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  1. #1
    Senior Member BillyBob58's Avatar
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    down compression

    From Pan's article "Nesting tricks from an old coot" there is this:

    Minor down compression (up to 50%) does not reduce warmth in down insulated gear according to studies conducted by the Department of Defense Cold Weather Laboratory in Natick, Massachusetts.
    So, how about that? I find that pretty surprising, but I am very happy to hear it. So, if I compress it to 2" under my back/butt from the original appx. 4" loft( or 2.5 down to 1.25 or whatever), it might be nearly as warm anyway?

    Another way of looking at it: If when attempting to adjust my PeaPod or a non-snug type UQ, I sometimes have to choose between maximum loft and the pods/UQ's contact with my back. Sometimes if I adjust the pod until it just barely contacts my back, I will then notice that loft seems to have decreased quite a bit. The amount varies with which hammock I use, sag angles and other variables. But maybe as long as I "only" lose less than 50% of the loft, I will be better off accepting this loss- which might have minimal effect- and maintaining a "no gap" adjustment?

    If this is true, is it maybe related to the benefits of increasing down density with overfill, as has been mentioned here recently? IOW, down that is compressed has lost some loft but gained some density, it seems to me. The same amount of down in a smaller space, rather than more down added to the original space when the max loft is already somewhat limited by baffles.

    Opinions? Thoughts, anyone?
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  2. #2
    Senior Member te-wa's Avatar
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    Quote Originally Posted by BillyBob58 View Post
    So, if I compress it to 2" under my back/butt from the original appx. 4" loft( or 2.5 down to 1.25 or whatever), it might be nearly as warm anyway?
    Opinions? Thoughts, anyone?
    given that your down doesnt just wiggle out of the way under pressure, yes. that is where the bonus of overfill and down density shine.

    fwiw, i had a tq that i made filled w/ 7oz down. it was fairly rated to 35°
    i overstuffed it recently w/ 3oz additional down and sleep comfy down to 22° as tested the other night. HUGE temp gain for a mere 3oz densly filled chambers. Worth the weight? you bet!
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  3. #3
    Senior Member Just Jeff's Avatar
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    Down works by creating small microclimes inside the cluster...basically, little pockets of warm air that can't circulate, so they stay warm. If you compress down a little bit, that cluster is still there, and still not circulating...it's just not as big. I bet this has something to do with the study. I also bet you can find that study somewhere online if you're really interested in the details.

    Of course, once you get too small there will be no microclime...this is where you get cold spots.

    I think this also relates to very high fillpower down and humidity. 800-900fp down has very fine clusters and no quills...i.e., no structure. So the water weight has a larger impact on each cluster, plus the viscosity making the cluster stick to itself...in effect, humidity causes compression in each cluster. That's why down that tests at 900fp in a lab won't test at 900fp on the AT. So we overstuff. I'd still rather have the highest quality down in an overstuff rather than starting with lower quality down. It should last longer anyway. Assuming I can afford it - the price difference between 750fp and 900fp can be pretty significant...Hungarian Goose Down is selling 2.2 lbs for $187. (Link, but folks have had trouble ordering from them before.)
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  4. #4
    Senior Member BillyBob58's Avatar
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    Quote Originally Posted by Just Jeff View Post
    I also bet you can find that study somewhere online if you're really interested in the details.
    Maybe the engineers here can simplify some of the studies mentioned here:

    http://www.backpackinglight.com/cgi-...e_pagination=1
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  5. #5
    Senior Member headchange4u's Avatar
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    Quote Originally Posted by BillyBob58 View Post
    Maybe the engineers here can simplify some of the studies mentioned here:

    http://www.backpackinglight.com/cgi-...e_pagination=1
    Interesting read. Thanks for the link.
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  6. #6
    Senior Member GrizzlyAdams's Avatar
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    Quote Originally Posted by BillyBob58 View Post
    Maybe the engineers here can simplify some of the studies mentioned here:

    http://www.backpackinglight.com/cgi-...e_pagination=1
    Found that interesting. I'll try to re-interpret.

    Each experiment involves a fixed volume where some down will be. One side of the volume is "warm". The other is "cold". The experiments measure how much heat from the warm side gets through to the cold side, per unit area, per unit distance between warm and cold sides. The precise thing being measured is thermal conductivity . The key thing to understand about this measure is
    that among other things, it is a "rate" with respect to the distance between the warm and the cold surfaces. More on this later.

    So, a number of experiments are done, that vary the amount of down in the volume. Given the volume size and amount of down, you can compute how thermal conductivity changes as the density of down in the volume changes.
    In layman's terms, with a quilt or sleeping bag, this computes how thermal conductivity changes as you increase the amount of overfill---more down in the same volume increases the density.

    memo to self : don't be afraid to overfill.

    The point is made that the inverse of thermal conductivity is a measure of insulation. And that (and this is really interesting part) for down densities ranging from the "natural" fill density (like 800 fp) to 2.5 x the natural density (i.e., as much as putting 2 and 1/2 times as much down in the chamber as the natural fill would call for), the insulation grows like a linear function of the density, over that range. In other words, each additional 10% overfill you put in there actually increases the insulation, and increases it by the same amount. It is important though to understand what this is not saying...if you double the down you don't a quilt that has twice the insulation as a non-overfilled quilt. By doubling the down you get twice the increase in insulation that you would get by overstuffing only by 50%, not 100%. It is the increase in insulation that increases in proportion to overfill.

    What this says is that within this range of densities, it is not the loft that matters, but the density of the down in the chamber. This has some interesting ramifications. The problem of "compressing" down and getting a cold spot is actually one of decreasing a volume locally, and pushing down out of the way by doing so. If no down moved out of the area, then as the compression brought the cold and warm surfaces closer, the density would increase correspondingly and the insulation remains the same. The problem we experience with cold spots due to compression are either (i) because the compression is so pronounced that it takes us out of this linear area of interest, and/or (ii) down shifts away from the area being compressed to areas not being compressed, thereby decreasing the density and lowering in the insulation.

    Going a bit out on a limb now, but it seems to me to argue for narrower chambers in our quilts, because as compression occurs there is more resistance to the down moving...the cross-section of area it has to move through is smaller. Which means that the density stays higher and the quilt is more resistant to being compressed. Ever wonder why the Snugfit has narrower baffles than other commercial quilts?

    I wonder what people here who really understand quilts think about these experiments and results...

    Grizz

  7. #7
    Senior Member BillyBob58's Avatar
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    Thanks, Griz. And as mikeinfhaz also said: "given that your down doesnt just wiggle out of the way under pressure, yes".

    Well, then, looks like there is not really much good news here. Because I'm pretty sure that if I get a decrease in loft due to too much tension, that the down is indeed moving to another spot within these pretty large baffles. Also baffles that only block in one direction, leaving a pretty long chamber for down to move around in.

    Though hopefully this motion of down in response to compression is limited with these snugly fitting quilts. Only because they limit the compression to start with.

    Some of the fancier sleeping bags I have used in the past used small baffles that blocked down migration both length and width wise. But also, some less expensive sewn through bags/clothing I have seen also kept the down within a fairly small area.
    Last edited by BillyBob58; 01-01-2009 at 22:16.
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  8. #8
    Senior Member lenle01's Avatar
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    Slightly off topic

    Has anyone tried to make a quilt like this? Smaller squares would reduce down shifting. The only way I could see making one would be sewn through baffles.

    http://marmot.com/fall_2008/equipmen...door/down_yurt

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