National diving incidents data compiled by the BSAC highlight an increase in the number of ascent-related incidents, where divers have shot to the surface following a regulator free-flow.BSAC safety and incidents officer Brian Cumming said that in most cases, divers could have dealt with the problem underwater rather than rushing to the surface, which greatly increases the risk of decompression illness. Most free-flow problems occur at the beginning of the season, when the temperature of the water is at its lowest, particularly in the case of inland freshwater lakes.
‘You are looking at a combination of factors,’ says DIVE’s Charles Hood. ‘The water is cold, gear may not have been serviced and there are many new divers in the water. Lots of diving is happening at freshwater sites, where the cold water, particularly at depth, can cause a regulator to free-flow. It is important to remember that salt water freezes at a lower temperature than fresh water, so the likelihood of a regulator freezing up is higher in a freshwater lake than in the sea.’
One of the most common reasons for a regulator free-flow is cold-water conditions, particularly in the UK. A first stage in a regulator functions to reduce air supply pressure from the maximum cylinder pressure. However, when air travelling from a cylinder is subjected to a large drop in pressure – say, from 230 bar to around 10 bar – the temperature inside the regulator decreases rapidly. If the water around the regulator first stage is cold, for example, below 5°C, then ice can form in the system, causing a malfunction. This in turn can cause a free-flow.
On the surface, in cold conditions, a free-flow may start because of the freezing of moisture inside the regulator. In other cases a free-flow underwater may happen when an alternate source is used to inflate a delayed surface marker buoy (SMB) or lifting bag.
If you experience a free-flow at depth, you need to know the best way to deal with the problem safely. Here we take look at free-flow and what action to take, as recommended by BSAC first class diver Richard Trevithick.
1 BREATHE THROUGH IT
For a very limited time, it is possible to breathe from the free-flow as air streams out of the regulator. The British Diving Safety Group (BDSG) recommends that divers practise breathing from a free-flow regulator (simulated by pressing the purge button) as indicated by their training organisation. However, if you do adopt this method, ensure that you do not seal your lips around the mouthpiece. ‘You may use your tongue as a splash guard to prevent choking on water,’ the BDSG advises. ‘This technique takes practice to perfect and to feel comfortable with, so take every opportunity to simulate it in safe conditions.’
2 BAIL OUT
Depending on your equipment configuration, switch to your bail-out gas, if necessary.
If you are using a twin-set, you can isolate the problem and shut down.
4 USE YOUR BUDDY'S OCTOPUS
If you are not carrying a bail-out gas, immediately switch to your buddy’s alternate air source – you are going to lose gas quickly.
5 STAY CALM
Take some calming breaths, assess the situation and when ready, abort the dive safely with your buddy.
Make sure your kit is in service. The most important consideration according to the British Diving Safety Group is to have the regulator serviced regularly and specifically for cold water. Also, leave plenty of time for your kit to be serviced, such as a month before use. ‘The valve can take a while to settle in,’ says Charles Hood. ‘Once the spring and seat haved settled in, it often needs tightening. If it hasn’t had time to settle before use in the water, this might cause a free-flow.’
The British Diving Safety Group has set out the following recommendations for cold-water diving and the prevention of free-flow:
• Ensure that your cylinder is free of moisture and has been filled with air containing as little moisture as possible.
• Try to keep your cylinder out of the cold before the dive (don’t leave it in your car overnight).
• Blow away any trapped water (or ice) that may be on your cylinder valve or regulator orifices, with air from your cylinder.
• Take check-out breaths submerged in shallow water immediately before diving, rather than above the surface in the cold air.
• Avoid the cooling effect of fast air flows caused by using the purge button, breathing heavily or filling delayed SMBs or lifting bags.
• Keep your second stage submerged while walking through the shallows.
• You may consider restricting yourself to no-decompression stop diving, and to a depth from which you are certain your are able to make a free ascent.
All diving valves have to allow water to act on a mechanism inside their first stage, which transfers the amount of ambient pressure to the valve workings. When the diver descends and the water pressure increases, the valve can compensate for this increase and deliver the correct pressure of gas to the diver.
As the valve regulates high-pressure gas from the cylinder to that of intermediate pressure, the gas cools and thus, in turn, the first stage cools. A problem can develop where this cooling actually freezes the water surrounding it. As the water freezes, it expands and exerts pressure of the valve. This then over-compensates for the ambient pressure and the valve starts to free-flow. The valve then cools rapidly and freezes more water and then the free-flow becomes uncontrollable.
To stop this, manufacturers have come up with a whole host of preventative measures. One method seals the first stage so that any water is kept out of the valve assembly. Water pressure acts on the seal and is transferred internally to the diaphragm. Another method is to make the orifice where the water enters large enough, so that the permanent flushing of water warms the first stage and prevents freezing.