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Autoclaving Big Beakers and Flasks? Use the Vacuum Function!

If you’re regularly autoclaving large Erlenmeyer flasks, graduated cylinders, or beakers—and it’s not practical to run the entire load lying horizontally—a vacuum function is vital to your consistent success.

Why Autoclaving Containers is a Hard

Autoclaving any container is inherently challenging—even when that container is open.  Why?  Dense cold air tends to pool at the bottom of your beakers and flasks.  Steam can trap this in the beaker, creating cold pockets that are unlikely to reliably sterilize.

To address this, Priorclave includes Freesteaming and Pulsed Freesteaming as standard functions on every autoclave.  During freesteaming, the autoclave holds open its chamber vent while flooding the chamber with steam.  Owing to Priorclave’s pressure vessel design and construction, this churns the air in the chamber.  It’s a fairly effective way to clear the chamber of cold air pockets and saturate many different types of loads, without adding any additional moving parts.  (Pulsed freesteaming is very similar, but pulses the steam stream, creating additional turbulence.)

As Philip Berriman, an instrumentation technician at California State University (Fullerton), notes “Priorclaves really are better than most when you use the freesteaming, especially if you use pulsed freesteaming.  It does a good job of mixing the air up in there.”

But even pulsed freesteaming can’t cure all ills.  Freesteaming has difficult mixing the air pooled inside flasks and beakers—especially narrow flasks and graduated cylinders.  Covering the mouths of beakers with foil while autoclaving—a common practice in many labs—further lowers the odds of your load sterilizing properly.


In running a series of trails, Phil found that large vertical flasks covered with loose foil always failed to sterilize during a standard gravity cycle (that is, one with no vacuum stage)—even with a five minute freesteaming period. 

Removing the foil and keeping the freesteaming stage improved outcomes greatly: 60% of the beakers were sterilized by the end of the standard cycle.  Nonetheless, without using a vacuum cycle, vertically oriented Erlenmeyer flasks always failed to sterilize, even when no foil was present and freesteaming was used.

Vacuum Autoclaving Increases Reliable Sterilization of Flasks and Beakers

Using a non-vacuum cycle, foil covered flasks were only successfully sterilized about 40% of the time.  Adding just one pre-cycle vacuum pulse improved sterilization by about 90%.   Using three pre-cycle vacuum pulses virtually guaranteed successful sterilization.

Avoiding foil and autoclaving beakers lying horizontally are best practices—but, if for some reason, that’s not possible in your lab, “a vacuum is able to overcome many bad practices.”