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John Carter
(jcarter1)

Locale: Pacific Northwest
NeoAir as stool; physics question on 05/27/2009 18:27:02 MDT Print View

It just occurred to me that since the NeoAir has horizontal baffles, it can be partially deflated and folded up into thirds to create a 6-7" thick stool. I just tried it and it makes sitting 'indian style' MUCH more comfortable. So much so that I don't think I would need a camp chair.

My physics question is this: If I fold the pad into thirds, so that the weight of my rear is on top of 3 sections of pad, does this distribute my weight over the three segments, thereby reducing the stress of putting most of my weight on a small surface area? Or, does each segment of pad receive the full weight of my body, thereby tripling the strain on the NeoAir? There are enough INTJ engineers on these forums to get an accurate answer from one of you. =)

EDIT: I just tried rolling it up like a cinnamon roll, and it effectively creates 4 complete layers, or 10" of thickness. It feels like sitting on a nice, soft log. This will make preparing dinner much more comfortable! (assuming I'm not going to blow a hole in the pad by quadrupling my effective weight on the pad!).

Edited by jcarter1 on 05/27/2009 18:33:17 MDT.

Ashley Brown
(ashleyb) - F
Re: NeoAir as stool; physics question on 05/27/2009 18:56:07 MDT Print View

The air pressure in your pad will rise in order to support your body weight. This pressure is evenly distributed across the whole pad, because if there were any areas of higher pressure then air would simply flow to an area of lower pressure. This assumes that you're not squashing it so much that air can't flow between the layers.

So to answer your question, you should be fine.

Dan Durston
(dandydan) - M

Locale: Cascadia
Pressure on 05/27/2009 23:21:21 MDT Print View

What is higher is the pressure between your pad and the ground. When your pad is laid out flat, the weight of you and the pad is dispersed over 1440 square inches (72" x 20"). If you weigh 199 lbs and the pad weighs 1 lbs, then that's 200lbs over 1440 square inches, or 0.13psi.

With the pad rolled up so it only has 1/4 the contact with the ground, you are effectively quadrupling the pressure between the ground and the pad to 0.52psi.

I suspect that normally this isn't a problem, but if you're sitting on a sharp stone or a pokey stick it will be pressing on your pad 4 times as hard, so it'll have a much better chance of puncturing your pad.

John Carter
(jcarter1)

Locale: Pacific Northwest
Re: Re: NeoAir as stool; physics question on 05/27/2009 23:47:39 MDT Print View

Ahh, that makes sense, and explains my confusion. So the air pressure will remain constant throughout the pad, which is fixed based on my total weight, whether I'm laying down or standing on one foot. But the fabric that is directly under my weight will be subject to the full weight of the part of my body above it, in terms of how hard it is being pushed onto the ground.

I will always use an 1/8" Evazote foam pad, so I'm not too concerned about punctures. I guess my real concern would be added pressure on the seams, where a pressure-related blowout is mot likely to occur.

I would imagine though, that being a Thermarest product, they must have a significant margin of error in the breaking point; whenever one rolls around or gets off the pad one is adding pressure points along the way. Assuming I am gentle when getting on or off the pad, I would assume I am within the margin of error when it comes to pressure.

Edited by jcarter1 on 05/27/2009 23:48:54 MDT.

Roger Caffin
(rcaffin) - BPL Staff - MLife

Locale: Wollemi & Kosciusko NPs, Europe
Re: Re: Re: NeoAir as stool; physics question on 05/27/2009 23:55:08 MDT Print View

> my real concern would be added pressure on the seams, where a pressure-related blowout is mot likely to occur

By putting all your body weight on a smaller area the airpressure will necessarily be higher. That's already been discussed.
But this increase in pressure happens anyhow when you sit up on the pad and effectively have most of your weight on your backside. So ... one trusts that Therm-a-Rest have allowed for this in the design. I imagine they have.

Cheers

John Carter
(jcarter1)

Locale: Pacific Northwest
Re: Re: Re: Re: NeoAir as stool; physics question on 05/28/2009 00:19:40 MDT Print View

Thanks Roger. So which one of you staffers would like to create a new test category; the high-jump-onto-the-rolled-up-pad bursting test? =)

Ashley Brown
(ashleyb) - F
qualification... on 05/28/2009 00:56:17 MDT Print View

Sorry John for not being clearer.

The pressure will rise and be higher than when you are lying down, because your weight is supported over a smaller surface area. So yes, more pressure on all the seams, but nothing to do with the number of layers. As Roger notes it shouldn't be any different from sitting on the pad normally, when you place your weight over a smaller area.

HOWEVER... when the pad is laid out flat and you put too much weight on it (eg. sit down heavily), usually you start to hit the floor and the pressure stops rising. When you roll up your pad as a burrito this is unlikely to occur, so there is no 'escape valve'. So certainly avoid 'bouncing' or sitting down heavily, or it may well go pop!

The key point is how heavily inflated the pad is when you roll it up. The pressure increases because as you sit down the volume of the pad decreases. The maximum amount of additional pressure that the pad can generate before "bottoming out" is roughly proportional to the surface area you are sitting on times the thickness of the pad. In the case of a burrito-seat the thickness is 3 or 4 times as great, so you can generate 3-4 times as much additional pressure (due to loss of volume) before hitting the bottom. Hence the need to start with a lower inflation level and/or avoid sitting down heavily.

Hope that is clearer.... cheers, Ashley

Edited by ashleyb on 05/28/2009 00:58:49 MDT.