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  1. #1
    Senior Member TeeDee's Avatar
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    Increasing fp and dwindling returns

    Increasing fp and dwindling returns

    I was thinking about my down top and under quilts today and the effectiveness of the 5.5" of measured loft with 900 fp down.

    I read somewhere in the web (Thru-Hiker?) that 4" of down loft is rated at -20° F.

    This got me to thinking about the fill power, fp. At what fp was that rating measured, leaving aside all consideration of warm/cold sleeper and what the rating actually means in terms of comfort.

    Consider a quilt with a volume of 20,000 cubic inches , 80"x50"x5". Here is a table of the weight of the down of varying fp needed to fill that quilt:

    1. 600 = 33.3 oz
    2. 650 = 30.8 oz
    3. 700 = 28.6 oz
    4. 800 = 25.0 oz
    5. 900 = 22.2 oz


    So in going from 600 to 900 fp down, we "lost" 11.1 oz of down or 1/3 of the fp and weight. That is a very worthwhile weight savings.

    But at what cost?? Putting aside the economic considerations, I want to consider only the insulation value of the down.

    As is obvious from the above, as the fp is increased, the amount of down decreases. That means that the volume that was occupied by the down is now occupied by air. The only way for that to happen is to have fewer air pockets of greater size.

    As you get fewer air pockets and they get larger, the heat transfer by conduction and convection gets more efficient and hence more heat is transfered, i.e., lost.

    In the extreme, you have the case of a large simple air mattress with, say, 4 or 5, large air chambers. In this case, for a quilt, the heat is conducted efficiently from your body across the shell fabric to the air chamber. In the air chamber, convection air currents are set up which carry the heat to the outer shell fabric, where the heat is efficiently conducted to the exterior environment. The large air chambers allow stable and very efficient air currents to be established to maximize the transfer of heat from your body.

    That is why air mattresses make poor top and under quilts in comparison to closed cell foam pads or down or synthetic filled quilts which have a very large number of isolated air pockets. The air isolated pockets greatly impede the transfer of air between the pockets and hence greatly impede the transfer of heat through the mass of down.

    As another example, consider large air pockets filled with crumpled up newspaper or leaves. You are closer to the situation with down, but you still have larger air pockets and a whole lot fewer of them. That is why such a method of providing insulation under a hammock is better than nothing, but far from adequate at or below freezing temperatures.

    So by going to down with a higher fp, we have decreased the number of air pockets and increased their size. The air pockets are still isolated, but the heat has to be transfered between fewer pockets across the mass of the down filaments. The larger and fewer air pockets allow more efficient heat transfer via convection within each air pocket and conduction between air pockets.

    The question is: how much more efficient is the heat transfer with increasing fp?

    If the increase in heat transfer is minimal (define minimal in this context) in comparison (define your method of comparison) to the weight decrease and cost increase, then the trade off is probably worthwhile.

    Does anybody know the fp of the down at which the various temperature ratings were derived?

    If the temperature ratings were derived with down rated at 600 fp, then what is the rating for 800 fp or 900 fp?

    I strongly suspect that the temperature ratings were determined for 600 fp down since the temperature ratings seem to have been around for many, many years and never mention fp - maybe because fp wasn't even considered when the temperature ratings were determined? In looking at fp and it's history, it seems that the drive for higher and higher fp is a relatively recent phenomena and if the temperature ratings were derived before fp came into vogue, then I suspect that the down used was not treated and prepped to get the best fp possible. All of which leads me to believe that 600 fp down was used.

    My meandering thoughts have lead me back to the temperature ratings I have seen for which 4" of down loft was rated at -20° F. So my 5.5" of loft is rated at considerably below that temperature. I seriously hope and doubt that I will ever inhabit an environment where the temperature rating of my down quilts is challenged, but I now have even less of a notion of what the temperature rating for 5.5" of loft with 900 fp down would be.

    What I would really like to see is a determination of temperature rating for various fp and varying loft. Has anybody ever seen such a table??

    I really would not be too concerned about how the temperature rating is defined so long as it was defined identically for the various fp and lofts thus enabling a means of comparison and a possible means of determining if weight savings was really worth the cost increase with increasing fp and the decrease in insulation value.
    Those who sacrifice freedom for safety, have neither.

    Do not dig your grave with your teeth. (Unknown)

  2. #2
    Senior Member MAD777's Avatar
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    Facinating subject.
    Richard Nisley, a professional that deals with materials and heat transfer, has discussed this concept extensively at BPL.
    The bottom line is that down can be compressed about 2.5 times from it's full loft and maintain an almost steady insulation ability. I won't attempt to get too detailed here, but as is commonly known, heat travels through various means including convection, conduction, radiation, etc. As I remember it, lofty down is good at preventing conduction but allows some convection. As the loft is compressed convection decreases but conduction increases, offsetting each other. Hence, some amount of compression doesn't significantly affect the outcome (up to 2.5 times according to Richard's lab measurements).
    But, 900 FP works better than 600 FP given the same conditions for both.
    Mike
    "Life is a Project!"

  3. #3
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    I'm not sure I agree with your logic that higher fill power means fewer air pockets of greater size. That sounds like the difference in fill power is that higher fp down is just lower fp down clusters that are expanded.

    I don't think that is the case. With all other things being equal, higher fill power means a higher percentage of down and that means less feathers. Feathers do not provide the insulation that down does. So maybe all you are doing by removing feathers is removing dead weight and not affecting the insulation capabilities. Heck, maybe feathers impede the insulation capabilities of down by providing an alternate and more efficient thermal path involving conduction, I don't really know.
    Youngblood AT2000

  4. #4
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    Not sure if these will help???

    http://www.idfl.com/
    http://www.backpackinglight.com/cgi-...hread_id=33188

    There are a bunch more on BPL about down fill, but none directly answering your question/s!!
    "yesterday is history, tomorrow is a mystery, but today is a gift---thats why its called a present" - Master Oogway
    It's always best if your an early riser!

  5. #5
    Senior Member packeagle's Avatar
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    I always figured that down is just as warm at a given loft no matter the fill.

    Vendors like Hammock gear measure their down by weight so they give two temp ratings because upgrading to 900fp gives you the same weight of down not the same volume. (I know he offers 900fp as standard now). If you were to get his 800fp Phenix it would have 11.7 oz of 800fp. If you uprgraded to the 900fp version you would get 11.7 oz of 900fp down. That is why he states that the rating is lowered by 5* with 900fp, because by upgrading to 900fp you are essentially overfilling the bag by something like 1170 cu in of volume with 0 increase in weight.

  6. #6
    Senior Member Syb's Avatar
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    Interesting timing on this topic. A friend just gave me a huge 650fp down quilt that I'm interested in cannibalizing for a UQ and a TQ. I'll be watching this thread for sure...
    Syb
    Enjoy the elevation

  7. #7
    Senior Member TeeDee's Avatar
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    Quote Originally Posted by MAD777 View Post
    Facinating subject.
    Richard Nisley, a professional that deals with materials and heat transfer, has discussed this concept extensively at BPL.
    The bottom line is that down can be compressed about 2.5 times from it's full loft and maintain an almost steady insulation ability. I won't attempt to get too detailed here, but as is commonly known, heat travels through various means including convection, conduction, radiation, etc. As I remember it, lofty down is good at preventing conduction but allows some convection. As the loft is compressed convection decreases but conduction increases, offsetting each other. Hence, some amount of compression doesn't significantly affect the outcome (up to 2.5 times according to Richard's lab measurements).
    But, 900 FP works better than 600 FP given the same conditions for both.
    Compressing the down to reduce the loft and maintaining the same loft with a differing fp is not really the same thing.

    So reducing the loft by compression may balance the competing mechanisms, but that is a different scenario.
    Those who sacrifice freedom for safety, have neither.

    Do not dig your grave with your teeth. (Unknown)

  8. #8
    Senior Member TeeDee's Avatar
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    Quote Originally Posted by Youngblood View Post
    I'm not sure I agree with your logic that higher fill power means fewer air pockets of greater size. That sounds like the difference in fill power is that higher fp down is just lower fp down clusters that are expanded.

    I don't think that is the case. With all other things being equal, higher fill power means a higher percentage of down and that means less feathers. Feathers do not provide the insulation that down does. So maybe all you are doing by removing feathers is removing dead weight and not affecting the insulation capabilities. Heck, maybe feathers impede the insulation capabilities of down by providing an alternate and more efficient thermal path involving conduction, I don't really know.
    Hmmm - that is the first argument that I have ever heard that increasing the fp is done by reducing the percentage of feathers.

    What I have read on the internet - admittedly only one company in Canada - says nothing about changing the mix of down to feathers to change the fp. Now maybe the Canadian company is playing a game and not telling the full story and still being honest. Don't know.

    I have always gone on the assumption that the percentage of feathers is equal irregardless of fp.

    Maybe you're right and maybe not - don't know. Until I know definitively, I'll have to go on the assumption that the mix is equal across fp in which case my logic still stands.
    Those who sacrifice freedom for safety, have neither.

    Do not dig your grave with your teeth. (Unknown)

  9. #9
    Senior Member TeeDee's Avatar
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    Quote Originally Posted by packeagle View Post
    I always figured that down is just as warm at a given loft no matter the fill.
    Yes - so have I - I have never really thought much about it before though.

    Quote Originally Posted by packeagle View Post
    Vendors like Hammock gear measure their down by weight so they give two temp ratings because upgrading to 900fp gives you the same weight of down not the same volume. (I know he offers 900fp as standard now). If you were to get his 800fp Phenix it would have 11.7 oz of 800fp. If you uprgraded to the 900fp version you would get 11.7 oz of 900fp down. That is why he states that the rating is lowered by 5* with 900fp, because by upgrading to 900fp you are essentially overfilling the bag by something like 1170 cu in of volume with 0 increase in weight.
    By using the same weight he is increasing the volume and hence the loft so he gets more insulation from the increased loft if conventional practice of ignoring fp except for weight is true.

    But then if the fabric measurements are the same, then the volume cannot increase by much and the 900 fp down is compressed somewhat compared to the 800 fp down. The loft would still increase slightly due to the pressure from the down. So considering the expert opinion cited in the first reply which states that the down would maintain the same insulative value compressed a slight amount and the fact that the loft would be increased slightly, he gets a slight increase in temperature rating. Although a 5 degree increase in temperature rating hardly seems like it would be worth the extra cost of 900 fp over 800 fp. From a customer perspective it would be better to decrease the weight to maintain the same volume if the temperature rating is maintained. That would bring the cost to the customer down and decrease the weight. Of course, the cost to make would increase since differing weights would have to be factored into the manufacturing process and differing processes cost money.

    What would happen if he decreased the weight and maintained the same loft?? That is the situation I'm really interested in.
    Those who sacrifice freedom for safety, have neither.

    Do not dig your grave with your teeth. (Unknown)

  10. #10
    Senior Member
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    I have to agree that 'higher fill power means bigger air pockets' is a gap in the original logic. If you look at a tiny feather vs comparable down, you'll see that the down has zillions of wisps along its arms vs the feather which has only a hundred or so hairs on a central spine. The hairs and spine are much denser than the zillion(approximately) wisps. Therefore where you can see 99 gaps between the hairs on the feather, there are probably literally many thousands of gaps between the wisps, so it would take a 100x increase in the volume of that single piece of down to get equal size air pockets to what's in the feather.

    Sorry I can't think of a better way to explain it.

    Jbo
    Last edited by jbo_c; 02-17-2011 at 20:39. Reason: clarification

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