Hi, I'm working on a project and need a bit of help, since I don't want to kill myself. My question is this: If I have a cylindrical tank (think a big scuba tank) half full of liquid, half full of gas (let's just say it's water/air) and start heating up the liquid, I know that pressure will go up as the liquid heats up the entire tank. But I also know water is (practically) incompressible. So is the pressure change only in the top half of the tank where the air is? Am I correct in assuming that as the pressure in the top (gas filled) part of the tank, the pressure in the bottome half (liquid filled) is more or less the same? I'm confused because as a diver, I know that when calculating pressure in terms of diving, we need to add 14.7 psi (1 atm) to the calculation, to compensate for the pressure of the air above the water. So at 33 feet, which is the depth that represents 1 atm, we actually have to calculate it as 2 atm with regards to figuring out dive total and avoiding the bends. This suggests that the pressure above a liquid translates into pressure throughout that liquid. On the other hand, I also know that when scuba tank manufacturers test tanks hydrostatically, they fill the tank up with water first. Because water cannot compress, the tank will hhave way, way, way less kinetic energy @ 3000+ PSI then it would if it was filled with air @ 3000+ PSI. So even if it fails the test and blows, the damage will be much less. To me, this suggests that air pressing down on a liiquid below does NOT translate that pressure throughout that liquid. The reason I'm wondering is, I'm doing a chemistry experiment where I need a tank that can withstand high pressures/temps, in the range of 1500 psi and 250 celsius. Pressure tanks are insanely expensive, and scuba tanks can easily handle that kind of pressure, so I'm thinking about modifying one to use as a pressure tank. I'll need to drill a few holes near the bottom, and weld on some npt threaded fittings. I need to know if the ball valves/piping etc connected to those fittings need to be rated to the same PSI pressures, or if they (because they are completely submerged inn the liquid) are "shielded" against the high pressures in the air pocket by the liquid. And I'd like to not maim myself, so any help from someone who understands pressure and fluid would be a big help. Thanks in advance.