Hand Warmers

[JAK]

Handy for first aid, and for day trips, but I'm curious about the science.
http://en.wikipedia.org/wiki/Hand_warmer

How many btu are released in terms of BTU per ounce?
Any ideas for DIY hand warmers, maybe one fueled with charred wood?
Also comments on practical applications, and pros cons vs more clothing, more food, candle lamps, hot water bottles, etc.

On cold mornings I used to hard boil an egg, two on really cold mornings, then eat them once I got to school.

Another comment. Always wondered my hard boiled eggs and potatoes seemed to keep their heat extra-ordinarily long. Always understood the heat capacity of water and the insulating effects of the potato and egg had a lot to do with it, but the other day I got to thinking that there might also be some change of state taking place, so there is some additional heat locked in with the heat of fusion. Not sure how much. Also there could be some pressure involved, not sure how much, but that might effect the heat stored in a baked potato or a hard boiled egg. So curious what naturally available material might work towards this end, and perhaps outperform a simple hot water bottle insulated with blue foam or whatever. Don't blow yourself up experimenting with metal canister packed with water and wool socks. Play safe.

[mikec]

I just use the store bought iron oxide hand warmers. If you look hard enough and buy in bulk, they are about 50 cents each. Throow a few in a sleeping bag at night and you can raise the temperature in that bag by at least ten degrees.

[JAK]

Thanks Mike. So I did a little refresher chemistry 101. I hate moles.Anyhow, 2 Fe + 3 O = Fe2O3 + 825 KJ per mole
2 Fe = 110g per mole, so potentially 825 KJ per 110g of iron oxide powder = 7.5 KJ per gram = 220 BTU per ounce.

Take off 10% for some salt and sand mixed in and you might get 200 BTU per 1 ounce packet. I think I got that right. This compares quite favourably to say 600 BTU per ounce for alcohol fuel when you consider you can put it directly into your pocket or sleeping bag compared to heat lost in heating up a water bottle. Compared to using wood heat to heat up a water bottle if that was practical, you might get the same heat from a 1 litre water bottle heated to 200F. So on a long trip wood heated hot water bottle might save weight, but a few of those packets might be more versatile when you consider you might only need them for the coldest nights. Neat stuff. Hot rocks work great also, but are often impossible to find in winter, but if you have the energy you can get pretty warm trying to pry one out of frozen ground. Definitely will try some packets.

[Sheriff Cougar]

I use the store bought kind when hunting in cold weather and they last a very long time. I love them!

[Wise Old Owl]