ScoobyPat once bubbled...
Dont breath on/purge reg (or use inflators) out of water? Where did that come from?
Aren't you supposed to check for proper inflation and breathing before hitting the water??
A free flow occurs because parts of the regulator become much colder than freezing due to adibatic cooling of the air going through the reg and dropping from tank pressure to intermediate pressure. Everybody knows your tank gets hot when it is filled as the air gives off heat as the pressure rises in the tank. The same thing happens but in reverese when the air drops in pressure when it leave the tank and goes through the reg.
To oversimplify the process, the air has to re-absorb the heat it lost when it was compressed and it does this by sucking heat out of the regulator and in turn out of the surrounding water. This normally works ok as the water has a very high specific heat compared to air and normally transfers the heat fairly efficiently. However when you test your reg out of the water, inflate your BC or drysuit on the surface, etc, the air cools the reg but the air outside the reg cannot transfer nearly as much heat to the reg as water could.
The result is that the first stage becomes supercooled far below freezing before you are ever under water. Consequently, water coming in contact with the ambient pressure chamber, the mainspring in a piston or diaphragm second stage or the piston stem or piston head in a piston second stage may form ice on the piston, diaphragm or spring and literally freeze it into position. This prevents the piston from moving back down to close the high pressure seat in a piston reg or prevents the diaphragm from returning to its normal position and closing the high pressure seat in a diaphragm reg.
So when diving in cold water and in colder air temps, you do not want to test the reg, breathe off the reg, or fill a BC or the dryusit off the reg until the first stage is completely submerged. It is a deviation from standard scuba pre-dive procedures, but you are diving in special conditions. If you like you can test the reg at home before you pack your gear. Then just be sure the tank valve is all the way on when you are ready to dive and you should be fine.
In really cold air temps, you may want to leave the reg under water a bit before breathing off it. Back inflate BC's or wings are great for this as you can float on the surface more or less face up for a minute or two (orally inflate your bladder) with the reg submerged before starting your descent. This will allow the reg to warm up closer to the water temp.
Higher flow rates equal more coooling and higher heat transfer requirements. This is a problem in cold water as the temperature differential between the water and a nearly frozen reg is less and the water itself is much closer to freezing so you only have a temperature differential of a couple degrees to perhaps 10 to 15 degrees to aid in the heat transfer process. So you want to keep your breathing under control and do not want to fill your BC or drysuit and breathe at the same time in order to inimize the heat transfer required.
You also do not want to make any long blasts with a dry suit or power inflator - make very small corrections or additions of no more than a second or two. Most freeflows will occur a few minutes after the dive as you get deeper and require more air from the reg. Flow rate increases with depth, the tank pressure is still high early in the dive, and the reg may still be colder than the surrounding water if the air temp on the surface was well below freezing. If you survive the first few minutes at depth and do not increase your air consumption you will probably get through the entire dive as the tank pressure falls further and further.
The higher potential flow rates of high performance regs also create a potential hazard due to the higher heat transfer loads required. The Scubapro Mk 25 is a good example of the problem as can potantially flow so much air that it's TIS kit can provide inadequate protection at very low water temps. In contrast the lower performing Mk2 and MK 16 are suitably freeze resistant (if used properly) with essentially the same TIS technology.
Low pressure tanks also mean less adibatic cooling as there is less pressure drop. A 2250 psi steel tank gives you a lot more margin than a 3300 psi AL 80 as there is a third less adibatic cooling involved. I used a MK 25 extensively this year with 2250 psi tanks with no freeze ups at all while my dive buddy with near identical air consumption and technique but 3000 psi tanks froze up on numerous dives in water temps below 45 degrees.
Lower tank pressure helps but I don't think short filling an 80 a few hundred PSI will give you much of an advantage. You would have to drastically underfill it and that is not a good idea.
Warm water poured over a reg can help but there are obvious pitfalls if you do not start the dive shortly after applying the water.
A long surface swim is not a problem if you swim on your back and are not breathing off the reg and can actually be a good thing if the air is colder than the water.
Silicone filled environmental kits are very effective at preventing free flows as long as they are kept filled with silicone, which tends to leak out over time and with use. The general folklore is that diaphragm first stages are freeze proof as no water enters the reg. This is not exactly true as it depends entirely on the specific design.
The Scuabapro MK 16 for example is often touted as a wonderful cold water reg and is regarded by many as being freeze proof. But it has the main spring and diaphragm exposed directly to the water. The parts are designed and coated to shed ice and the ambient chamber has really big holes in it to encourage a lot of water flow and keep the temp above freezing, but a freeze up is still possible if poor technique and excessively high flow rates are used. Piston second stages on the other hand are regarded as cold water hazards by many as water normally enters the ambient pressure chamber where it can freeze on the piston head, piston stem, or mainspring. However if the ambient chamber is filled with silicone grease water entry is prevented and the reg is very, very freeze resistant. A SPEC (Silicone Protected Environmental Chamber) equipped Mk 10 piston reg in practice is much more freeze resistant than a Mk 16 even with the Mk 16's TIS (Thermal Insulating System) kit.
Some diaphragm first stages use a special cold water kit where the ambient chamber is filled with silicone oil and sealed with another soft rubber diaphragm to transfer ambient pressure while keeping water out of the second stage. (Dacor did this a lot) These work in the same fashion as a piston reg using a silicone grease filled ambient chamber and has the same limitations. If the silicone oil leaks out, a freeze can occur as water will get in. The silicone oil can expand in high temps and can cause the oil to leak out during warm weather. Generally speaking it is easy and user freindly to top off the grease in a piston reg, but more difficult to top off the oil in a silicone oil filled diaphragm reg.
Avoiding storage of your tanks at temps below zero sounds good but in practice they are going to most likely be at the same temp as everything else by the time you start your dive, so I would not worry about it.
I should add here that Sherwood used a different method of freeze protection on their cold water first stages. They allowed a small amount of air to leak into the ambient pressure chamber which kept the water out. It works well but one caution would be to avoid extremely rapid descents that may outstrip the flow of air into the ambient chamber. Under normal conditions the excess air dribbles out in a very slight stream of bubbles which often prompts the occasional dive buddy to tell you that you have a leak.
