Ordinary Banana

I accidentally fell down a deep rabbit hole while trying to figure out how pH meters work. I thought this would be a really nice simple example of a scientific instrument, suitable for didactic explanation as part of an exposition of the failures of logical positivism. FAIL

I mean how hard could it be to measure pH? Turns out there are two (or more) types of meters that work on entirely different principles. That they are measuring the same thing depends on masses of theory and experimental confirmation that the user doesn’t know anything about.

It appears to be literally true that no one knows how the better type works. They use a “glass electrode” that is semipermeable to hydrogen ions. The nature of the electrode is a closely-guarded trade secret and they appear to be discovered by trial and error, not theory.