Whilst I might accept that your improved explanation makes it more plausible
since I forgot to factor in the 200L/8 hours thing, you most certainly did not fail to asphixiate people because "air is mostly nitrogen". That's irrelevant. You still can't respire nitrogen. You failed to asphixiate people because you were extremely fucking lucky.

However, I'm confused about this still. Dewars don't pump liquid out. They allow a tiny proportion of the internal liquid to vapourise, then vent this gas slowly to allow themselves to self-cool because the latent heat necessary for vapourisation needs to be sucked from the surroundings. Leaving a dewar to stand around in your office might cool the office (although we'd be getting into a bit of a 1st/2nd law of thermo situation so it's probably not that simple) and will produce a cool gas layer on the floor, as cool gas sinks, but it's demonstrably not pumping LN2 anywhere. And it's still really dangerous unless your office is the size of a hanger.
( the mighty badger Aphrodite, on a bar stool, by your side, Wed 21 Sep 2011, 9:06, 2 replies)

I think word dewar is often interchangeable (even though it shouldn't be)
Some of our lot call the 200L storage tanks dewars, when they're not. Our magnetism lab wasn't much bigger than an average sitting room, but with the windows open, we'd pump ~120 litres through the SQUID when cooling it down over 6 hours, all of which came pissing out the exhaust port. Still didn't manage to set the O2 alarm off.

Still, not as much fun as the time the techie was a lazy shit and forgot to fill the N2 dewar in the SQUID. When it boiled off, the vacuum jacket decided to desorb all of its contents and create a heat sink for the helium dewar, which resulted in a screaming jet of helium boiing off through the safety valve. Fun times.
( Colonel Boris "...a desperate Buzzfeed imitation...", Wed 21 Sep 2011, 10:55, closed)

Helium's a different matter
I used to use a lot of helium too - in fact "helium evaporator" was more or less my job title for a bit. Any significant disparity between the liquid I collected and the metered gas I returned brought a furious technician to my lab door with forthright suggestions about my gas seals and my parentage.
(ubergeekian, Wed 21 Sep 2011, 12:07, closed)

This reminds me
of helping fill a superconducting electromagnet with liquid helium.

I was on work experience (back when I was 16) at a university and they let me play with high intensity lasers, cray computers, superconductors, extreme vacuums and liquid helium.

Not at the same time, unfortunately, that WOULD have been fun...
(Imaginary Ed was dreamed up on, Wed 21 Sep 2011, 13:50, closed)

There's a 1 kW unshielded laser lying around in one of my labs
it's aimed at the door. as you do. Fun times.
( the mighty badger Aphrodite, on a bar stool, by your side, Wed 21 Sep 2011, 14:06, closed)

Liquid helium
is an awful good way of making high (if not very extreme) vacuums. Cryopumping, it's called. ah, happy days and great fun.
(ubergeekian, Wed 21 Sep 2011, 16:35, closed)

Dewars
There are lots of different designs. For fast filling of the test rig I had a small centrifugal LN2 pump which was lowered through the neck. We tried (sorry, would have tried) that for the air conditioning experiment, but it was (would have been) a bit noisy. So we tried (hell, I can't be bothered pretending) method two. You seal the neck with a rubber bung containing a dip tube which goes to the bottom of the flask. As heat leaks in the thing self-pressurises and drives liquid out, just like the LPG tank on a car. That's how LHe dewars work, by the way, though you sometimes have to encourage things with a bit of work on a gas-filled bladder.

Whether this works for LN2 depends on how good your dewar is. Too good and it doesn't push enough out: too bad and it empties too quickly.

You are quite right about the physiology. We weren't poisoned because nitrogen is not (at those pressures) poisonous, which is where the air-is-mostly-nitrogen comes in. We weren't asphyxiated because the room was well ventilated, we weren't putting that much more nitrogen into it, and because the nitrogen rich area would have been at the floor. Touch shit for the mice, possibly.
(ubergeekian, Wed 21 Sep 2011, 12:05, closed)

Interesting.
you'd need to watch it, though, - the N2 rich air is only at the floor because it's cooler. N2 is marginally less dense than air, all else being equal, so it's really the room vents that matter.

I'm surprised by the amount in both yours and Boris's posts though - I can trigger a full 02 alarm just by mis-filling a small storage dewar, although all our labs run odd sealed air handling systems to be fair.
( the mighty badger Aphrodite, on a bar stool, by your side, Wed 21 Sep 2011, 14:05, closed)

Sure
It was the coolth what done it. It was just a normal office, so no alarms of any sort. None in the helium labs either - you knew something was leaking when your voice went funny. Seriously.
(ubergeekian, Wed 21 Sep 2011, 16:36, closed)