Here’s one you’ve no doubt experienced first hand if you cave dive: It’s a hot day. Your tanks have been sitting in your vehicle all the way to the dive site and are now out in the open on a picnic table or tank rack. While attaching your regulators you note the pressure. It’s a very comfortable 248 bar/3,600 psi. After suiting up, you enter the water and begin your pre-dive checks — including checking your final starting pressure. You notice that your tanks now read 227 bar/3,300 psi. What happened?