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GEDinLA
07-31-2012, 10:46
I've been researching down bags online via specs (no stores near me) and tons of good advice and opinions gathered here (thank you all). Being a numbers-type person, I set out to develop a simple but meaningful method to compare different bags based on specs. Essentially, I was trying to do better than just comparing the temp ratings of one company with those of another and hope they were comparable. Instead, I've considered the amount and quality of down used along with the dimensions of the bag. I (only) looked at 17 bags from a 6 manufactures but remarkably, the results look meaningful and interesting (to me at least). Have a look if you are interested and comments are welcome!

Odd Man Out
07-31-2012, 14:28
This is all very interesting. I too am a "numbers-type person". Do any of these bags have an EN rating. Since the EN rating is based on actual heat transfer data, it might be a good way to validate your assumption that actual rating should be proportional to effective loft. If true, there should be a good correlation between Effective loft and the EN ratings (better than the spec. ratings).

The last few graphs suggest to me that among the bags you chose, there isn't a lot of variability in efficiency. In the plot of wt vs loft_eff, most the points were very close to the same line. It would be interesting to see how bags that are not marketed as UL bags would rate. Also, quilts claim to be more efficient (more warmth per weight). It would be interesting to see if they were outliers on this graph.

GEDinLA
07-31-2012, 15:34
This is all very interesting. I too am a "numbers-type person". Do any of these bags have an EN rating. Since the EN rating is based on actual heat transfer data, it might be a good way to validate your assumption that actual rating should be proportional to effective loft. If true, there should be a good correlation between Effective loft and the EN ratings (better than the spec. ratings).

The last few graphs suggest to me that among the bags you chose, there isn't a lot of variability in efficiency. In the plot of wt vs loft_eff, most the points were very close to the same line. It would be interesting to see how bags that are not marketed as UL bags would rate. Also, quilts claim to be more efficient (more warmth per weight). It would be interesting to see if they were outliers on this graph.

Thanks for the comments - you make good points.

Comparing non-UL mummy bags should be possible. I may try it if I get a chance. As for quilts, that would be VERY interesting, however, being of a different shape the quantity I call the "bag area" wouldn't be comparable. If manufactures would actually spec the area of their bags, it allow bags of different shape to be compared using fill_volume/area as a figure of merit.

I thought about the EN ratings after I had posted. Six of the bags have EN ratings (5 MBs and the Valandre). The first thing to note is that this only affects the linear formula used to relate effective loft to comparative temperature. It wouldn't affect the calculated effective loft nor the bag weight (obviously)....so the close correlation noted between bag weight and effective loft would be unchanged. Attached are two files showing the linear fit to the EN rated bags and the linear fit using ratings from all of the bags (as was in the original document). You can see that the different fit spreads out the range of T_comp above and below 20*F. Bags that fell near 20*F are hardly changed. It may be worth including this (or changing to this) in the orignal document if I can figure out how to edit or change the attachment in the original post.

Feral Bill
07-31-2012, 16:05
I do not see bags of different widths as being comparable. If you need a bigger bag, it costs in neccessary weight.

Tom Murphy
07-31-2012, 16:23
When comparing bags, I would also list construction features like integrated hoods, chest baffles, zipper baffles, footbox design. These features become critical when the temperatures dip below zero.

MuddyWaters
08-12-2012, 14:06
The basic info of how much loft = what temp rating is well known, thats how mfgs decide how to make a quilt or bag.

Actual use is a lot more complicated. Higher quality down, de-lofts MORE with humidity than lower quality down. I actually think 800 may be better for sleeping bags than 900+ for this reason.

A cheaper colder bag can actually keep you warmer in some conditions than a more expensive "warmer" bag.

Tipi Walter
08-12-2012, 14:15
I think the best way to determine bag warmth is to first get a bag from either Feathered Friends, Western Mountaineering or Valandre. Then figure out the amount of down inside the bag by ounces. 35 ounces or above is a bomb-proof winter bag good to 0F or below. High quality companies for the most part use the best quality down and so you don't have to spend a lot of time researching the "best down bag", let these guys do it for you. Find the range you want, i.e. -20F or 0F or 20F and go from there, and always try to get the fill weight of the down inside your bag. This is the most important number, along with the source and quality of their down. Campmor bag goose down vs WM and it's not even worth a discussion. Etc.