Does this seriously work? I'll give it a try I think, would be incredible if it does.
I tested one racket that was fairly newly strung (some days earlier) that I hadn't played with and the app gave 1 kg lower than what I had requested from stringer. And I find it credible that a polyester string had lost a kg in a couple of days. And my other more used rackets measured lower as they should.
But what impressed me most was that I could test a racket several times and get same result every time. This was far from a scientific test, but there is definitively something to it.