[browniee]

I realised that there are alot of pharmaceutical industries that set the number of rotations for the friability tester at 100 revolutions. Why is this so? why must it be at specifically 100 revolutions ?

[sjb]

Possibly a chicken and egg situation? The earliest machines allowed testing at this setting (due to electric frequencies? I don't know), so SOPs were written and applied with this in mind, and as such it became a de facto standard?

[Borek]

I doubt it "must be". But definitely you want tests to be normalized, that means using identical settings everywhere. 100 rpm is a nice, easy to remember number, plus, as sjb wrote, it could have some historical reasons.

I know a story of an industrial lab where they run viscosity tests during resin synthesis to tell when to stop the polymerization. Viscosity was measured at some strange temperature, say 67 °C. Technically it could be any temperature from the 40-70 range or something like that, so why 67? Well, the only reason was, at the time the synthesis was set up (and the viscosity test was calibrated) that was the only setting available (because of some malfunction of the thermostat). It stick, as whole procedure was designed around it.