste precious tank pressure. Let’s take a look at why regulators free-flow at the surface and how to stop them from doing so.
Why Free-Flows Happen at the Surface
Most of the time divers experience a regulator free-flow at the surface after a dangling second-stage reg touches the water. What gives?
When you breathe off a regulator, your inhalations lower the pressure inside the second stage; this is what pulls the diaphragm in and opens the airway. When the diver stops inhaling, the incoming gas — now with nowhere to go — builds up and creates positive pressure which pushes the diaphragm back out and closes the airway, ceasing the gas flow.
To lower the work of breathing many regulators are designed to take advantage of what's called the Venturi effect. Essentially, they use the high-speed gas flowing through the regulator to maintain a low-pressure area. That helps keep the diaphragm pulled in so the diver doesn’t need to suck in as hard keep the air flowing.
Water is much denser than air, and if the second stage hits the water while the mouthpiece is facing up it can activate the purge, which presses in the diaphragm and starts the air flow. Gas will escape into the sky and won’t build up the positive pressure needed to push the diaphragm and close the airway. If this happens the Venturi effect quickly takes over and your regulator can free-flow.
How to Stop or Avoid a Regulator Free-Flow at the Surface
Pick up the second-stage and then submerge it with the mouthpiece facing down. The reg will quickly build up positive pressure and stop the gas flow.
You can minimize the risk of free-flowing by entering the water with both your primary and alternate regulator facing down.
If your regulator has a Venturi switch, flip the Venturi to pre-dive mode. (On regs that use "min/max" or "+/_" markings rather than "dive/predive," this is the min or minus position.) This will make the reg less sensitive to the Venturi effect, usually by raising a deflector into the path of the incoming gas to disrupt it and prevent it from creating a low-pressure area.
If your regulator still free-flows, turn off your tank and inspect the regulator. It is possible sand, salt or other debris have gotten in to the reg. Make sure to have your gear serviced regularly, at intervals recommended by the manufacturer.