/ Down Garment - Warmth Comparison
200g 675+ fill power versus 130g 850 fill power.
Which is warmer? and how do you make the comparison?
How do you make the comparison? With great difficulty... Sleeping bags at least have EN13537 which should help a fair bit, as in theory the results of one manufacturer should correlate with those of another, but it is very difficult for jackets as there is no standard test. Some manufacturers will get their very warm kit tog-tested (see the recent UKC articles on the subject) but these values are not often made public.
I suspect that the two values you've stated above would be very roughly similar, disregarding the design thing, which as I've said is absolutely crucial.
Don't you just get horrible sweaty knackers?
Interesting. I'd always assumed the warmth factor (eg 650g fill) came from the amount of feathers in a given volume. More feathers within the same volume gives a greater warmth. The things you can learn on here...
[Converting the weight from grammes to ounces and then multiplying by the fill power will give the lofted volume in cubic inches, which can easily be converted to litres to give a more understandable number.]
However, as others have said, things are a lot more complicated than a simple calculation can determine.
One simple metric might be the lofted volume. You can assess this by multiplying the fill weight and fill power (you could convert to litres as per Ex-Engineer, but the simple numbers will do for comparison). With all other factors identical, the one with the larger value ought to be warmer.
However, as Mr Fuller says, there are so many other factors...
For instance, a bag filled with more, but lower fill power down will not compress as much where you lie on it, so might be warmer under the body.
The problem with maths converting to volume is the assumption that the pressure exerted in one baffle is the same as that exerted in another baffle. These pressures will also be different to those exerted in the fill power test. The numbers the Ex-Engineer generated above, though, fit with the idea of these two down assemblies being roughly the same warmth: the greater mass of the lower-quality down will exert a greater pressure on the down below it, thus reducing its ability to loft and so perhaps making those two volumes very roughly similar. This all gets horribly complicated with the down underneath the other stuff being able to loft less than the stuff above it. Of course, this is for a static system before you fit in things like knees compressing the down, and this is way more complicated still. This is one reason why fit is so crucial, and a reason why down clothing has barely changed in so long - it's unbelievably complicated to get 'just right' and if it ain't broke, don't fix it.
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