On Friday afternoon, I was looking for one of the technicians which is quite aspirational because The Institute has many of the attributes of the Mary Celeste after lunch on Fridays. I noticed a lonely white-coated body in the lab and went over to commiserate about the loneliness of the long-distance scientist. Turned out he was an outside contractor at the end of his day 'calibrating the autoclaves'. I've written quite a bit about autoclaves on The Blob, partly because I am terrified of the things and partly because I want to make sure none of my students have a damaging hot, wet, pressurised accident while using one. Accordingly, I pointed to the beige splodge on the lab ceiling where a student had launched boiling agar skywards as well as catching some on her face. Then I told him about appointing students Autoclave Liaison Officer so they'd learn how to use that equipment safely. Having unloaded on the poor visitor, I then asked what he was doing and how he was doing it.
heat - hold - cool cycle. tsk, tsk, he shared, that autoclave is not actually holding its temperature for the full 15 minutes. But he wasn't about to fix it, he was only the calibrator. The SOP standard operating procedure says to heat the thing up until it achieves a pressure of 15 p.s.i. and a temperature of 121oC. In any closed system these two statistics are locked together by Gay-Lussac's Law: P/T = constant. On our autoclaves, because the pressure P is measured on a gauge [L: Top L] we don't really need the temperature gauge [L: Mid R] because we believe St Joseph of Gay-Lussac. When the needle reaches the red zone on the dial, we say, then start timing 15 minutes. But one of our autoclaves never quite reaches the red zone because the gauge is set low, and a certain type of student sits around waiting for
Because 15 p.s.i. and 121oC are only rough rules-of-thumb that have worked in most cases, most of the time since 1884 and therefore they must have a good whack of tolerance built in. It's a racing certainty that your Petri dishes won't be contaminated if only 118oC is achieved. You sort of know this is a makey-uppy seat-of-pants measure because 15 p.s.i. is as near as dammit 1 atmosphere (or 100 kiloPascals in new money). The value, originally chosen for convenience, has now become an article of faith. Part of the training that scientists should get is that they learn when to cut corners safely rather than slavishly following