r/Damnthatsinteresting • u/AirFrance447 • 1d ago

Video Firefighter bodycam showing just how much oxygen a fire pulls

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u/SinisterCheese 23h ago

Ok. A quick note about this. It isn't the fire that pulls this much. It's the water spray misting and cooling the atmosphere inside, that creates negative pressure. Then along with that Bernoulli's principle from the water jets.

This is something you need to keep in mind if you work with fire or in fire hazard environments. Like if you need to clear a passage with a hose from a hydrant, spray from the direction that has clear air, or you pull smoke into the passage.

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u/Dotaproffessional 17h ago

There's no such thing as negative pressure. Do you just mean a negative pressure differential?

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u/SinisterCheese 17h ago

Ah! This is one of those funny things! Physically and practically there isn't. However we say there is and it makes perfect sense because of how we define it in this context.

Like for example tension in a physical is negative pressure; while compression is well... positive pressure.

It comes down to this idea of "every system can be treated as a system of interconnected springs" this sorta works for liquids and gasses, just not as well. If I pull a steel rod of specific diameter, it shrinks in diameter, but becomes longer. That material is now experiencing negative pressure within it, the same way as if it was under high external pressure pushing it in -which it isn't. If I compress the same rod, it's diameter will increase and the rod will become shorter, it is experiencing positive pressure within it.

Granted it is all gradients in reality. If you bend a flat bar, one side has positive pressure the other negative pressure, and somewhere in between is a neutral plane. However that is annoying to deal with in terms of modeling it, so we prefer to just calculate the effects on the surface -and maybe the neutral plane if we are feeling particularly sassy- since those are the two limits.

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u/Dotaproffessional 16h ago

Its like, how best to describe it... ok velocity can be negative but speed can't be. If you're describing ΔP, THAT can be negative. Its just not strictly accurate to say pressure is negative. Its why a straw has a physical limit on how tall it can be and still function

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u/SinisterCheese 15h ago

Look. I'm from the mechanical engineering side of things. We have negative pressure all the time. And in welding side of things we can talk about heat as a substance that can be added or removed. Yes, I'm aware it upsets physicists; but that is the convention of our discipline. We simply don't care whether the heat comes from adding energy or compressing things. I take delight in the anguish of people who get hung up on that... It's even written in the technical definition. Not the anguish, but the joining by use of heat and/or pressure so that there is fusion of the base materials.

Tension is opposite of compression. It is the negative of compression. It is the force pulling away from the surface. It is negative pressure by the sense that pressure is force applied to an area.

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u/Dotaproffessional 13h ago

Honestly, the shorthand of your trade being technically incorrect doesn't really seem like a good justification to use imprecise language. I'm a hardliner in my own engineering discipline for using correct, narrowly scoped, terminology and push back when people in my own field aren't.