I can't really find too much really technical information of value about down, its volume or "fill power". TNF site says that one ounce of down fills one ounce of volume fully compressed and that one ounce of 900 down will expand to 900 cubic inches. So it sounds like the specific gravity of fully compressed down is 1.0 like water which sort of makes sense so lets accept that. But it also sounds like they weigh one ounce of uncompressed down and stick it into a large graduated container with no weight on it and eyeball it to see where the top is. What I wonder is for instance if this say a 9" diameter cylinder and they put one ounce of weight on it (including the weight of the cardboard disc), the area of a disc being pi x r x r, or 3.1 x 4.5 x 4.5 or 63 square inches, thats .016 ounces of pressure per square inch, what would the new volume be? Since all down IS stuffed into a shell and since it is the pressure against the shell that makes it fill, then we have to assume that there is some pressure in all down garments and the down will not loft to its full lofting capability.
My old REI down jacket dropped into a cardboard box sort of lightly so that it is pressed a bit on the sides, enough to say assure that most of the air space not in the jacket is closed, balanced against the fact that the jacket itself has some "lofting" to it, I measure a volume of 8" x 12 x 20" or 1920 cubic inches. I sort of think based on a big TNF coat of the same era having 9 ounces of fill that this jacket has at least 4, maybe 5 ounces of down, say five ounces of 550 down. It may be 600 down and it may only 4.5 ounces of it but it sort of averages out similarly. Then negating any volume of the jackets nylon shell, that would be 384 cubic inches per ounce after being in the dryer for a few minutes to loft it, and the jacket has a thickness of about 1.25 inches, or 1.5 inches where there are larger "bags" for the down since it has sewn through construction. Since baffled construction has that icky "weight" and time consuming construction, I'll assume that most jackets vs coats have sewn through construction and therefore have more pressure against the down near the seams. So assuming all of this it appears that the down in the jacket is exhibiting and effective loft power of" 384 vs Say 600, which would be 64% of its rated "full loft power".
Glenn was looking at a WM jacket with 3.5 ounces of 850 down so that should be 2975 cubic inches under no pressure and should have a loft of 2975 / 1920 times the thickness of my jacket or 1.5 times more thickness, or 1,9 to 2.3 inches of thickness measured laying on a flat spot. I wonder if that is true, and if Glenn gets that wm jacket if he'll measure it for us. The photos do not appear to bear this out. It looks like the flash has less loft than my jacket. This tells me that the stuffing pressure of down garments is FAR MORE CRITICAL TO THE GARMENTS VOLUME THAN THE ACTUAL "LOFT VALUE" OF THE ORIGINAL DOWN. aLSO i WONDER IF THE HIGHER LOFT DOWNS ARE COMPRESSED MORE BY THE SAME AMOUNT OF PRESSURE ON THEM, MEANING THAT LOFT NUMBERS ARE REALLY MARKETING AND DETERMINING NUMBERS RATHER THAN AN INDICATION OF THE WARMTH OF THE GARMENT. iT IS SAID THAT THICKNESS= WARMTH, HOWEVER SOME MATERIALS MAY HAVE A HIGHER R VALUE PER INCH.
JUST SOME THOUGHTS
jIM

Inside the paper given in the link below is some data on the thermal conductivity of various insulating materials for cold weather clothing under differing degrees of compression. Phil Gibson is the thermal insulation guru at the Army Natick labs which investigates such things.

A key finding is that loose clothing insulation has its thermal conductivity decrease as it is compressed - as you can see in all of the Gibson figures.

www.jeffjournal.org/papers/Volume2/Gibson.pdf


You will be confused by this paper, partly because the materials being used are not carefully identified, and partly for other unknown reasons the writing is sometimes unclear.

By looking at all of the figures, and looking for the down data point corresponding to the lowest density, you can calculate that the goose down used was 550 fill or better.

The old books on backpacking/outdoor clothing told you AIR!! is the thing that insulated, and "dead air" was the key and down clusters were what gave you that "dead air". Therefore, LOFT was king, because LOFT measured how much dead air you had.

Later, some darned scientific rats came along and measured the thermal conductivity of different down products and down under different conditions. As is typical of those scientific rats, they showed that the old easily quoted oversimplifications were just oversimplifications.


I'll bet that you never heard of the "International Down and Feather Laboratory".

They have a bunch of articles on the testing of down which you can read at:

www.idfl.com/articles/
www.idfl.com/

Did you know that different countries have different standards for measuring the properties of down? Well, they do.

What that partly means is that with the looseness of standards, some advertising/marketing tricks can be played.

Of course, the down in a standardized loose pile, or graduated cylinder, has different thermal properties depending on conditions of compression, as illustrated in the Gibson paper.

Most commonly, high quality down is somewhat compressed in the normal state in any outdoor gear. Partly because it doesn't take much pressure to compress it a little.

Because the thermal conductivity decreases as the loft is cut down by the pressure of the restraining cloth, the heat flow out of the garment may not be too seriously affected by the slightly reduced thickness. In fact, the heat flow could even be reduced if the thermal conductivity dropped in greater proportion to the loft decrease.

Can you use the Gibson data to understand this? Maybe. Remember, you don't actually know what down product they used, because they didn't tell you. The figures are small scale and hard to read, so getting the data you hope for out of them might not be so easy.

Richard Nisely over at www.backpackinglight.com has written that the down goods manufacturers are well aware of this sort of thing, and have learned to fill the down chambers "to the optimum". Some have found that these writings are difficult to understand.

You can learn a little more about the compression of down from the masters thesis to be found and downloaded at:
http://smartech.gatech.edu/handle/1853/4899

You can also learn a littme more about down from this link and the paper you can download....
http://www.highbeam.com/doc/1P3-1395016301.htm
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There has been a lot of "Hot Air" written about goose down in the past.

Good hunting.

Jim

Wow, Jim, now my head hurts! Engineer, right? (You're probably old enough to appreciate the line the female lead delivered to John Wayne in The Horse Soldiers: "A railroad engineer? It must be marvelous, drivin' that locomotive and ringin' that bell - Ding!Dong!, Ding!Dong!" Needless to say, Duke got his revenge: they fell in love at the end of the movie.)

If I get it, and can remember, I'll be glad to measure the loft. Since I'll be measuring from the floor, and the garment's front will be folded to lie on top of the back, should I divide by 2?

The one difference I have found, based on an old TNF Bluc Kazoo and a WM Megalite bag, is in feel, not quality or quantity of warmth. Both kept me equally warm, but the WM bag (850 fill) sure feels lighter when I'm under it than the old TNF bag (which was "the" bag to have when I got it in the mid-80s.) I have no evidence, but my own feeling is that the higher fill powers are lighter to carry, feel like you're sleeping under nothing - and, since there's less down filling the same amount of space, is probably less long-lasting than lower fill power down. I tend to prefer the higher fill powers for subjective, not objective, reasons - realizing that I'll probably have to replace it sooner.

I'll probably make a decision right around New Year's - maybe someone will put one on sale. I did notice WM has (or will) come out with a Flash XR that has some sort of W/B shell material - not sure that really adds any function for me, since I don't wear it to hike in, but I bet it runs the cost on up there.

No, not an engineer. A Materials Scientist, Retired. When I introduced myself that way, once long ago to an older lady at a party, she said ... "It must be marvelous. You get to see and feel all the new materials before we who sew with them ever hear about them." We never fell in love, either.



If you have a sleeping bag with an insulation volume of 8,000 cubic inches you would need 10 oz of loose uncompacted 800 Cu.In./oz. down to fill it. If you replaced that with 8,000 cubic inches of 600 fill down, you would need 13 1/3 ounces of loose uncompacted down. So, the same volume of loose down would weigh about 33% more.

In addition, the modern bags most likely use somewhat finer texture in the cloth yarns, for a somewhat lighter cloth shell, and in many cases they use a more modern smaller and lighter size zipper, not to mention virtually no more metallic zippers in sleeping bags.

So far, this is not much more than saying that lighter materials available today result in lighter sleeping bags.

What is left out of the discussion above is the explicit knowledge of the thermal conductivity of the different grades of down, and how that varies with compression or overstuffing that may give rise to the "puffy" feeling of some bags, vs the "Limp surface fabric" of others. So far, I haven't seen real data on these factors, and don't see the need to guess.

It isn't true that I got to look at and feel the new fabrics before they were available to the general public. However, years ago, the thought that I would be caught dead sewing anything was sheer fantasy.

Things and people change.

Actually, the engineer thing was aimed at Jimshaw - but sounds like I got two for the price of one! All intended in good fun - I'm a CPA, so there's lots of room for shots back ("Auditor: the guy who comes in after the battle is lost to bayonet the wounded." And the classic movie line, from Same Time Next Year, when Alan Alda plays an accountant and delivers the classic line about impotence: "My wife said she thought that when she married me, my eyes would be the first thing to go.")

I hadn't thought about the lighter nylons used now - that may well be part of the difference in feel. But for me, at least the fill weight makes a difference: I have an MSR Ventra, which is, I think, 600 fill in a light nylon, and it "feels" heavier when I use it. (It also isn't as warm as the WM, though it's rated the same temperature - but I suspect that's more a function of self-assignment of ratings as opposed to actual physical properties.)

Good info - thanks.