A few words about free-flows
One of the problems caused by cold water is the greatly enhanced risk of a regulator free-flow, which drains your gas faster than you would want.
Sometimes free-flows happen at the surface already but in these cases we are talking about ice diving in freezing surface temperatures (and the water is relatively warm then). A good ice diving course would cover surface operations (including but not limited to: do not exhale into the reg while in freezing cold air and once the reg submerges, it stays submerged).
Free-flows need three predisposing factors to happen
- moisture in the wrong place (cylinder or 1st or 2nd stage)
- cooling (either in the 1st or 2nd stage)
- time for the ice crystals to form in the wrong place
Scenario: 1st stage free-flow
If the compressor used to fill your cylinder is not maintained and operated well, your cylinder might contain moisture. The situation is worse if you use rented gear and someone had an out-of-air event before and some water found its way into the 1st stage or maybe the cylinder. Now that you own your gear you just need to make sure the compressor is operated properly. Whatever the reason, the moist air cools down a lot in the first stage due to decompression from 200 ATA to about 10 ATA. If the water surrounding you does not warm the first stage enough, icing could take place
inside the first stage - and leave something open. This would cause a wildly free-flowing regulator. No number of second stages will help you now. You will need another first stage, which hopefully is drier, or at least does not free-flow quite yet. Your options: close the cylinder valve and wait for the ice to melt and the first stage to recover its operation... although this could take time and the problem would probably repeat itself. And who knows if your other first stage is similarly affected... It's time to abort the dive nicely - and to use better maintained gear & compressor next time.
Scenario: 2nd stage free-flow
The very air you exhale is moist. This moisture will make the inside of your second stage moist (moist, but not flooded with warm, above freezing, water). You cannot stop this from happening. It's just a matter of time. At the same time a pressure drop from the "intermediate pressure" to the "ambient pressure" happens at the regulators second stage and the air thus cools down a lot. Unless the water around you is capable of heating your regulator enough, the moisture inside the second stage will cause small ice crystals to form. Those will unfortunately make your second stage leak air. This problem would probably manifest itself as some hissing and a minor air leak first, then a little bit of bubbles, then more bubbles, and if the situation grows worse, a full blown free-flow. How quickly the situation develops and grows worse varies. Solution: close the cylinder valve and wait for the ice crystal(s) to melt away. The liquid water that enters your reg will help to melt the ice crystal(s) away - it is above freezing temperature anyway [and no, we do not discuss Antarctica now]. You can use the other first stage + reg for breathing while waiting. Depending on the type of the dive (open water vs. ceiling, shallow vs. deep) and diver (used to free-flows, not used to them) aborting the dive may be a wise thing to do, but it is not always strictly necessary. Rule of thumb: abort the dive if there is no immediate access to surface or if the problem repeats. Some advanced techniques exist to manage gas flow would aborting not be possible (feathering).