I dunno, once you do any of those you influence the outcome. I'd just leave it alone. Once you open it up you can no longer observe it in the state it was, think Schrödinger's Law or like Charles' Law and Boyle's Law and stuff. If there is moisture in the tank, which is what it sounds like, then it's probably better to estimate the moisture amount using the sound of moisture. If you open it up, and it has pressure, you'll never be able to effectively measure the moisture. If there is no pressure and there is moisture then that's a really odd situation as well and best to leave it alone? *shrugs*
The tank could easily have 15mL of water (as is common) or worse 15cc or even worse 15 cm^3 of water.