Speaking of Loft (Sleeping Bags)

[Mitchell Keil]

Is anyone Average??
And is such a comprehensive table (good to an STD of 14%)of any real value when 65% of the American Population is overweight and backpackers probably occupy the tails of the distribution for the most part?

[Richard Nisley]

Mitchell-A method for adjusting the “average” temperature rating was covered in prior forum posts. http://www.backpackinglight.com/cgi-bin/backpackinglight/xdpy/forum_thread/3249/index.html?skip_to_post=23873#23873 is one of my posts.

The 2004 version of ISO 8996 defines the "average" individual: – a man 30 years old, weighing 70 kg (154.3 lbs), 1.75 m tall (68.9”), and a body surface area of 1.8 m2 (19.83 ft2); – a woman 30 years old, weighing 60 kg (132.3 lbs), 1.70 m tall (66.9”), and a body surface area 1.6 m2 (18.12 ft2). The ISO 8996 standard states that users should make appropriate corrections when they are dealing with special populations.

I will go through an example of making the appropriate correction for a Marmot Hydrogen. See my post at http://www.backpackinglight.com/cgi-bin/backpackinglight/xdpy/forum_thread/5533/index.html?skip_to_post=39365#39365 for the “average” rating of 30F. Let’s assume I want to know at what temperature this bag will keep me warm, assuming a similar environment specified for EN 13537. I am a 63 year old male, I weigh 180 lbs, and my height is 5’ 11”. My BMR calculation is 1475.6 C. The “standard” male calculation is 1698.32 C. I wouldn’t be thermal neutral at 30F because I have a lower BMR than the bag was rated with. My personal rating would be ~13.1% worse or 34F. If you were the same body size but 18 years old, your BMR would be 1779.92 C. Your personal rating would be 4.6% better than the standard or 28.6F.

From this exercise in triviality, you will hopefully see that using a theoretically “average” male is a reasonable guide for most consumers. If you want a customized rating, it is quite easy to calculate.

[Mitchell Keil]

Richard:
Thanks for the reference to the BMR calculator. My BMR turns out to be 1733.04 or about 2% better than the "standard" male. I guess that accounts for why I have felt that MontBell and WM are fairly accurate in their ratings. I also sleep warmer than most because I have a higher metabolic rate and lower body fat than most.

What still concerns me is that there are so many variables to the "real world" experience of warmth in a bag that a normalized "standard" seems hopelessly idealistic.

[Steve .]

Richard – thanks, your post helped me to :)

[Richard Nisley]

Mitchell-Yes there are a number of real world variables that can raise havoc with a rating. Those that come to mind in priority sequence are as follows:

-The standard assumes you are using a pad equal in R-Value to what the bag was tested with. In the BPL world I see people routinely using a fraction of the R value used to rate the bag to save weight.
-The standard assumes you are sleeping on your back.
-The standard assumes you are sleeping in the equivalent of a jogging outfit (.5 clo).
-The standard assumes you are out of the wind. Forced convection will significantly reduce the rating.
-The standard assumes you are not using a bivy. The dead air space between you bag and the bivy will add about .5 clo in additional warmth.

I have looked at the detailed EN13357 reports for quite a few bags. The test uses a sophisticated thermal dummy with 20 different zones. On the plus side I have been amazed to see how many the rating drops with a poorly fitted foot box, the absence of a zipper draft tube, a poorly fitting hood, etc.

The major benefit of EN13357, in my humble opinion, is that it clearly defines the test variables so you can accurately compare bags. I have attached a chart, which I created, to illustrate the point. All of the lines represent standards or recommendations from various experts. Most of them don’t mention the pad’s R-value, sleeping posture of the tester, clo value of the clothing worn, tester’s BMR, wind speed, or if a bivy was used. In other words, as you stated, “There are so many variables to the "real world" experience of warmth in a bag that a normalized "standard" seems hopelessly idealistic.”

EN13357, in my opinion, defines the majority of the variables so as to not be hopelessly idealistic.

[Michael Martin]

Great post, Richard!

Your chart looks similar to the one I made to generate the BPL table. Please check your data for the BPL curve, though. Our chart is non-linear.

Cheers,

-Mike

[Richard Nisley]

Mike,

I just re analyzed your numbers and came up with a R2 .9919 linear regression fit. The non linear regression fit yielded an R2 of .9935. I thought the small difference was in the noise level and made the curve linear for simplicity. Can you see where I messed up?

[Dondo .]

>>If available, the EN 13357 rating is the most accurate way to determine a bag's relative warmth.

Richard, I've only been able to find the EN ratings for TNF and Marmot of the U.S. manufacturers. Mountain Hardware also sells in Europe but I haven't found their ratings yet. Have you found ratings for MH or any other U.S. companies?

[Richard Nisley]

Dondo-Laurence should be able to provide the numbers for MH in Europe. His contanct info is as follows:

Laurence Ohlmann, Mountain Hardwear Coordinator
Mountain Hardwear, GmbH
Espace Europeen de I’Entrepreise
2 rue de Vienne
BP 30004 Schiltigheim
67013 Strasbourg Cedex, France

Phone: 33/3.90.22.08.88
FAX: 33/3.88.81.06.89

[Michael Martin]

Hi Richard-

The BPL table is piece-wise linear. Both linear and non-linear regression will yield curves that approximate it pretty well — certainly well enough given the assumptions and limitations of the model we used.

Our curve was developed empirically from the sources cited here. As Benjamin mentioned, our curve closely resembles Jardine's and Weiss' (Natick) data at higher temperatures. (By the way, your Jardine curve looks different than mine. Did you use his ETR=100-(40xT) formula, or something else?) At very warm temperatures, we digress from Jardine as we consider the effect of wearing a base layer. At cold temperatures, we digress in the other direction by considering the increased surface area of the bag as loft increases, and also the increased heat loss caused by stronger thermal convection due to the larger delta T. (Although this second factor can be mitigated by careful bag design and construction details to minimize drafts.)

Bear in mind that our chart (and, in my opinion, any chart for that matter) is only a crude guide. As you know, the differences just between being well rested, well fed, and dry vs. tired, hungry, and wet can skew a bag's perceived rating by (oh, say) 20 degrees or more.

Cheers,

-Mike

[Richard Nisley]

Mike,

My Jardine plot was derived from an old article pointing out the discrepancies between the Jardine and Jordan rating systems. They were the two acknowledged gurus during the bygone era when the report was originally published. It showed the original Jardine formula as Tempc=-41.19*(m2.K/W)+38.212. When I plotted it, it closely paralleled the EN13537 lower comfort line and other peer reviewed standards so it didn't raise any concerns with me that it might be in error.

[Erin McKittrick]

Looking at the numbers here, I see a good argument for dragging one's husband (or wife) along…

I just measured the loft of our summer synthetic quilt at about 1 inch. Which, if I look at the charts/tables here, should only keep me warm to about 55 degrees. (as a small woman my BMR is less than most of you, so it should probably be worse). However, with two people, this quilt is easily warm to 40 or 35 degrees – and goes about to freezing if we wear our fleece clothes inside…

Has any one done any calculations comparing 2 person sleeping to 1 person sleeping?

-Erin
http://www.groundtruthtrekking.org/WildCoast.html

[Richard Nisley]

Erin-I will assume the couple is in a spoon or back to back position. That would reduce their respective quilt exposed body surface areas by about 40% and hence their respective insulation requirements by the same amount. My calculated results jive with your personal experience. If you were under the quilt by yourself you would be warm down to 49.92 F. Sleeping with your husband you would be warm down to 33.98 F.

EN13537 assumes you are using a Thermarest Standard pad. You are probably using a pad with insulation less than that. That will account for the minor difference between the calcuation and your experience.

[Brett .]

Richard; just found this post while memorizing some bag loft 'rules of thumb'.
You explained that putting two people in one bag will lower the comfort range by about 10'F, and this is an easy number to remember and plan for. Two people sharing a semi-rectangular bag, or a Montbell super stretch bag is a very light weight insulation option, about 11 oz per person down to 20'F. (MB#2 long, for example)

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