Baffle height vs chamber height vs number of chambers, oh my...

[Merganser]

I've read that there should be some extra loft in a down UQ chamber, beyond what you would need purely to satisfy a differential. My first question is why? Is this just wiggle room because everything is an approximation and no-one sews perfectly? Obviously if you come up short on the differential the down will be compressed and that's bad. Does the extra loft play a role beyond just making the average loft greater?

I'm working with the UQ Calculator (awesome by the way). I've made a few mods to it to suit my taste and construction methods. One thing that I added was a differential calculation (I base that on a radius of 14" over 180*) as check to ensure that the outer shell is wide enough not to compress the down below the baffle height. This way I can make sure that "max chamber height" is at least large enough to match the theoretical differential.

I sleep cold but always with a base layer and don't expect to use this much lower than 40* so I think targeting 20* is good. A friend was good enough to loan me his 20* Incubator so I could analyze it. I'm not planning to copy the tapers (though I did measure them out) but I want to use measurements from the widest part of the Incubator as a guide for my build.

BTW, I'm assuming 850pf down for all of this.

This is what I've measured:
Baffle height = 1.75" (I keep reading 2" but I measured this in a number of places and it was pretty consistent)
Inner baffle width = 4.889" (44/9)
Outer baffle width = 5.75"

The min outer baffle width according to the differential calculation = 4.498", there's an extra 1/4" of fabric per baffle, so this seems consistent with the advice to allow a little extra loft. (If I set the baffle height to 2" there's really no extra loft.)

Matching the measured outer baffle width requires a max chamber height of 2.601". And yields an average loft of 2.175" and temperature factor of .00307, close to spec.

Does this seem sound so far?

Taking the next step then...

I want to make my UQ with 11 baffles instead of 9. I think this will control the down a little better and my width of 44" divides nicely by 11. Opinions on that?

Trusting the 1.75" baffle height I plug this into the calculator.

Baffle height = 1.75"
Inner baffle width = 4" (44/11)
Outer baffle width = 4.75" (Min outer width per differential = 4.498 + 1/4")

The outer shell is a 1/2" wider overall than the Incubator's, but to get the outer baffle width of 4.75" I only need a max chamber height of 2.506. The result is 2.128 average loft and a temperature factor of .00300.

While I don't suck at math, it's not part of my day job either. I can deal with some trig but that's about it. I sure as heck don't have a while pile of formulas memorized.

It does make sense that with a narrower baffle you would need a steeper curve to get to a given height, and that would require a bit more fabric. I don't quite understand what the calculation for chamber width is doing but it has squares, square roots, 2s and PI in it and that smells like something that would include curvature. Am I right on that? Anyone what to educate me on what's going on there?

When it comes to the average loft, on which the temperature factor is based, the average height is just MEAN of the baffle and max chamber heights. To me this seems like an approximation that would not track with changes in curvature. Now perhaps its close enough but I'm curious as to if there is a more accurate way? For example, would it be better to divide the cross-sectional area of the baffle by the inner width or is that just a different approximation?

[Merganser]

OK, I think I figured out a piece of this. The equation was written a little funny but the UQ calculator is assuming the curved portion of the baffle corresponds to an ellipse. So its calculating the corresponding ellipse and adding half of that area to the area of the rectangle formed by the baffle height and inner chamber width.

The outer chamber width then must be based on the circumference of the same ellipse. I goggled that and it made my head spin. The formula looks similar to an "approximate" version I ran across. I'm a little puzzled though why it multiplies by 2 then immediately divides by 2. Seems a little redundant.

[Double]

When i used it I would play with the max height till I was able to get a baffle width number thats not a 5 diget decimal. I went with 3/4 of an inch extra width on the outer shell (inner was 5 inches and the outer 5.75) it made the outer fabric 55 inches plus you add in for rolled hem and i didnt have to cut the width at all only to lenght.

[dudeman_atl]

Lol. Math is fun, but you can't stuff down that accurately.

Leave a little extra fabric on the outer shell and it's all good.

"Measure with a micrometer, mark with chalk, cut with an axe"

[Merganser]

When i used it I would play with the max height till I was able to get a baffle width number thats not a 5 diget decimal.

Yeah, I found myself doing the same thing. You do have to be able to measure things. Also tweaking the % overstuff to get a reasonable fraction on down.

I went with 3/4 of an inch extra width on the outer shell (inner was 5 inches and the outer 5.75)

Was your baffle height close to 2"? I assume you got the results you were after...

it made the outer fabric 55 inches plus you add in for rolled hem and i didnt have to cut the width at all only to lenght.

Those allowances are important. Depending on the fabric I choose I may be very close to roll width as well.

I removed all of the allowances from the "output" section and added 2x baffle height to both length and width. So now the "output" section has just the fabric dimensions up to the outer seam so its independent of how the edges are finished. I added all of the allowances for my build to the "fabric" section instead.

[Carbon]

I'm not being scientific at all with my current UQ project. I'm strictly going by feel and what looks reasonably right. But I do have a general idea of how low it can take me to.

[Merganser]

"Measure with a micrometer, mark with chalk, cut with an axe"

That is certainly how it works out when it comes down to it. I figure my odds of having a good result are better if I start with a good plan and that's the point I'm at, measuring with a micrometer.

[dudeman_atl]

The design exersize is fun, to be sure. I think I poured over my last quilt for three weeks.

It seems to me that plus or minus a quarter inch on the outer baffle radius determines the look of how puffy the baffles look, but has little to do with the performance.

[Calait]

Starting to work on my underquilt, have to start out with a sleeping bag converted to a very narrow underquilt and pray for the best at this time. Thanks for the information placed in this thread.

[Double]

Yes it was a 2 inch baffle height. I used the 2.5 inch baffle material from dutch. Most people go with the longer 3.5 and put a rolled hem so that you are sewing 2 layers of mesh but I haven't seen the need cause if it rips one layer it's going to rip 2 layers just as easy. Plus less hassle of trying to sew the rolled hem while keeping it lined up or first sewing the mesh then sewing it to the shell. It all seems like a lot of extra work for little return. Don't forget to dart your outer shell before you sew on the baffle material, or at least before you attach to the other. Ask me how I know. lol