Cavechat.org

There has been some discussion in the Simmons-Mingo thread regarding space blankets. So I thought I'd start the subjects it's own thread here.

Adventure Medical makes a nice quality emergency bivvy sack that is much more cave worthy than most common "space blankets" the Thermo-Lite® 2.0 Bivvy seen here can be used as a sleeping bag over and over.





The Heatsheets® Emergency Bivvy is only 3.5oz and is much more durable then the typical space blanket.






IMHO both are caver worthy.

I've heard good reports from a caving acquaintance on using a Blizzard Survival Bag while in Camp One in the Gouffre Berger.

The local cave rescue team always include a Bothy Bag to shelter casualties while they are awaiting evacuation (and also unfortunate cold Heyphone operators!).

There is an intersting article on space blankets on TraditionalMountaineering.org:

http://www.traditionalmountaineering.or ... ankets.htm

"A thin plastic sheet has no insulating qualities, indeed, the plastic immediately becomes as cold as the snow or the wind blown air and will immediately conduct that cold through wet clothing to the body. A space blanket is not a blanket! If you lie down on a plastic space blanket placed on the snow the cold will immediately be conducted to your compressed clothing. You must carry extra insulating clothing layers and an insulating ensolite pad!"
"Note: An emergency sheet of thin Mylar or Kelvalite plastic (an "Emergency Blanket"), properly used, minimally can help reduce heat loss a bit in four out of five heat loss categories."
Of course this is a mountaineering site, but the principles still apply.
This site also reccomends the emergency bivy sac.

I would have to agree with the above, the thin plastic coated space blanket will not provide any insulation between you and rock or what ever you are sitting or laying on. You need a closed cell foam pad, your knee pads, your pack, another caver if you can get them to cooperate.

The general term (space blanket) comes in a variety of forms, thickness, and weight. The one thing they do best is stop the movement of air around the body. They do reflect some of the heat back to you and can add in keeping you dry. Stopping that movement of air is very important, if you have ever been in a snow cave you will understand how much warmer you are because ot this. The plastic garbage bag can also do this, when worn like a poncho.

Adding a heat source(candle, carbide lamp, small stove) will add greatly to the comfort range and length of time you can remain stationary. Just remember you need ventilation when adding the heat element.

If you choose the thin plastic coated blanket, I strongly suggest that you unfold it completely, refold it and place it in a zip lock bag. When these come from the manufacturer, they are tightly compressed, trying to unfold it with fingers that are going numb is not a good idea. There have been reports of people being found dead with one of these partially unfolded next to them(above ground).

If several cavers have space blankets you can build heat tents, reflectors, over head cover, double walls which will decrease the heat loss.

Remember, doing all these things will keep you buzy and hopefully build or keep a positive attitude.

nordicjw wrote: The one thing they do best is stop the movement of air around the body. They do reflect some of the heat back to you and can add in keeping you dry.

No doubt a shiny plastic sheet is good for blocking wind and serving as a vapor barrier. But I have never been convinced that the shinyness has any advantage over a black garbage back for reflecting Infrared heat back to anyone. Anyone know of any data about this? I've heard confident statements going both ways.

You should try the Blizzard sleeping bags...they work great!!

http://www.ps-med.com/blizzard/bag.html

Much better than other ones I have tried, they are very durable and warm.

So the underlying question here is about the best way to bivouac in a cave without carrying 40 pounds of gear.

#1. You need insulation from the cold cave floor/walls

I carry an ensolite pad in my cave pack. It pads my back and gives me a pad to sit on that insulates me from the cold cave floor. You can forget about lying on the ground, unless you carry a full-length pad. You can use your knee/elbow pads, but if these are wet/muddy they won't do you a whole lot of good.

#2. You need protection from air currents

I carry a space blanket. While I'm sitting on the pad, I can wrap the space blanket around me to prevent air currents from sucking heat from my body. It's not meant to insulate me from the cold, it's to stop wind.

#3. You need an internal heat source

Food = internal heat.
If the cave is dry, you can pack a small down sleeping bag into a bag the size of a grapefruit. Otherwise, the only insulation that you'll have is the clothing you brought with you. A neoprene hood will greatly add to your internal heat preservation, and works even when wet. Neoprene kayaking hoods are smaller and more appropriate for use under caving helmets.

#4. You need an external heat source

My third source is always a small carbide lamp. I carry a small container of carbide that should give me 24 hours of heat at minimal flame. Put that carbide lamp inside of your space blanket and you've got a small space heater. Be mindful of carbon monoxide poisoning.

Anyway, that's my take on the subject.

cavedoc wrote:

nordicjw wrote: The one thing they do best is stop the movement of air around the body. They do reflect some of the heat back to you and can add in keeping you dry.

No doubt a shiny plastic sheet is good for blocking wind and serving as a vapor barrier. But I have never been convinced that the shinyness has any advantage over a black garbage back for reflecting Infrared heat back to anyone. Anyone know of any data about this? I've heard confident statements going both ways.

I can tell you with 100% certainty that aluminized mylar is far and above more effective than plain black garbage bags. Aluminized mylar "space" blankets reflect up to 97% of infrared back to you, black plastic is nearly 0%. That is exactly why satellites use it for insulation, reflects solar radiation back out into space, keeping it from heating the satellite. Ever see the protective clothing worn by foundry workers? It's shiny for a reason...

Marduke wrote:I can tell you with 100% certainty that aluminized mylar is far and above more effective than plain black garbage bags. Aluminized mylar "space" blankets reflect up to 97% of infrared back to you, black plastic is nearly 0%.

So I googled this to look for a source and most of what I found agrees with you, but it's all by marijuana growers trying to hide their grows from IR sensors. Forgive me if I remain suspicious. Can you direct me to a good primary source on this? You sound like you know what you're talking about.

Just google "aluminized Mylar" and nothing else. It will pop up exactly how it work.

So aluminized mylar does appear to reflect back most IR. I'm still wondering how important this is. I can see it if you sit in a parabola of the stuff facing a fire. I can see that for space applications it would be very helpful as a satellite is going to get brutal IR, and convection in the vacuum of space will be neglible. But underground how important is radiation compared to other heat loss mechanisms?

I can put a liter of fluid next to me and warm it by conduction, a little. I haven't done the experiment but I doubt that I could do much to a liter of fluid a foot away from me by radiation, even if there was some mylar behind it. I"m not saying it's wrong to use the stuff but if the benefit is unsubstantial it would be good to know so that other actions could be prioritized.

Let's say there's a caver sitting in a passage with a small quantity of wind. 20% of his surface area is in contact with the rock. The rest is exposed to air. An aluminized mylar sheet is used or an equivalent non-aluminized plastic sheet is used to cover up and block wind/convection. Conduction will not change. Evaporation will be improved equivalently with both products. The only difference would be the amount of IR reflected. I"m willing to assume that much more IR will be reflected by the aluminized mylar (let's assume 97% reflection vs. 0% reflection) but how much is that difference compared to the total? The denominator would be heat loss prior to any use of the sheet. (I'm making up numbers now) If a plain plastic sheet reduces heat loss (by convection and evaporation) by 70%, does aluminized mylar reduce it by 71% or by 85%? I guess the question really is what is the magnitude of each of the forms of heat loss from the beginning?

Where this could be pertinant. If you are extracting someone in a litter who is wrapped in a vapor barrier, would adding another layer of aluminized mylar make a difference large enough to add it? If someone was already packaged would the difference be big enough to justify stopping to repackage if aluminized mylar showed up?

Any caver biophysicists out there?

In a temperate environment, 50-65% of heat loss is via radiation from the skin, with the remainder being mostly evaporation.

In a colder environment, convection (via wind usually) is a much bigger player. By the time a patient is able to use any sort of warming blanket/plastic, I would assume they have been removed from a situation where conduction directly into rock is a major issue, by simply putting some additional clothing between them and the rock/ground.

So, heat loss via radiation is a much bigger player than I believe you assumed.

Interesting. Do you have a published/peer-reviewed source for that information?

I'd like to use it for cave rescue training, and I suspect Roger would too. But I really need something more concrete than forum hearsay.

This is perhaps the best article. Please see figure 3 for a comparative amount of heat loss for radiation, convection, and evaporation at various temperatures.
http://www.pnas.org/content/23/12/624.full.pdf

Some other excellent references:

http://en.wikipedia.org/wiki/Black_body ... human_body

http://hyperphysics.phy-astr.gsu.edu/hb ... od.html#c1

http://academic.uprm.edu/~mgoyal/fluids ... apter5.pdf

http://books.google.com/books?id=ayeerc ... &q&f=false

http://books.google.com/books?id=dWVJPY ... on&f=false

http://www.sciencedirect.com/science?_o ... archtype=a

http://www.drphysics.com/convection/convection.html

http://www.dsto.defence.gov.au/publicat ... R-0849.pdf

Thanks!