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Want to buy a new bag and will buy down. There is a nice looking Robens bag on field and trek, seems similar in spec to the Alpkit bags (all sold out)
Whats the story with fill power, fill weight etc etc.
http://www.fieldandtrek.com/robens-lite-700-sleeping-bag-781092
http://www.alpkit.com/shop/cart.php?target=product&product_id=16321&category_id=286
Mammut's 'Sleep Well' document is your friend.
[url= http://www.mammut.ch/images/Mammut_Sleep_well_pt1_E.pdf ]Part 1[/url] (general info, measurement, history etc)
[url= http://www.mammut.ch/images/Sleep+Well_down_pt2_E.pdf ]Part 2[/url] (specifically around Down bags)
The 2 bags are actually quite different.
Down v feathers, the smaller the gap in ratio the better: AlpKit 90/10 Robens 80/20 (X % down + x % feathers).
Fill power, it's a complicated test but in straightforward terms, the higher the number the more loft you have: Alpkit 650fp Robens 500fp (this really is pretty low)
Down quantity: Alpkit bag contains 800g of down, the Robens contains 700g. Also, remember that the Alpkit bag doesn't just contain more down, it contains more better quality down.
I know it doesn't really sound like much but the 2 bags will behave very differently