I've used iAnalyzer ($19.99) to check timing belt tension. Set for no weighting, FFT, and 3 peaks, it seems to give repeatable results. Peak 1 at 110 hz agrees with the 5 mm wrench measure. Anybody else tried this?
This looks like the best app I have seen for tension. I tried a bunch of free apps with little success last belt change so I just went with the 5mm that has worked in the past.