Ok, so nothing in this thread makes me think that I should build a dry testing unit. A decent gauge is $300 and it's still not nearly as sensitive as the purpose built Witschi one. On top of that there is no software to do proper calculation for deflection, pressure and rate of change tide into the system. In all, I'd be guessing if the watch is water tight or not. On the other hand - no bubbles and we are good to go. I'll stick with my water based tester for now.