I appear to be the tent king.

by Alex Hern

Pete Stevens left a very well-reasoned comment on my second tents piece yesterday, laying the physics out. I believe he’s wrong, but it’s worth giving it a bigger platform. Here’s a short excerpt:

A sleeping person produces around 100W of heat. If you’re in a sleeping bag so good you don’t give off any heat, over the 8 hours you expect the person to have warmed up by 100W * 3600 * 8 = 2880000J = 2880kJ. The specific heat capacity of water is 4.2kJ/kg/K, a person is roughly 75kg. That’s 315kJ per kelvin increase in body temperature. Overnight with your perfect sleeping bag we expect therefore the people to heat up by 9C and have a resting body temperature of 46C which would kill them.

On one level, of course Pete is correct; the mere fact that people in tents don’t suffocate means that air, and thus heat, must enter and leave them. Being opaque to IR is not the same as being totally thermally insulated.

And yes, after a full night in a tent, I think the heat probably would be visible on an IR camera.

But the IR footage was not shot after a full night; the only time we have for any of the ‘evidence’ is for the photo the Times triumphantly put on their front page, which is from 11:12pm:

Obviously, the main point about this photo remains the fact that no one is in bed at 11:12pm, but the further point is that if that one glowing tent does have someone in it, either they’ve been inside all day, or they are using an artificial heater. If you go to bed at 10pm (which is still pretty bloody early), your tent is not going to be glowing on IR by 11.

As another commenter said (I believe trying to discredit me):

It’s not an xray.

This is correct. You can’t simply point a camera at a tent and see inside it, and not seeing heat being given off a tent does not prove that the tent is empty.

The tents outside St Paul’s are full every night. The protesters are here to stay. So can people stop trying to discredit them with tangential issues, and start dealing with them fairly?

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