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Brad Groves
(4quietwoods) - MLife

Locale: Michigan
Re: reflective coating in sleeping bags on 05/25/2010 18:40:34 MDT Print View

Lynn, I think Colin Krusor posted something awhile back about aluminizing cuben. Lotsa $$$ though. Cubic Tech does produce an aluminized cuben, but the sample I have is much heavier than the .33...

Frank, thank you. Pretty much my point. You can stop heat loss two ways or three ways.

Kendall Willets
(KWillets) - F

Locale: San Francisco
Metallized Fabric on 05/25/2010 23:40:01 MDT Print View

I was curious after reading about the Fugu here the other day, and some web searches turned up the following patent: http://www.freepatentsonline.com/6824819.html. It sounds like the fabric used in the jacket, but I haven't found any available. The patent holder is Milliken, http://milliken.com.

FIELD OF THE INVENTION

This invention relates to metallized fabrics which are durable to washing and wear. They can be used to down-proof articles in which they are used as linings, e.g., down and fiber filled, insulated articles of clothing and sleeping bags.

Jason Elsworth
(jephoto) - M

Locale: New Zealand
Mountain Equipment on 05/26/2010 01:57:19 MDT Print View

Almost 30 years ago now I had a Mountain Equipment synthetic bag with a silver radiant barrier in it. The concept didn't seem to catch on widely, but I did find this online http://www.livefortheoutdoors.com/Gear-Reviews/Search-Results/Sleeping-bags/Mountain-Equipment-Mithril-II-/. They claim it adds 3 degrees to the bag.

James Klein
(jnklein21) - M

Locale: Southeast
relevent info on 05/26/2010 07:06:33 MDT Print View

Bob G., I would like to know what the conditions were for your quoted value. Until those are given, the number you presented and your resulting claims are irrelevent.

Richard Nisley estimated the IR barrier on the FUGU was equivalent to having ~2oz extra 800fp down (see his post on 01/10/10 16:18:39 MST):
http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=27537&startat=20

I think this idea has merit. It is certainly easier and cheaper than ripping open a sleeping bag and adding some equivalent amount of down.

Chris Lucas
(ChemE) - F

Locale: SC
Misinformation on 05/26/2010 08:25:52 MDT Print View

I'm seeing a scary amount of conjecture and misinformation in this thread. Recommend everyone read this:

http://en.wikipedia.org/wiki/Black_body#Radiation_emitted_by_a_human_body

Radiation is a very significant heat loss mechanism which is why Aerogel jackets are so insanely warming because Aerogel is a near perfect IR barrier and thus prevents much heat loss from blackbody radiation.

Edited by ChemE on 05/26/2010 08:34:00 MDT.

Rog Tallbloke
(tallbloke) - F

Locale: DON'T LOOK DOWN!!
Re: Misinformation on 05/26/2010 08:51:21 MDT Print View

"radiation accounts for about two-thirds of thermal energy loss in cool, still air."

Unless of course the subject is lying down on a sleep mat.

Chris Lucas
(ChemE) - F

Locale: SC
Stefan Boltzmann Law Example on 05/26/2010 08:56:11 MDT Print View

Please note that the Stefan Boltzmann law which determines the difference between radiation absorbed and emitted by an object only depends on the area of that object, the temperature of that object, the emissivity of that object, and the temperature of the surroundings. In our reflective sleeping bag example, the object is the person + sleeping bag and the surroundings is the ground and air.

It makes absolutely no difference whatsoever if the object is standing up or lying down nor is there any physical reason why that should matter. The radiation leaving your skin doesn't care if you are hiking, sitting, sleeping, or recently deceased. All that matters is skin temperature which admittedly does change depending on activity level.

Chris Lucas
(ChemE) - F

Locale: SC
Re: Re: Misinformation on 05/26/2010 08:58:33 MDT Print View

"Unless of course the subject is lying down on a sleep mat."

Right. That portion of your body doesn't radiate to the surroundings because there is no gap. That area of your body transfers heat by conduction only. Side sleepers will experience more radiative heat transfer than will back/stomach sleepers.

EDIT: Oops, let me correct myself. The portion of a person in contact with the ground will still transfer heat through radiation to the ground provided there is a temperature difference between the object and the surroundings. So if you warm the mat/pad/ground under you to exactly your skin temperature, then radiation ceases. Most likely this is not the case though and so you continue to pump out heat to the ground via radiation.

Edited by ChemE on 05/26/2010 09:02:34 MDT.

Jeffs Eleven
(WoodenWizard) - F

Locale: Greater Mt Tabor
Re: Re: Reflective emergency bivies on 05/26/2010 09:05:18 MDT Print View

Some guy somewhere told me he uses an emergency blanket on snow for a groundcloth and it is very warm. Any validity? I know I'm forgetting "facts" from other threads, but doesn't it have to be close to you to reduce radiation heat loss. (VBL properties aside)

In other words... Space blanket, then pad, then bag wouldn't make much of a difference?

Richard Nisley
(richard295) - M

Locale: San Francisco Bay Area
Re: Misinformation on 05/26/2010 09:17:04 MDT Print View

Chris,

Aerogel is a very poor block of IR. Aerogel's primary benefit is that the average cell size is smaller than the mean free path of air. This dramatically reduces conduction.

Another material, such as carbon black, etc., is commonly added to aerogel to achieve a better IR block. Carbon is an effective absorber of infrared radiation and, in some cases, actually increases the mechanical strength of the aerogel.

James Klein
(jnklein21) - M

Locale: Southeast
surroundings on 05/26/2010 17:19:26 MDT Print View

Chris,

When applying the Stefan–Boltzmann equation for the situation you described (hiker in sleeping bag) the immediate surroundings would be his jammies (assuming he's not naked). His jammies would exchange IR with the first layer of his sleeping bag, the inside layer would exchange with the bags insulation.....

James

Roger Caffin
(rcaffin) - BPL Staff - MLife

Locale: Wollemi & Kosciusko NPs, Europe
Re: Re: Re: Reflective emergency bivies on 05/26/2010 19:07:27 MDT Print View

> he uses an emergency blanket on snow for a groundcloth and it is very warm.

A bit of creative imagination there imho. In contact with the ground? One degree from the space blanket, if lucky.

Cheers

Jeffs Eleven
(WoodenWizard) - F

Locale: Greater Mt Tabor
Re: Re: Re: Re: Reflective emergency bivies on 05/26/2010 19:28:51 MDT Print View

What I figured...

Bob Gross
(--B.G.--) - F

Locale: Silicon Valley
Re: Re: Re: Re: Reflective emergency bivies on 05/26/2010 22:10:43 MDT Print View

"A bit of creative imagination there imho. In contact with the ground? One degree from the space blanket, if lucky."

Roger, I think they have their minds made up differently.

That sounds like another creative marketing victory to me.

--B.G.--

Jeffs Eleven
(WoodenWizard) - F

Locale: Greater Mt Tabor
Re: Re: Re: Re: Re: Reflective emergency bivies on 05/26/2010 22:42:15 MDT Print View

I think the guy that told me (a co-worker) has won a victory in creative BSing

Chris Lucas
(ChemE) - F

Locale: SC
Re: surroundings on 05/27/2010 02:39:42 MDT Print View

James,

Not so. One can arbitrarily define the system boundary wherever they would like. You just need to be aware of what you are solving for when you do so. We could set the system boundary at the skin, the jammies, the bag's interior surface, the bag's exterior surface, the tent wall, etc.

Chris

Stuart Allie
(stuart.allie)

Locale: Australia
Re: reflective coating in sleeping bags on 05/27/2010 04:19:48 MDT Print View

There seems to be confusion about absolute and relative losses.

Maybe this will make things clearer...

A naked person in still air loses (depending on the experimental conditions) 50% to 80% of their body heat by radiation. The absolute amount is of the order 100 Watts by radiation, the remainder by conduction and convection.

Now lay that person down on the cold ground. They will now be losing somewhere of the order of 400 Watts or more by conduction with the ground if the ground is at, say, 40F. So even though they are still radiating 100W, that is now a much smaller proportion of their total heat loss. So when a person is laying on the ground, it's much more important to deal with the conductive heat losses before worrying about the radiative losses.

As soon as you add moving air (increasing convective losses), clothing, the layers of sleeping bag, mat, etc. into the equation it all gets complicated of course, but clothing reduces radiative losses too. The essential point is this: in real world conditions, radiative losses are (relatively) small. For most purposes, you are best to focus on reducing heat lost by conduction and convection.

Hope that helps.

Roger Caffin
(rcaffin) - BPL Staff - MLife

Locale: Wollemi & Kosciusko NPs, Europe
Re: Re: reflective coating in sleeping bags on 05/27/2010 05:59:53 MDT Print View

Hi Stuart

Those numbers seem very high to me. Can you quote a good reference for them?

Otherwise I agree about the real world bit.

Cheers

James Klein
(jnklein21) - M

Locale: Southeast
relative losses on 05/27/2010 07:48:55 MDT Print View

Stuart, a nude person laying on 40F ground would be losing much more than 100W via IR losses. Even assuming the radiation exchange, for skin not in contact w/ ground, is occuring at the immediate air temp (must be ~32F for 40F ground temp)....the total IR radiation would be much closer to 300W.

James Klein
(jnklein21) - M

Locale: Southeast
creative marketing on 05/27/2010 07:54:17 MDT Print View

Yeah Bob, I curse these creative marketing depts are every time I wake up to a frosted tarp even though ambient temps are above freezing. I wish I had been smart enough to invent radiation heat loss so I could have patented various magical products that block it.