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For diving in the UK in colder weather the water temperature can easily be below 10C. Warm water regulators seem to be 10C +, and then cold water regulators for below that as they need to be able to limit or prevent free flow.
Is it possible (and / or acceptable) to use a set of cold water regs in warm water, or is it better to just get two sets of regs?
UPDATE: We got mares abyss 52 regs (second + octo) with a DIN first stage. Worked fine in 8C water.
It is likely your cold water reg has diaphragam-like first stage which makes it particularly suited for cold water as well as contaminated water (which is not necessary the case for piston first stage). Usually, first stage designed for cold water have more surface area and mass in order to allow for heat absorption from water (insulation is NOT a good idea!) In other words, first stage for cold waters will do just fine under warm conditions.
Second stage icing -- which is the most common problem -- may lead to blocking of inlet valve from closing, and therefore leading to free flowing air. Chances of freezing can be reduced by better isolation of divers exhaled breath and the area where cold inlet gas comes in. This also depends on thermal properties of components from which the second stage is build of. Just keep in mind that simple metal/plastic design does not tell which is better. Metal freezes faster but also heats up faster by taking heat from water, in the end everything depend on the overall design. All this does not influence warm water operation.
There is more to this though; Sometimes free ice formation can take place, when ice builds up in the second stage but does not lead to free flow. The diver may be not aware of this and can accidentally inhale chunks of ice which can be very dangerous. In order to avoid this, some manufacturers use Teflon coating in order to prevent ice formation (which obviously still can happen). Clearly, this modification does not concern usage in warm waters.
Summarizing, I can't think of any theoretical issues with cold water regs operating in warm conditions. Form my practical point of view, I use my cold water rig regularly in lakes in the northern Europe and it works flawlessly e.g. in the Red See.
Just keep in mind that even the best regs may freeze and this can be a stressful situation (I can tel from my own experience).
