How should I go about loosening knurled knob on centrifuge that's been overtightened?

[EddyCurr]

Good practice on the part of the designer/manufacturer calls for markings to designate left hand threads. Big fail in this instance.

The circumferential slots in the faces of wrench flats for hex fasteners is one method of marking (hose-to-gauge fittings on welding gas bottles is a familiar example.) Crescent shaped marks on the tops of fasteners is another and would have possibly prevented the situation in this case.

Of course, awareness that fasteners are not always 'lefty, loosey' and that marks may or may not be present is essential.

.

[BCRider]

Another idea for things like this would be directional knurling or fluting that tends to let the hands slip more easily while tightening but obtain a harder grip for loosening.

A bit of lettering tape on the lid might aid in this not occurring in the future.... well, perhaps not as often at least.

It would not be that hard to make up a simple small strap wrench for something like this which uses a short length of 1/2 or 3/4" nylon webbing and a saw cut in the end of a bit of steel rod with a hole on the other end for a cross handle.

[A.K. Boomer]

This is exactly why I stated the "obvious" back in post 6

even if there's no protruding threads you can still for the most part figure out what direction thread it is - even if all you have is the end of the nut to look at - take your time, get a magnifying glass if you have too, whatever it takes because nothing is really more critical than knowing what direction to turn when going to "loosen",

also - as I stated - any time something itself is rotational (centrifuge) there should be a big red flag going off about it possibly being a specific direction thread... many a apparatus is broken due to someone not wanting to take a quick few minutes to analyze the specifics...