Hacker News Reader

Comment by bombcar | Hacker News Reader

Those emergency blankets don’t work like normal blankets - by trapping air and preventing it from moving - but by reflecting heat and keeping it in.

View on HN

I'm not a physicist but wouldn't this reflective effect be tiny compared to the prevention of convective heat loss?

I spent a night in the woods (on purpose, not a real emergency) sleeping with an emergency blanket. It was not at all comfortable, but the blanket seriously worked. I would wake up whenever it moved to stop covering me because it would instantly become cold, then feel much warmer as soon as the blanket is back in place.

The reflective action is impressive. They also act as a wind break and turn whatever clothes you are wearing into a more effective measure against convective heat loss.

A long time ago I similarly tested a (non-heat-reflective) orange plastic survival bag, sleeping overnight behind some rocks on exposed moorland (something like https://www.military1st.eu/cs037-highlander-emergency-surviv...).

The poor thermal insulation and condensation were predictable. I hadn't predicted the shifting wind which chose to balloon my orange cocoon with cold air every 20 mins no matter which orientation I chose to lie in. I finished the night with a bungie cord around the sack, under my armpits. That of course left my head, shoulders and arms exposed but kept the wind out. Not recommended.

After this experience I upgraded to reflective mylar survival bags. NB "bag". These are a little harder to find than survival blankets but if a bag can be so badly affected by moderate wind, a blanket would be so much worse.

I sewed one into my ski jacket. Always good to have in case of injury and help an hour away.

Considering average human body area is around 2 square meters and body temperature is around 37 degrees Celsius I get around 1000 W of radiation from the human body according to this calculator: http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/radfrac.h...

Or to put that in terms of net heat loss, in the 0-30C range you're looking at about 10 watts for every degree of difference between your skin and the environment.

Backing out human power consumption from caloric requirements comes closer to 120W constant output, at 2,500 kcal/day.

The range is a low of about 50--75W (1,000--1,500 kcal, effectively starvation levels), and a maximum ceiling of around 485W (10,000 kcal day, perhaps a elite endurance athlete). Well below 1 kW in all cases.

Human skin temperature is considerably lower. (Or to think of it from another perspective, the system which is insulating your core temperature includes layers inside your body – skin, etc. – in addition to clothing, blanket, etc.)

And they are not comfortable, as this post shows:

https://www.reddit.com/r/CampingandHiking/comments/4wtetp/im...

But if it is an emergency, and that's all you've got, it's a lot more comfortable than not having one. It's almost like they are purpose designed by looking at the name of the product.

> Those emergency blankets don’t work like normal blankets - by trapping air and preventing it from moving - but by reflecting heat and keeping it in.

This reads as if to imply emergency blankets vent air freely, that's obviously not true.

Thermal insulation, whether we're talking about a blanket, a winter jacket, or the foam stuff in your walls, generally involves a layer of some thickness within which there are many tiny spaces full of trapped air. It's difficult for heat to propagate across those spaces. The author's point is that mylar emergency blankets do not provide this thick layer of "trapped air". It's true that they prevent air from crossing, but only for the very thin layer of the blanket.

I'm not sure exactly how effective the mylar blankets are and under what circumstances, but this is what "trapped air" refers to.

Heat is transmitted by radiation, by conduction and by convection. A foam jacket blocks heat transmission by preventing convection. A mylar blanket prevents transmission by radiation and convection. Yes, it still loses heat to transmission but it isolates rather well. Unsurprisingly, that’s why they are used as emergency blankets.

You'd be surprised how hot surrounding oneself with mylar gets. I've been inside a grow tent lined with mylar, and even though it was open on one side, it felt like a sauna within minutes!

Homeless people use garbage bags to similar effect, you don't even need fancy mylar, it's just an optimization.

The main problem with these solutions AIUI is the trapped humidity.

Reply on news.ycombinator.com