True Lab Stories: The Plastic Lens

(Series explanation here.)

The lab I worked in in grad school contained a bunch of miscellaneous objects whose purpose was a little hard to discern. One of the oddest was a big heavy acrylic lens. It was probably an inch thick, and two or three inches in diameter, and had four screw holes around the outside edge. It wasn't a terribly good lens, but it was a lot better than you would've expected from the origin story.

(More after the cut.)

The experiments we were doing required ultra-high vacuum, despite a fairly high gas load, so we had a variety of big vacuum pumps. One of the pumps we used was a cryopump, which works by getting a metal plate extremely cold-- down to liquid helium temperatures. You fill it up with ultra-pure (and ultra-expensive) helium, and there's a compressor inside that liquifies the helium, and pumps it down to a metal plate that sticks out into the vacuum. Pretty much any other gas you put in the system will freeze out onto the wall at that temperature, which gets you a nice low vacuum, and it's exceptionally clean.

Of course, like any other vacuum component, you occasionally want to clean it up even more, which you do by shutting the compressor off, and heating everything in the system up while pumping on it with another vacuum pump. You take your whole vacuum system, wrap it up in these flexible heater tapes and aluminum foil (to distribute the heat a little more evenly), run it up to a fairly high temperature, and leave it there for a weekend.

During one of these bakeout cycles, a visiting post-doc was given the job of baking out the cryopump section, so he looked up the bakeout specs for the system, and found that it could be heated to 400 degrees. The higher the temperature you can get to, the better, so he ran the heaters all the way up, and left.

Of course, being European, it had never occurred to him that the temperature might be specified in anything other than degrees Celsius. And 400 C is a good deal hotter than the 400 F that was the actual spec... The heater tapes left scorch marks on the casing of the pump.

For no reason I can fathom, these pumps always have a Plexiglass window through which you can watch the workings of the compressor. I have no idea what purpose it serves-- for all I know, it might just look cool. I don't know the purpose, you see, because the window wasn't there on the pump that I used. When it got heated to 400 C, the plastic softened, and the vacuum on the inside pulled it in, turning the thick acrylic window into a surprisingly good lens.

When they called the manufacturer to ask what to do, they stumped the engineers. They had never heard of anyone doing anything remotely like that, and had nothing to suggest. The guys in the lab cut a piece of aluminum to the same diameter as the window, and stuck it in there, and to everyone's surprise the pump worked fine after that (and the tech support people at the cryopump company probably got free drinks at the next trade show, for having the best dumb-ass scientist story ever...).

And we had a mediocre acrylic lens lying around the lab for the next several years, as a conversation piece.