Anatomy of a free-flow
Divernet
Ever found yourself in that embarrassing situation in which your regulator behaves like a shaken champagne bottle on a Formula One podium John Bantin wouldnt want it to happen to you

ITS A COMMON SIGHT. A diver is about to enter the water when suddenly his octopus or even his primary second stage suddenly does an impression of a jet engine at the end of a runway just before take-off.
The sudden roar as gas at 10 bar of pressure rushes out uncontrollably often leads to the diver grabbing the offending item and shaking it violently, or even frantically smacking it on something hard. In fact, all he needs to do is put his thumb over the mouthpiece, but usually hastily fumbling with the tank valve solves the problem instead, even if it means a substantial part of the tank-fill is lost.
Another, more serious, situation can arise if a second stage suddenly starts to free-flow during a dive. This is usually associated with icing of the regulator during a dive in cold fresh water. Seawater around the UK rarely gets cold enough, so this usually occurs in the static water at inland sites
Although every diver will have been trained to breathe from a free-flowing regulator, the remaining air-supply can be reduced to zero very quickly.
Swapping to breathe from a buddys alternative second stage is not often the answer either, because presumably that regulator will be close to free-flowing too, and doubling the air-flow for two divers will only make matters worse.

WHY DOES IT HAPPEN
Take a little time to consider what your regulator does. Your tank is filled with gas at a very high pressure. Your regulator takes the role of a couple of pressure-reducing valves set in series in the air supply path.
The first stage reduces the pressure to around 8-10 bar more than ambient pressure, and it does this by a pressure-sensitive diaphragm or piston pressing on a pre-tensioned spring. Just as your bicycle pump gets hot due to increased pressure when pumping up tyres, the reduction in pressure causes a drop in temperature at the first stage.
If the water or air surrounding it is already very cold, the increased drop in pressure can cause that temperature to be very low indeed. Water surrounding the first stage can freeze, and the ice so formed can interfere with its action.
For this reason, many regulators intended for use in these conditions are environmentally sealed, so that the mechanism is kept dry at all times.
Other designs use a large heat sink to transfer heat from the water once the regulator is submerged, and this is used to raise the temperature of the moving parts, which may be subject to very cold airflows. In the latter case, the pre-tensioned spring is usually visible through large slots through which the water gets pumped as the valve spring moves with the action of breathing.
If the first stage fails, the increased pressure of gas that is allowed to pass has an effect on the second-stage valve, which itself is normally held closed by its own pre-tensioned spring. An unstoppable free-flow is the result.
Free-flows due to icing usually start at the first stage, and the high flow of air can then cause the second stage to ice up.
However, this doesnt explain why a regulator suddenly goes into an unstoppable free-flow when a diver is standing on the swim-platform of a boat in the Red Sea.
Modern regulators are designed to make the work of breathing as easy as possible. Once the gas pressure is reduced from that in the tank, in the medium-pressure hose that connects the two parts of the regulator, it is delivered on demand at ambient pressure each time the diver inhales.
How does it do that A simple valve is pulled open by a lever that in turn is pressed upon by a pressure-sensitive diaphragm, usually at the front of the second stage and also available to be pressed to purge the regulator.
The valve controls the airflow. This is adjusted so that the diaphragm is in equilibrium with either the air pressure or water pressure on it.
When the diver inhales, there is a drop in pressure inside the body of the second stage, the outside pressure presses in the diaphragm, and this operates the lever, which in turn opens the valve and lets the air flow through.
Regulator designers strive to make their regulators breathe as easily as they can. The effort needed to crack open the valve is kept low using a highly sensitive diaphragm with a good mechanical design. This cracking pressure can be increased, using an adjustment knob to increase the tension on the spring that holds the valve closed.
By no means all regulators have this breathing resistance adjustment knob. There are philosophical arguments surrounding the need for it. Some people would argue that if you need to reduce the airflow, simply inhale less heartily.
The designer must also consider that once the air is flowing through the body of the second stage, it should be as unimpeded as possible. He uses a venturi effect to keep the airflow clean.
Unfortunately, if there is a dramatic and unimpeded fast flow of air across the back of the pressure-sensitive diaphragm, this can cause an apparent drop in pressure that is taken up by the pressure on the front of the diaphragm. This causes it to move inwards, putting pressure on the lever that in turn opens the second-stage valve further. The effect is exponential.
To counteract this effect, designers often add a venturi plus/minus lever that simply positions a vane within the flow of air, breaking it up and stopping the venturi effect. Its like putting a thumb over the mouthpiece!
However, unlike with a thumb, you can still breathe from the regulator.
In fact, there are those who cant tell the difference in the breathing characteristics of their regulators and permanently leave the venturi lever in the minus position.
So if your regulator suddenly free-flows before diving, put your thumb over the mouthpiece to increase the pressure within the body of the second stage and on the back of the diaphragm. This allows the diaphragm to reposition itself and the valve to close.

DONT GET ICED-UP
Free-flows caused by icing are another matter. The pressure at the valve of the second stage has become too great for the spring to keep the valve shut, because of the increased pressure of gas from the first stage. Again the effect is exponential, because the fact that nothing impedes the airflow means that the icing effect is increased, and ice will then begin to form also at the second stage. The only way to stop free-flow due to icing is to eliminate the chance of any icing happening.
Firstly, choose a regulator designed to be used in cold fresh water. This may have an environmental isolation kit to keep the works dry, or a designed-in heat sink. The second stage should also have plenty of metal parts to act as a heat sink.
The next thing, which can be even more important, is to take precautions to stop the circumstances that cause icing. Do this by ensuring that the air in your tank is properly dry à get it from a well-managed compressor.
Avoid inhaling moist air back into your regulator at the surface before diving. Avoid purging a regulator in cold air before diving. And avoid breathing from your regulator in the air if the first stage is damp. Take those initial test breaths only when the first stage is fully submerged.
Avoid big airflows by minimising demand. Breathe gently under water, and do not double airflow by allowing use of your alternative air source unless it is a dire emergency. Do not use your regulator to inflate an SMB or lift bag.
Practise breathing from a free-flowing regulator. You may even have forgotten how to do it. Plan dives that do not require mandatory decompression stops, and keep to a depth from which you can make an emergency swimming ascent if need be.

COLDWATER DIVING: DO
  • Use a coldwater reg
  • Breathe gently during the dive
  • Practise breathing during an underwater free-flow
COLDWATER DIVING: DONT
  • Get air from dodgy compressors
  • Breathe from or purge the regulator until submerged
  • Use your octopus unless you must
  • Inflate an SMB or lift-bag from your octopus
  • Do unnecessary deco diving
  • Dive deeper than you have to

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