Our lab focused on ultralow temperature physics. We ran experiments using dilution refrigerators (also see here), and these each required multiple vacuum pumps running continuously (in our case, each fridge needed a helium-leak-tight, sealed, mechanical "roughing" pump, a big conventional mechanical pump (for the "1K pot"), and a large diffusion pump as a "booster"). The mechanical pumps were housed in a cabinet in a room one floor below the main lab, and even with that kind of distance and insulation they provided a continuous background hum to the room. That basement room also contained our group's helium liquefier, an ancient beast of a machine (a twin is shown here) that took in recycled helium gas from our experiments, cooled it by using pistons to drive a big flywheel, and then liquefied it by squirting it through a tiny, cold orifice. The liquefier provided something between a wheeze and a heartbeat to the lab, a steady state "pachooka pachooka" sound with a repetition period of around one second when it was working well. The muffled version of this noise also permeated the lab. After being in the group for a few months, I understood completely where Scotty was coming from. It was deeply disturbing to walk into the lab and realize that something, somewhere was amiss because the sound or extremely subtle floor vibrations felt "off".
The liquefier (officially the Demon Liquefier From Hell [DLFH], or The Liquef--ker) was a formative part of our lab's grad school experience. Running the system, which predated any serious automated controls, required some amount of fiddling in the best of times, interpreting half a dozen cryptic gauges ("inches of water" as a pressure unit? Really?), with the only useful diagnostic being whether the liquid level in the big helium storage dewar is increasing or not. A period preventative maintenance every few months meant replacing press-fit bearings, cleaning amazingly stinky phenolic parts, and worrying that we would bend a cam "wrist" and be out hundreds of dollars for a spare as well as having the system be down for a week. Even before helium prices rose dramatically, recycling helium was a good idea if you could do it. One of the most depressing calculations you could do as a student in our lab, as you were listening to the intake purifier blow moisture like a sad sneeze and wondering why the hell the DLFH wasn't making liquid, was to compare the cost of your time, recycled helium, and externally purchased helium, and realize that it was clearly financially smart for your adviser to use you to maintain the system.
The DLFH was certainly educational. I learned a lot about engines and big mechanical systems. I learned that it is only marginally cheaper to build a heavy crate and ship via an express carrier than it is just to buy a plane ticket for a 130 kg flywheel. I learned what it feels like to take a jolt of 208 V (not recommended) and that yelped curses from that room could still be heard up in the lab. To this day I still reflexively shudder a bit when I hear that "pachooka" sound when I visit a place with a similar gadget.
The DLFH was certainly educational. I learned a lot about engines and big mechanical systems. I learned that it is only marginally cheaper to build a heavy crate and ship via an express carrier than it is just to buy a plane ticket for a 130 kg flywheel. I learned what it feels like to take a jolt of 208 V (not recommended) and that yelped curses from that room could still be heard up in the lab. To this day I still reflexively shudder a bit when I hear that "pachooka" sound when I visit a place with a similar gadget.
Fantastically recognizable. Including the economics...
ReplyDeletePlease spread these anecdotes out over a certain timeframe; it'd be a shame if we get an overdose now and are back to only physics later on.
Maybe I'm overly sensitive, but I think your remark about 208 V appears a bit too jokingly despite you saying it's "not recommended".
ReplyDeleteTalking about potentially dangerous situations in a less than serious way does not invite people to be careful about these things.
All in all (if this was 208 V from a wall receptacle, i.e. with a potential for high currents) you could have died - depending on the floor you were standing on, the type of shoes you were wearing, (or whether you were better grounded with your other hand!) and the humitidy in the lab.
I once got in contact with an (invisibly) damaged ion pump cable at 7000 V. Luckily I had good shoes, did not touch grounded metal with my other hand, and was standing on a raised polymer floor. Hence it only "tickled all over my body". Had only one of these thing inadvertently been different, my heart would have stopped.
I'm more about common sense than about safety rules, but the way we talk about these things have the potential to instill the right or the wrong attitudes.
Anon@12:16, a fair point. It scared the tar out of me at the time, and I probably should not have made light of it.
ReplyDeleteFor the curious: There was a pressure switch that had to be adjusted with a screwdriver, and basically the whole system had to be fully powered and running to know when the adjustment was correct. I was smart enough to wear rubber shoes, put on an insulating glove, and of course use an insulating-handle screwdriver. However, when reaching into the bowels of the machine to do the adjustment, my sleeve pulled back, and some bare skin on my wrist brushed between something (electrically) hot and something grounded. I yelped very loudly, basically flung the screwdriver across the room involuntarily, and started swearing. One of my lab mates dashed in, out of breath, to make sure I was ok. The real moral of the story: never EVER do something like that while alone. That was really stupid and dangerous of me.