MET’s, CLO’s and BMI’s?

There is at least one error below, but this is what I understand.

1 and 2 – R can be either imperial (F·ft²·h/Btu) or metric (m²·K/W). It's ambiguous. You have to infer which one from context. In the U.S. you use imperial and everywhere else you use metric. Sometimes metric R is denoted RSI.

1 CLO is the insulation value of a standard business suit. 1 CLO = 0.88 imperial R. 1 CLO = 0.155 RSI.

CLO is used for clothes. R is used for U.S. construction. RSI is used elsewhere.

MET is the metabolic equivalent of task or the amount of heat produced. MET = 1 for a person at rest, by definition. MET = 0.8 when you're sleeping. MET is like 10 or more for vigorous exercise.

Richard posted this chart http://www.backpackinglight.com/cgi-bin/backpackinglight/xdpy/forum_thread/9378/index.html

If you're talking about a sleeping bag, then you want a MET of 0.8. Choose that line from Richard's chart, and for a particular minimum temperature, you get the required CLO.

Like for a temperature of 30 F, you need a CLO of about 6.3.

I think this is overly conservative, you actually need less than this, like maybe a CLO of 4 for 30 F.

3 – Thru hiker says 1.2 inch Apex synthetic has a CLO of 4.1 so that's 3.4 CLO per inch. Some people say that down has less CLO per inch than synthetic, and some people say it's the same. Your 3.9 per inch is probably optimistic. I don't think it matters too much which down – it's the same CLO/inch.

4 – Twice the loft produces twice the CLO.

Another thing you could do is look at sleeping bag specs. Look at bags with your desired temperature rating. Make your bag with the same loft.

Jerry,

Thanks for responding. I think I am pretty good on the definitions part of things. And I did see Richard's chart. That, along with his treatise on down density, and the Mammut Sleep Well document have given me a good foundation on the theory.

I guess what's missing for me from Richard's MET chart is the mathematical relationship between MET, Temps and CLO. I'm assuming a regression equation underlies each MET line. But I don't know what it is. Richard hinted at it in the Q&A below from that same post.

Again, I'm no expert here, just trying to figure it out too, and I don't think there's a complete, consistent story here but…

Richard's formula is something like

CLO = (T – 90) * 0.08 / MET

where CLO is the CLO you need
T is the minimum temperature
the constants 90 and 0.08 I derived just by looking at the chart
MET is the metabolic level

Regarding CLO/inch for down, I have not found any consistent value.

Like I said, for Apex, it's 3.4 CLO/inch, using thruhiker's numbers. Some people say down has the same CLO/inch than synthetic and some people say it's less.

Because of the uncertainty of CLO/inch for down, It seems like all this analysis is not going to get you your final answer.

Maybe you're best off looking at down bags, finding several rated at the temperature you want, see how much loft they have, and use that.

What is the BPL position statement?

What is 3.5 – 0.05 * T? I haven't seen that. That would mean 3.5 inches for 0 F. From Richard's chart you need a CLO of 9.5 for 0 F. That would mean a CLO/inch of 2.7.

If you get answers of 2.5", 2.2", and 1.9" using different methods, that might be as consistent as you'll ever get.

I assume all of these assume you're wearing minimum clothing inside your bag. If you wear insulated clothing you can extend the range down 10 or 20 F.

“Richard’s formula” – That’s exactly what I was looking for. Now if I can just figure out the mathematical relationship between MET and M2K/W (if any).

The Backpacking Light Position Statement on Sleeping Bag Temperature Ratings is found
here.

The publish date was back in 2004. This is the table from the article.

The Loft formula comes from a Thru-hiker.com forum post here.

The chart from the post was this.

Thanks for following up. I’m really trying to understand the science behind constructing garments and sleep systems that will keep me warm under targeted conditions.

I’ve got a great table/chart I’m working on that pulls together the temp ratings on down, synthetics and the EN13537 Comfort ratings in a format even I can understand. I just want to make sure I’ve got good numbers to go on it.

M2K/W is square meters * degree Kelvin / Watt – the insulation value that will produce one degree Kelvin difference if you pass through one Watt of power through a square meter area.

MET is the rate of energy produced where a value of 1 is defined as the amount an average person sitting produces, which by convention is 4.184 kJ/kg/hr.

Thanks for the references, post your tables.

You had better add a rather large fudge factor to your graphs. How cold or hot a person sleeps depends not only on the person (with big variations between males and females being well known), but also on what their current physiological state is and what they have eaten recently (among other things).

Of course, the weather is also significant, and highly variable. Temperature, wind, rain, humidity … cloud cover …

For this reason, once you venture into the colder regimes, trying to fine-tune your gear to be exactly right is an invitation to disaster. It is not possible and may even be foolish. You will need a safety margin.

Cheers

Roger,

Understood. YMMV.

However, if I understand correctly many of the graphs Richard and others post based upon EN13537 or the Mammut Sleep Well study show results that are based upon the "average male" under very specific indications of humidity, wind speed, etc. in order to get a baseline relationship. For example, Richard's MET chart reveals profoundly useful relationships for warmth under varying workloads for a given CLO. It's left to the individual to adjust for their own conditions and specifics. (And we've seen dozens of individuals doing just that on the thread.)

That said, it would be equally valuable to me (and perhaps other newbies) to understand the comparative warmth per weight relationships between and among the widely used synthetics and various down fills, regardless of their other characteristics, in constructing gear. (I'll get schooled on that part next.)

As you point out, it is precisely because I need to build in a safety margin that I feel the need to understand what I'm working with. That's why I'm on this perhaps Quixotic mission to figure out how warm a given measure of a given fill power of down is under standardized conditions, so I can compare it thoughtfully to the synths.

This part is classroom for me. I understand that the practical labs occur on the trail. It's because I'm now responsible for building in my own safety margin, rather than just accepting that North Face or WM are right in their stated ratings, that I care to "know the math". And trust me, I really don't do decimals (or math) well so this is really work for me to try and get my arms around the concepts.

Thanks for the insights. -L

Maybe I missed it above, but are you a "typical male"? Are you generally comfortable at 25° in a 25° EN rated bag?

I discovered, after many surprises, that my resting metabolic rate is about 20% lower than that of the "average male". I need a 15° bag for 25° temps. (Thank you Richard N. for the push to get a simple test.)

hmmmm.

I live in Florida so the lowest temp I typically see on a campout is maybe low 40's. But I'm going to the Smokies this summer and Vermont this fall. I have a general idea of how warm a bag I will need for both, but in reality how will I know I got it right until I get there?

I read the thread that led to your test. Pretty cool. Now you know why the norms didn't apply to you.

But I have to ask. If you wanted to make a down bag good for 15d so you would be comfortable, how much loft would you need? How would you know? Based upon what?

Conventional wisdom? If you were still going by conventional wisdom you'd still be cold in a 25d bag. [smile]

I'm just trying to look behind conventional wisdom and see how the science works so I can make the right adjustments for my own circumstances.

So, how about it. Anyone want to offer up their opinion on how much down is needed for a given temp and how you got to that number? I don't have the answers. I'm looking for them.

First off I need to do some friendly ribbing of you: You're an economist used to rounding to the millions. You are into making your own gear and thru-hiking which indicates you have a surplus of time and frugality. So I ask you (jokingly) How'd this economy treat ya?! ;)

Mainly that's to get back at you for pulling me back into the DDDD (Can we go with D4?).

As far as the math goes, you're right. It's nice to have a formula for comparing apples to apples, and then from there tweaking the results for your orange self.

As for the down clo there exists a severe lack of research into clo of various fp of down. Most studies take down as down regardless of quality (or don't mention quality) which is basically an indicator of how poorly executed most scientific studies are, but I digress. The conclusion is that higher fp does have higher clo. This is due to mechanical differences that create higher fp also contribute to insulative properties (more trapped air, less shaft for heat conduction and etc). I was one of the people that halfway detracted from Richard on density, pointing out that he was absolutely correct (compressing down increases clo-inch as studies have show) but it decreases clo/weight so it's not a useful technique for garment/bag construction. It has secondary benefits such as keeping down from shifting and insuring no loft compression due to shell fabric weight which are definitely useful and why a 10% overstuff is not a bad idea (maybe even preferred?). Richard and I actually talked at length on this issue (and amicably!) and I think he has pretty much put to rest the issue of compressed down. Makes sense seeing as he has been right all along, just focused on one property over another (max clo instead of min weight).

As for the MET to clo formula, I don't remember what it was, but Richard I believe derived it from industry standards or from the EN standard? I do think it was/is a generally agreed upon formula and not proprietary but I could be wrong. Either way, I would trust it as a guideline.

That said, 2.53 is based of clo/oz-m^2 (strange mixed units that I question) and converting into the correct units I was getting 4.61clo/in for 850fp down. This is much higher than what seems probable and common knowledge/usage dictates. I can only think that the perfect down is just that much better than a real world environment where the down has to deal with humidity and dirt/oil which degrades it's clo performance severely from ideal (but still better than synthetics). Now if I take the data as mislabeled and assume it's clo/oz-YD^2 I get more reasonable clo values for 850fp down (2.86clo/inch or 3.86 depending on the linear regressions used). So the data is all over the place.

So how much loft should you use? Well for PRACTICAL purposes depending on which clo/inch you use you can get calculations for anywhere from 1.6" to 2.5" which is disconcerting to say the least. I would actually say with 850fp you can probably go as low as 1.6" but this would be survival level, not comfort level unless you're a very warm sleeper. If you look at WM bags they use 2.5" single layer loft for 20F and most say they are conservatively rated. So with your calculations saying 1.9"-2.5" I would just suggest splitting the difference which gives you 2.2" and happens to fall in line with BPL's stance. Also 1.9" with a 10% over stuff would give you ~2.1" so 2.2" looks even more attractive.

Depending on how you sleep this should be more than warm enough for 20F. It's also always nice to end up with a bag warmer than intended than cooler.

Sorry if this post was rambling but it's the first thing I've worked on this morning. I hope you were able to glean some useful info from it.

Dustin-
Thanks for the insights. Actually I'm still among the 90% of Americans still employed. I have just been doing decimal math as kind of an anesthetic to dull the senses so the Fed's policies appear to make more sense. .. but I digress ..

Actually your regression work on down CLO's for Rog last October was part of my early reading on the subject before I went down the rabbit hole. Trying to pin down a reasonably useful number on a down CLO reminds me of George Bernard Shaw's observation on my profession => If all economists were laid end-to-end they'd never reach a conclusion."

I'm quickly discovering there's little data, lots of opinions and few conclusions (mostly of the pick-a-number and go with it variety). That said I have another day or so before my Thru-hiker kit gets here so I'll chase my tail a bit more on this.

I'll go back and take a look at the formula work I did on the EN stuff and see how I could work it back towards the MET work Richard did. (And if you think it's scary watching an economist do decimals you don't even want to contemplate my algebra skills.)

If I can get to a reasonably respectable, acceptable down CLO number I've got what I think will be a really useful chart to share. But given that economists forecasting abilities make weathermen look good, I'm trying to draw on the observations of the BPL savants to get closer to a "real" number.

Can you confirm/refute that CLO properties for down would be cumulative for loft, perhaps linear? Using your 3.26 number from your regression for 800fp would one assume that 2" was 5.52, 3" was 9.78, and 4" was 13.04? That works out to EN LL comfort ranges of 56f, 24f, -9f and -40f? Does that seem reasonable?

Hi Lane

> the lowest temp I typically see on a campout is maybe low 40's
And that really is your biggest problem.

In all seriousness, it would seem from what you have written that you have never really experienced COLD. It is going to be a shock to the system, and academic discussion is not a substitute for getting out there and freezing once or twice.

No, I am NOT having a go at you; I am trying to explain the need for first-hand experience to calibrate yourself. There is no substitute!

Last week we went on a pleasant alpine walk in our late Autumn. It hit -8.6 C one night. The tent was frozen solid in the morning. Mind you, the next night we got 8" of snow, so things do happen in the mountains.

Bottom line: go slightly over-weight the first few times, for safety!

Cheers

Greg
How did you measure your RMR and what is your 'simple test'?

I don't think I match the "typical (sedentary?) male" either: I get cold at night and when stop/start walking with kids but when going at full throttle I need very few clothes, even in winter. I think this is related to physique (6'1", BMI 22).

Lane

If you have estimates in the range of 1.9 to 2.5 inch – well, you're not going to get anything tighter than that with better estimating.

Just choose 2.2 inch, or 2.5 inch.

Then take an extra jacket to wear inside in case you need it.

I think you're right, lots of opinions but not so much data.