Tents and Tents and Tents
by Alex Hern
Between comments and responses to my piece yesterday, as well as Occupy London’s official statement, a few other explanations have come to light for the scarceness of glowing tents in the photo and video I linked yesterday:
- The camp is very active at night; as can be seen by the number of people wandering around, at the time the telegraph turned up, there probably were a few empty tents, because their occupiers were still awake, out and about. Doesn’t mean the camp is full of people going home in the evening.
- The properly glowing tents are likely using propane heaters, which is why they show such an even heat density.
- If you are in a sleeping bag in a tent, you shouldn’t be giving off any heat. Heat that leaves your sleeping bag and warms up the tent is wasted heat, and heat that makes it all the way to an IR camera is very wasted heat.
Although I stick with my explanation yesterday as at least partially responsible (although I realise that rather than calling them “IR translucent” and “IR opaque” tents, I should have just referred to them as what they probably are – summer and winter tents), the third point above is likely also part of the reason.
Regardless: Rather than proffering a multitude of explanations as to why some tents glowed and some didn’t, we should remember the important point. Every paper that reported this story as fact was performing shoddy, shoddy journalism. The original claim still seems to have come from one city councillor looking at some footage from a police helicopter, and only the Telegraph even bothered to do more than print that entirely unsubstantiated claim. And their fact-checking was, well, poor:
The journalist in question was found by our Tranquility Team (night-time volunteers who look after the peace of the camp and who keep it ship shape in the twilight hours), shaking the tents of occupiers, who initially thought they were police and understandably approached them to enquire what was happening. The journalist explained that he was investigating a claim that tents were empty at night, and argued that it was correct to check this claim.
It is clear that there is a strong desire to discredit Occupy London on the part of many on the right. It is to be expected from the Telegraph – who are, lest we forget, edited by the ex-assistant editor of the Mail – but the Times remains, just about, the newspaper of record, and the BBC is supposed to be impartial. They should know better.
Interestingly, it appears the Times at least know they have been a bit dodgy. Compare and contrast the original piece,”Nine out of 10 campers quit protest camp at night” with this new one, which went up at 11pm last night: “Labourers are few on anti-capitalist night shift“.
Both broadly the same story, by the same journalist, illustrated with the same image – but the nine out of ten tents empty claim is quietly dropped.
One thing stands out to me – the BBC is supposed to be impartial – ha ha ha – hasn’t been for a long time.
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.
The absence of dead people in the tents suggests that their heat output is being lost to the environment, once it’s progressed through the sleeping bag it must progress into the tent and then out of the tent through the skin of the tent. This can only happen if the tent is hotter than it’s surroundings. Therefore a tent occupied by living people must be hotter than the surroundings.
To a first approximation the rate of heat loss is linear with temperature difference and area. From the photo we can estimate that the temperature of the surroundings is about 14C and the temperature of a person is about 20C as they show at the white end of the spectrum. The surface area of a person is about 2m^2, the surface area of a small tent maybe 6m^2. Doing the maths we get
2 x (20-14) = 6 x (T-14)
where T is the temperature of the tent.
Solving we get,
12 = 6 x (T-14)
2 = T-14
=> T = 16C.
Obviously if there are two people inside, there is the same heat output but a source area of 4m^2,
4 x (20-14) = 6x(T-14)
=> T = 18C
On a thermal camera caliberated between 14C and 20C we’d expect this to be visible.
To quote Homer Simpson, “In this house, we obey the laws of thermodynamics!”.
[...] Stevens left a very well-reasoned comment on my second tents piece yesterday, laying the physics out. I believe he’s wrong, but [...]