sodium-acetate heat pads

[spectrum]

I used one years ago in a wetsuit up here. It was advertised to last for an hour, but in our cold water it only lasted for about 10 minutes. I only used it about 5 times and gave up. It didn't really make any difference to me.

Something was different in what you tried. The product we have runs for a solid hour+ and is not depth or atmosphere sensitive. We have taken them to 40+ feet with only good results. Towards the end of the dive they do solidify but they do so contoured to your body and gear so it's barely noticeable. It too is activated with a little internal clicker disc.

One "diving doctor" thing I read was that having something warm next to your skin tricks your body into sending blood to the surface of your body causing more rapid cooling of your core (unless that warm object was covering a large portion of your body.

I think that would be true if it were a single "dose" of heat. In this case the warmth is delivered for most - all of the dive and you come out ahead. For instance if I am going to spike a wetsuit with heated water to start the dive without a major influx of cold I will use very warm but not hot water for the reason you state.

Pete

 

[MarkH]

I believe that there is a different type of heat pack that relies on air for teh chemical reaction. Were you by any chance using one of this type?

~Jess

I used the kind that is activated by the metal disk. It was made for diving (by Bare). I believe they made a neoprene belt to put them in under your suit. It worked for an hour on the surface (fairly linear drop in heat output), but when submerged in a bucket of cold water, it only lasted about 10-15 minutes.

 

[fnog]

Hi all. I am reactivating this thread 7 years after because I am considering to use one of these in a wetsuit.

So, let find the answer to JessH question:

If you do a google search, you will end up on an *very bad* article from the Irish Examiner. An illustration of what happens when you mix a complacent journalist and a surgeon who pretends to be a physicist. No the kind of chemical reaction we like to see.

Obviously the accident happened with an air-activated warmer, and these contain plain sea salt (sodium chloride), but no sodium acetate whatsoever. An air-activated warmer functions along the same principle as your regular open fire: it is indeed a chemical reaction of oxydization that liberates some heat while turning steel into rust, and is proportional to the amount of oxygen available. These warmer types are dangerous and you can even find reports of compression chamber fires caused by these. I remember in highschool the demonstration where the teacher would let steel wool burn in pure oxygen (that is only 1bar PpO2).

Sodium acetate cristalisation is *not* a chemical reaction, it is just that : cristalisation. When ice forms from water, it also gives out heat, and that is why the temperature stays at zero celsius instead of carrying on down, for as long as not all the water is frozen. Lookup "SatNaOAc.pdf" for a more accurate explanation. These types of sodium acetate warmer pads are safe even if you pump your drysuit with Nitrox at 40m.

If you seal them in 1mm neoprene, to avoid burns, and in cordura, to avoid puncture, you can safely put them in your suit just as you activate them. (Be careful not to permanently fold the steel capsule through the added thickness, else the pad will not stay "charged": it will cristalise as soon as you pull it out of the kettle).

The ideal model for diving would have :
- those two layers of neoprene and polyurethane
- multiple connected compartments that "hinge" so as not to hinder movements when they are cristallised
- a high quality reinforced steel capsule that is in fixed position, with a yellow dot to indicate it.