Air activated pocket & foot warmers - Are they dangerous?

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paDsy

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One for the science nerds perhaps....

I've read a few posts with people saying the use of those 'use once' air-activated hand and foot warmers are dangerous. I've not been able to find much to support the claims in the posts, just the mention of increased PP02 possibly causing the formation of iron oxide (Fe2O3) to happen more aggressively which could result in greater heat output and nasty burns 😕

I very much like the idea of being able to stick these warmers onto my socks and glove liners to keep my extremities warm with the mimimim of faff (and cost). But I'm not about to head down to 30m with the looming possibility I might re-surface with chemical burns! The inquisitive side of me must know what the actual risks are and how/why they happen. Has anyone on scubaboard actually used these warmers at depth and lived burn-free to tell the tale? If so, under what conditions did you use them (depth/gas/location in the suit)

In my normal scenario the drysuit would be inflated using a gas mix of 21% O2 (regular breathing gas). My questions really asks what the potential affect of the increased partial pressure of oxygen (PPO2) might have on the chemical reaction taking place in the hand warmer, and is this reaction safe at depth knowing that the PPO2 increases in the breathing gas (used to inflate the suit) in direct proportion to the absolute pressure, i.e depth.

Adam
 
Are the single use ones that much cheaper than the reusable ones? The reusable ones are a completely closed cycle that might be effected by pressure but not by gas mix.

I'd be worried just because you'd have no easy way of removing them if they do get too hot.
 
Are the single use ones that much cheaper than the reusable ones? The reusable ones are a completely closed cycle that might be effected by pressure but not by gas mix.

I'd be worried just because you'd have no easy way of removing them if they do get too hot.
The pads work out about 30p each (60p a pair) so fairly inexpensive and of course you don’t have to remember to recharge them… however, as you say, there’s no easy way to remove them if they over-heat.

Under normal circumstances they get to a maximum of 56c (according to the manufacturers guidelines) which is fine if they are not directly next to the skin. I use them like this snowboarding in the minus temperatures.

Interestingly, the manufacturers state there is a difference between the foot warmer and the hand warmer in that they expect the foot warmer to receive less oxygen in a boot so it’s designed to heat more easily with less oxygen - if that makes sense.
 
Without knowing exactly what are the limiting factors in the reaction it's wise to assume that the ppo2 will increase the reaction rate.
Just how much is an unknown. It might be linear, or exponential (additional heat accelerates most reactions).
Another factor is the limited o2 inside the drysuit.
As in, the warmer might use up a significant amount of the available o2, which isn't being replaced. But without doing some maths that I'm waaaaaay to rusty (ha!) to do, I don't know. If it does use up enough o2 it might run hotter for a while before just stopping.

I'm also curious now to know if anyone has tested them at depth.
 
I have been told that at higher pressures (PO2) they react way too fast and get way to hot. They have burned people. Be careful, having a burning hot packet stuck inside your drysuit would be awful.

If they were safe and effective I imagine we would see them used all the time. Never seen anyone use them underwater.
 
I’ve used the foot warmer version for years stuck on my liner socks, then weezle socks on top. Nitrox to air in my suit. Buy the ones that say “for enclosed spaces”. Try at your own risk.
 
https://www.shearwater.com/products/peregrine/

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