I am familiar with them, they are a heavy but flexible clear plastic sealed package, filled with a hypersaturated solution of Sodium Acetate. The metal tab inside allows you to disturb the solution causing an exothermic reaction, which releases heat. You can also spontaneously initiate the reaction from shock.
Though great for home and car, I wouldn’t take them on trail. Weight to functionality for starters, they are initially quite warm but it does trail off quickly, my experience is maybe 15 minutes of warmth and then more or less finished (this also depends upon surrounding environment temps as well).
Secondly, recharging them. You will have to carry a significant amount of fuel to recharge them, they must be in boiling water (rolling boil at that) for 8-10 minutes, then allowed to sit without movement to cool and allow for supersaturation.
third, they can spontaneously go off, as from shock in your pack or pocket while carrying them, a pain since you don’t even get the benefits of utilizing the heat energy released.
fourth, they are in a heavy plastic, and can be placed in another plastic bag or similir, but they can leak over time, or get punctured, or any number of things.
I keep them at home for really cold days walking to school, or in the car. The dry heat disposable packs Bon Scott speaks of are also nice, come in various sizes, not shock sensitive but cannot be re-used, heat up nicely too. I have carried those on weekend hikes for emergencies, maybe two at most. Also have solid fuel handwarmers (popular with hunters) that I have used for, well, hunting, winter day hikes, etc. I had all three with me when I went Polar Bear swimming last month in Michigan. Those re-usable packs only stayed warm maybe 7-9 minutes in those temps, sitting inside my warmer blanket.
If you want an emergency heat source, I would consider a space blanket (those silver mylar types, weigh 2 ozs) and maybe 2 smaller or mid-size dry disposable heat packs, maybe 1-1.5 ozs each. Beyond that, layering, movement will keep you plenty warm.
An old trick, put your sleeping bag and a pair of dry socks inside the bag, put in your stuff sack as you normally would, and put all of it in a contractors heavy garbage bag. No matter how wet it is outside, how wet your pack gets from rain or stream slips, you will ALWAYS have a dry bag to get into then, and dry socks to put on. And, if you get too cold at night, you can use the bag over your toebox of your sleeping bag to block wind coming into the shelter, or into your tent, or just put your pack inside it and outside your tent (sans food of course) to keep it dry in case it rains that night.
My thoughts,
-xtn :boy
airferret
