[pct-l] the use of sleeping bags, design, choice, etc.

[Jim Eagleton]

Have any side sleepers had trouble with cold butts?  I used to be a stomach sleeper, but have "graduated" to side sleeping for my senior bladder.  I also got a narrow FF Hummingbird bag this year.  I know a differential cut is suppose to keep the down from compressing, but I have had someone problems this year.  It's hard to do a side by side comparison with the standard cut mummy bag.  
Rambler
 
> From: elee at microsoft.com> To: ned at pacificcrestcustombuilders.com; peprmintpati88 at yahoo.com; pct-l at backcountry.net; paul_c at tuxcnc.org> Date: Fri, 12 Dec 2008 12:28:32 -0800> CC: public at postholer.com> Subject: Re: [pct-l] the use of sleeping bags, design, choice, etc.> > Mtnned wrote:> >> The body radiates heat into the space between the skin and the down and> eventually it is held by (or bonds onto) the fine fibers of the down quills.> The down holds it undisturbed better than does the space between the skin> and the down, thus, allowing the heat to be held in the down is more> efficient. The closer the down can be to your skin, the less energy the body> has to expend on heating the space between.> >> > Thanks, Ned! I certainly don't have anywhere near your experience with outdoor skills or gear design, and I'm a software developer, not a physicist, but I want to point out a couple small corrections in terminology that might help people to reason about this stuff in a more accurate way, and maybe get better at solving problems in the field.> > First, insulation of any type doesn't hold or soak up heat, at least, not if it's any good. Insulation is not a heat *sponge*, it's a heat *barrier*. It doesn't keep you warm by soaking up heat and then giving it back to you later. It works by not letting heat leave the area you're trying to keep warm (in this case, your body). Insulation only works precisely because it doesn't transfer much heat into or through itself, or at least does so very slowly.> > Imagine that you put a block of metal and a bag of down in a freezer and leave them there for a week. Then you take them out and hold one in each hand. The hand with the metal block will get really cold really fast because metal is a heat sponge; heat flows into it easily and it can absorb a large quantity of heat. The hand holding the down may get somewhat cool momentarily, but it'll warm right back up again. That's because the down is a heat barrier; heat doesn't flow into it easily and it doesn't absorb much heat. Your hand feels warm even though the bag of down itself will still be below freezing in the middle.> > The other correction is that heat doesn't magically disappear, and objects don't consume heat merely by existing. Once something is warm, it stays warm forever unless the heat is allowed to travel somewhere else. Heating up the air between your skin and the bag is a factor when you first climb in, of course, but once it's warm it stays warm except for heat leaking out through your insulation. I've heard people suggest that you should make sure to pee before sleeping because that way your body doesn't have to waste heat keeping that urine warm. That doesn't make any sense.> > Here's another thought experiment - imagine that you're trying to fill up one of those big plastic swimming pools you can buy for your back yard. The goal is to get the water deep enough for you to swim in. But you have two problems; one, the walls of your swimming pool leak. Two, the hose you're using to fill it isn't very big and supplies only a trickle of water.> > There are three ways to solve your problem. One, you could get a bigger hose. As long as you have more water coming in than is going out, you'll be able to fill the pool. Two, you could fix the leaks in your swimming pool walls so that they lose less water. Or three, you could get a smaller pool. Even if the walls of the smaller pool are made of the same leaky material as the larger pool, it will leak less water overall because there's less leaky wall area so your hose can keep up.> > In any case, the water in your pool doesn't magically disappear on its own. You wouldn't say, oh, I need to get rid of this bucket that's submerged in the pool because otherwise my hose will have to constantly spend water to fill up the bucket over and over again. Once the bucket is full, it's full. It's no longer a factor in the overall equation of water coming in and water going out. The only things that matter in the long run are the input source and the barrier.> > Eric> _______________________________________________> Pct-l mailing list> Pct-l at backcountry.net> http://mailman.backcountry.net/mailman/listinfo/pct-l