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.

10 thoughts on “True Lab Stories: The Plastic Lens

  1. And that is why any other system than the metric system is wasteful, confusing and useless.

    Dagonzus of the Borg.

  2. Or why European postdocs are wasteful, confusing and useless. That’s not quite fair. Perhaps the story shows why postdocs are useless.

  3. Why “ultra-pure” helium for the refrigeration?

    Would a part in a hundred of N or Ar freezing out really cause problems for the system?

  4. I’m not sure how big a problem the impurities would be– I just know that the pump manufacturer specified ultra-pure HE, and we didn’t feel like finding out what cheaper gas would do. I suspect that enough ice in the wrong places could really do a number on the compressor, but I’m not really sure.

  5. At least he was smart enough to realize it wasn’t 400K. The real problem to the situation though, is that whomever wrote the bakeout specs didn’t specify the units. This is yet another example of why we always specify units.

  6. Well, it would have been better for the pump, but worse for Chad’s blogging career, if the postdoc had thought it was 400 K ~ 260 F. It’s a very good thing he wasn’t a theorist, because a temperature without dimensions *should* mean natural units (speaking as a theorist), and the Plank temperature unit is something like 10^33 K.

  7. US engineers use degrees F, feet and pounds. Sensible people use metric units. Remember that Mars probe that burned up in the martian atmosphere? Feet do not equal meters.

    And here’s another imponderable for you: why do math books consistently use 32 ft/s2 for g? Are they training only engineers?

  8. Ha ha, what a great story!

    I’m always afraid I’m going to overcook dinner, I have a French stove and American recipe books. I’m always too lazy to do the conversion in my head, but luckily I have a theorist husband around to do it for me, and I haven’t yet forgotten. But life would be so much easier if there was just one units system.

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