Increasing fp and dwindling returns

[TeeDee]

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!!

Thanks for that info, but they don't really address the same problem.

Quoting the BPL reference:

300g of 800 fill power in a quilt provides an average clo of 6.46 and an EN13537 LLimit temp rating of 24.2F. For every 50 fill power difference the same amount of down provides approximately .1 clo difference in warmth which equates to approximately 1.01 F difference in a LLimit rating. So, the same amount of filling in 850 fill power quilt versus a 700 fill power quilt will be accurately rated about 3F different.

and here:

There are a number of separate studies that I combined to answer the question. None of which are related to thermal conductivity vs down density. The study that was most relevant was a private study paid for by a group of manufacturers for their personal use.

Five mummy bags were made exactly alike by the manufacturers and filled with 0.624 kg (22 oz.) of down. The only difference between the bags was the fill power of the down which ranged from 600 to 800 in.3/oz. in intervals of 50 in.3/oz. An EN 13537 certified lab was paid by the manufacturers to test the insulation value of each bag. The data showed that the bag insulation increased systematically with the fill power of the down in the bags. The bag insulation value increased about 0.1 clo for every 50 in.3/oz. increase in fill power.

In both quotes, he's addressing the problem of equal weights of differing fp down. The problem I am curious about is equal volumes of differing fp down.

The rest of the thread seems to be asking the same question I have without really resolving the question.

1 or 2 of the respondents seem to have arrived at the conclusion that the lower fp is a better choice due to lower cost and actually insulating better than the higher fp. Nisely, the BPL expert, seemed to duck that question by not responding.

[TeeDee]

I have to agree that higher fill power doesn't mean 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 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 spaces to what's in the feather.

Jbo

Okay - by going to the higher fp, you have reduced the mass of the filling down and kept the volume filled constant.

Less mass at the same mass density for the down means less volume occupied by the down itself.

So, unless you posit that the down increases in density (mass/unit volume of the down), what do you posit occupies the filled volume left vacant by the reduced mass of down??

I maintain that the air in the air pockets has to increase to do that - hence more and/or bigger air pockets to fill the filled volume of the quilt.

This is the old trick question: what is heavier? a lb of lead or a lb of feathers/down??

Reread your post and for your post to make sense to me, you have agreed with youngblood's assertion that they change the mix of feathers to down to change the fp. Sorry, but I have read nothing to indicate the validity of that assertion. That does not mean that the assertion is false (or true) only that I have no reason to believe it true presently and so continue with my assumption that the mix is constant.

[jbo_c]

What would happen if he decreased the weight and maintained the same loft?? That is the situation I'm really interested in.

At some point(I don't know where.) this would be problematic because the higher fill power is so light weight that is compresses too easily to maintain the loft.

I just finished two down items made with 800fp from StormCrow and I can tell you from experience that without a fair amount of overstuff, the down has some trouble maintaining loft against just the weight of the fabric.

Wherever that point is, you begin to get diminishing returns(original point of post) because you would have to increase the weight of the down(overstuff) to maintain enough pressure from the inside to support the fabric.

I think around 800fp is where that tipping point is. That said, there may still be an advantage because overstuff is some 'insurance' against natural loss of loft.

Jbo

There you go: A plus at the beginning, a minus at the end - adds up to nothing.

[Joey]

Copied from wikipedia, with highlighted points of interest:

Fill power is a measure of the loft or "fluffiness" of a down product that is loosely related to the insulating value of the down. The higher the fill power the more insulating air pockets the down has and the better insulating ability.

Fill power is the most frequently used measure of down quality. It involves measurements taken of a one ounce sample of down in a plexiglas cylinder with a weighted piston compressing the down. The test requires controlled temperature, humidity, and preparation of the sample. All other things being equal a parka (duvet or sleeping bag) made with high fill power is lighter and more compressible than an equally warm one made with lower quality down. Fill power is expressed as cubic inches per ounce (in³/oz)—a lofting power of 400–450 is considered medium quality, 500-550 is considered good, 550–750 is considered very good, and 750+ is considered excellent.

750+ fill is quite different from 400 fills. Almost all down commercially available is a secondary product of geese raised for consumption. It would be prohibitively expensive to raise geese for down alone. The geese that are the source for lower fill down are about four months old when they are "harvested" for food. Down from these geese can be carefully sorted, washed, and blended, but it will never loft like really mature down. The 700+ down fill comes from a small number of birds kept for breeding purposes throughout the year. These geese molt naturally in the spring. While their down is loose it is collected by hand. It is very rare and, of course, expensive. The larger individual plumules are what gives the greater loft. The only way to get down of this quality is by careful hand selection which is the major factor in its scarcity.

Depending on the intended application, the highest available fill power may not be necessary. A high fill power will provide more insulating ability for a given weight of down. The warmth of a down comforter depends not only on the fill power, but also on the quantity of fill. So the difference between a comforter with 550 fill power down and a comforter with 700 fill power down is that the 700 fill power down comforter will be lighter for the same warmth rating. High fill powers are most important for weight sensitive applications such as backpacking.

Down (or any insulation) provides warmth by trapping a layer of air that separates the cold side from the warm side. A thicker layer of trapped air gives more insulation. This thickness is often called "loft." A comforter that uses 550 fill power down, for example, would have to use approximately 40%-50% greater weight of down than a similar item that uses 800 fill power down to provide the same loft.

[Joey]

Here's a chart I found on BPL that may be of some help.

[jbo_c]

I maintain that the air in the air pockets has to increase to do that - hence more and/or bigger air pockets to fill the filled volume of the quilt.

Exactly, the higher fill power gives you more, but much smaller air pockets = increased insulation efficiency.

That is also why high quality ice cream generally won't melt as fast on the way home from the grocery store. It has more air entrained(which could be spelled wrong) in the cream - creamier mouth feel - the small melted layer on the outside of the carton serves as an insulator for the ice cream further in - less melting.

Jbo

[jbo_c]

Had to go back and reread Youngblood's post to see whether or not I agreed. I do. Having seen a couple of different qualities of down, the primary difference is the amount of feathers. I'm sure there are some actual down clusters that are different qualities, but once you get into the higher fill powers, it seems your mainly looking at how many feathers are in the mix.

Remember that "feathers" isn't referring to something like what you find stuck in an urban cowboy's hat. Feathers in down are very small, like 1/4" to 1/2".

I can't tell you that is in anybody's spec, but I'll hold my position that the main difference in quality is amount of feathers.

Jbo

[Youngblood]

http://www.westcomb.com/content/FAQ/13

"Fill power is a measure of the loft or “fluffiness” of a down product that is loosely related to the insulating value of the down. The higher the fill power the more insulating air pockets the down has and the better insulating ability. Fill power ranges from about 175 cm³/g (300 in³/oz) for feathers to around 900 cm³/g for the highest quality down. Higher fill powers are associated with a larger percentage of down clusters and a larger average down cluster size.

Simply the higher the number the better and more expensive. The higher the number the higher it lofts and better it insulates.

At Westcomb we source only the highest grade Canadian Hutterite Down that yields a fill power of 850+."

[BillyBob58]

Yes - so have I - I have never really thought much about it before though.



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.

Wow, I admit I had never thought of the theories proposed in this thread. But it does make some sense.

10 oz of 900 vs 10 oz 600, much more loft thus most likely a good bit warmer. ( at least if the enclosing fabric also allows for more loft)

BUT 10 oz of 600 FP vs a lower weight of 900 = same loft, BUT maybe not as warm as the 600 fp of equal weight? Never figured on that one!

I guess the increased number of smaller air spaces in lower fill power ( as opposed to fewer but larger spaces with increased FP at equal loft/lower weight- if that is actually the case) helps explain why I always found the HH OCF pad- at least when used with a space blanket- to be so much warmer than it LOOKED to me like it should be. It never looked kike it would be warm past 50 to me, but it is (for me). Or for that matter why CCF pads are as warm as significantly thicker down. Or for some synthetic bags are rated as warm as somewhat thicker down bags?

Speaking of ratings for X inches of loft being established a long time ago: My 1st way high quality bag was a North Face minus 5 I bought in 1983. I was thrilled with that bag. It was something like 575 FP. It was just below ( in price) a Marmot bag with about the same temp rating that had a GTX shell and a whopping 625 FP, top of the line!

[JohnSawyer]

When I recently harvested some down as a side-effect of converting my old REI mummy bag to a TQ, I noticed a few things:

Some of the "down" clusters could be described as 50% feather. A fluffy component and the top was far more Feather-like than down-like..

like this:

Where other down clusters looked like this:


(ok, none of the stuff in this old bag looked THAT good, but you get the idea...)

In this case, I don't believe we're looking at a situation where lower quality down is trapping air in smaller bits, but rather the down is incapable of expanding due to inefficient material...

If that's the case, then 2" of 600fp or 2" of 900fp would have similar insulating qualities. The only difference would be that the 900 would actually trap more air, where the 600 would have a higher volume of feather-material consuming space.

To me, the question to determine which is a better insulator is: compressed feathers, or trapped air...

My $0.02...