My Space Blanket adds 2.3 degrees C

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My Space Blanket adds 2.3 degrees C

Postby JohnnoMcJohnno » Sun 29 Aug, 2021 3:25 pm

Not 2.2, not 2.4, but 2.3 degrees C. I own two very different space blankets, The one to which the 2.3 refers is what you might call a heavy duty type, Grabber brand, 150 x 210 mm, and it weighs 333g. I have carried one of these (or similar) around since I was in Scouts, for no good reason other than it might come in handy. The other space blanket is a more recent purchase - it's a SOL Emergency Blanket XL, 150 x 244mm, and it only weighs 88g. How does it compare to the heavier blanket? Here is what I did to check them out.
Space Blankets.jpg
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I built a heat box out of plywood powered by three 25W incandescent bulbs. The heat output should be about 70 Watts. I read somewhere in the past that the heat output of a sleeping person is around 75 W, so it's close. The lid has been removed for the photo, but it too is plywood. There are holes in the top and sides to allow air to circulate.
Heat Box.jpg
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I bought four Inkbird temperature sensors. These interface to my phone. I set them to log at 10 minute intervals. Put them in the same spot and they read within half a degree of each other, accurate enough for me. One of them went through the wash cycle when I left it in my pocket, but I can't remember which. I dried it out, replaced the battery, and it still seemed to work - although it may explain some glitchy data that I got during the tests.
Sensors.jpg
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I set up a tent with sleeping bag and sleeping mat in the backyard. The tent is a Tarptent Motrail, so single skin. The sleeping bag is a Roman Extremelite Zero with synthetic insulation. I only took this hiking once and nearly froze - it's not very good. The mat was a Naturehike folding CCF mat, so nothing fancy.
The heat box went in the sleeping bag with the end of the box sticking out - the intent was to replicate the fact that your head is usually out of the bag when you sleep. I put one sensor on top of a double layer of CCF foam on top of the heat box. This was intended to try and measure the air temperature in the bag rather than the lid of the heat box. Sensor number two sat on top of the sleeping bag (but under the space blanket), sensor number three in the tent and sensor four measuring the ambient outside. All tent doors and openings were shut during the tests.
Tent.jpg
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The bureau forecast a run of cold weather (well cold for my place) with predicted overnight temperatures of between 4 and 5 degrees. That's about as cold as it gets where I live. I carried out four tests - 2 with the heavy duty space blanket, one with the light duty SOL Emergency blanket, and one with me rather than the heat box inside the sleeping bag. For each test I started without the space blanket, and put it on at around 1:30 am in the morning to see if it made any difference. The actual time varied depending on when I could drag myself out of bed. There was no significant wind during any of the nights that I am aware of or noticed. Here are the results:

Day 1 (Heavy Duty Space Blanket plus heat box - Test A)
Data Day 1.PNG
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Temperatures inside the sleeping bag increased by 2.32 degrees C when the space blanket was put on. Note also that the tent is on average 3.5 degrees C warmer than ambient.

Day 2 (Heavy Duty Space Blanket plus me)
Data Day 2.PNG
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Not sure what happened here, but I suspect I kept rolling from one side to the other affecting the results of the sensors on and inside the bag. At 3:30 I required a comfort stop and decided I'd rather sleep in my bed at that point. The tent temperature drops back to ambient as expected once I leave, but then jumps unexpectedly - sensor glitch? I can't draw any conclusions about the space blanket. However the tent averages 4.0 degrees C warmer than ambient (for the period before I leave the tent) which is close to the Day 1 result. Based on that it looks like my heat box is a reasonable approximation of me as far as heat output is concerned.

Day 3 (Heavy Duty Space Blanket plus heat box - Test B)
Data Day 3.PNG
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I decided to repeat the first day test to see if I could get repeatable results. Temperatures inside the sleeping bag increased by 2.37 degrees C when the space blanket was put on, so pretty close to the Day 1 result. However the tent sensor starts playing up at 2:30 and goes well below ambient - not sure why. If you exclude what looks like bad data then the tent is on average 4.2 degrees C warmer than ambient

Day 4 (Light Duty Space Blanket plus heat box)
Data Day 4.PNG
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Temperatures inside the sleeping bag increased by 4.3 degrees C when the space blanket was put on. The tent is on average 4.7 degrees C warmer than ambient (ignoring what looks like another data glitch from the tent sensor).

Conclusions:
1. Obviously I am really bored in lockdown
2. The heavy duty space blanket was not as effective at retaining heat as the light duty one. Not sure why. I'm guessing but the light duty blanket is much more pliable than the fairly rigid heavy duty one, so it may just be providing a much more effective heat seal when draped over the sleeping bag.
3. As such, it doesn't seem like much point carrying a heavy duty space blanket, unless you want it to provide double duty as a tarp or groundsheet.
4. A single skin tent with one person inside should be roughly 4 degrees warmer than outside, in the absence of wind.

Not convinced by my methodology so any comments are welcome.
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Re: My Space Blanket adds 2.3 degrees C

Postby slparker » Sun 29 Aug, 2021 4:50 pm

Thanks for this, it is an interesting experiment. What you have done, I suppose, is measured the prevention of radiative heat loss from a radiating body at higher than ambient temperature. The increase in ambient temp within the tent is also interesting.

I'm not a scientist, but 2.3 C might lie within the expected margin of error for such an experiment - which means that the result might not be statistically significant.

Also, 2.3 degrees C is not a lot.... IRL is it enough to justify carrying a space blanket for prevention of radiative heat loss?

Thinking physiologically (more down my alley), the best use of the space blanket might be to wrap it around the individual under the quilt/sleeping bag so as to limit convective heat loss. The space blanket shiny surface is designed to prevent radiative heat loss, but most heat loss from a human is not radiative but convective (indirect loss through the movement of cool air past the skin, evaporating the sweat or moisture from the skin surface) and, in instances where the person has wet skin/clothes there is also conductive heat loss (directly through wet skin/clothing to the outside environment).

Given this, wrapping the space blanket against the clothed person, and then covering with an insulating layer, is probably more effective than laying the space blanket over the quilt. This effect is also called a vapour barrier and prevents convective and conductive heat loss. This is one of the the recommendations when caring for a hypothermic casualty and is also recommended in very cold and dry climates as a strategy for staying warm.
Last edited by slparker on Sun 29 Aug, 2021 5:02 pm, edited 1 time in total.
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Re: My Space Blanket adds 2.3 degrees C

Postby Lamont » Sun 29 Aug, 2021 4:57 pm

Onya JmJ.
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Re: My Space Blanket adds 2.3 degrees C

Postby wildwanderer » Sun 29 Aug, 2021 5:13 pm

Great testing! Thanks for sharing. The 4.2c increase with the sol light blanket is reasonably significant.

It would be interesting to see the effect of a rainy or dewy night on the results and see what that did to comfort levels of you with the light space blanket on and the effect on temperature levels.
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Re: My Space Blanket adds 2.3 degrees C

Postby Ms_Mudd » Sun 29 Aug, 2021 5:57 pm

I love your first conclusion point. Yes, I concur.

Very impressive effort to have gone to, it was a great read.
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Re: My Space Blanket adds 2.3 degrees C

Postby jobell » Sun 29 Aug, 2021 9:10 pm

Damn.... Got my vote for best lockdown project. Not to mention best lockdown read! Thanks for sharing.

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Re: My Space Blanket adds 2.3 degrees C

Postby JohnnoMcJohnno » Sat 04 Sep, 2021 12:54 pm

slparker wrote:Thanks for this, it is an interesting experiment. What you have done, I suppose, is measured the prevention of radiative heat loss from a radiating body at higher than ambient temperature. The increase in ambient temp within the tent is also interesting.

I'm not a scientist, but 2.3 C might lie within the expected margin of error for such an experiment - which means that the result might not be statistically significant.

I'm not a scientist either, it really needs someone who knows a lot more than I do about heat transfer to do justice to your comments. But yes I thought the heat retention in the tent was particularly significant given it's only a single skin tent. Whether the relationship holds for lower ambients, well I'm not so sure. Can't offer any comments about using the space blanket for hypothermia either. I did have an occasion once where one of our party suffered an injury, not life threatening, but they appeared to be going into shock. I wrapped them in my down sleeping bag which corrected the situation. Didn't even think about using the space blanket at the time.

wildwanderer wrote:Great testing! Thanks for sharing. The 4.2c increase with the sol light blanket is reasonably significant.

Yes 4.2 degrees C is a lot. Bear in mind though I was using the worlds crappiest sleeping bag. With something decent I'm guessing the increase would not be as significant.

Ms_Mudd wrote:I love your first conclusion point. Yes, I concur.

I am still bored! Thank God for bushwalking forums.
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