Hi @Agaricus, not sure if it was already mentioned, but did you give a try to this app? https://play.google.com/store/apps/details?id=com.adrenergic.tremorsense&rdid=com.adrenergic.tremorsense&showAllReviews=true
It seems to have ok reviews, I just installed it and tried it and got scores between ‘1’ and ‘3’ on my tremor on both hands (which is apparently higher than zero but lower than significant. At least in line with my subjective experience).
They seem to use the accelerometer from the phone, take 1000 readings within the recording time (which can be adjusted to be 2,3,5,7 or 10 seconds), calculate the standard deviation of the values, and scale the result according to a modified arctangent plot, thus ending up with a number between 0 and 100 which they use as an estimate for tremor severity.
Plus point: their code is on GitHub. (Big) minus point; it does not record readings! So screenshotting/manual logging would be needed in a scenario in which this is used to compare tremor data with other data sources.
Fun fact: they seems to suggest a way of grabbing the phone while taking the measurement which is roughly in line with @Agaricus’s intuitions so far.