View Full Version : PSI Question and Buoyancy Questions

10-12-2007, 12:34
Hi kids,

I apologize in advance for my ignorance, but I have a few stupid questions. I've been looking into purchasing a steel tank, preferably the HP X7-80. It says in the chart, which I've posted below that the Service Pressure is 3442 PSI. This is the service pressure for all of them, so for the first time, size doesn't matter? Does each size tank carry the same amount of PSI?

Also, as of right now, I'm diving in New England with either a 7mm or a 7mm Farmer Jane. I need roughly 30 pounds to start me on my way down the anchor line. Now, I was thinking that steel tanks are heavier and I wouldn't need to carry as much weight with me in my weight pockets. But, then I looked at the chart and realized that I would lose roughly 6 pounds of weight as the tank emptied out. How do I compensate for the weight that I would lose in the tank? Just overcompensate in the beginning?


10-12-2007, 12:51
size matters. 3442 psi is the max pressure any tank above will handle as far as LDS filling it and DOT is concerned. however, there will be more air in x7-100 than in x7-80 because the volume is different.
you are saying that you're diving with 30 lbs. what tank do you use? if you're using Al, then it's llikely that you'll take out ~7 lbs off your belt if you get x7-80. the most important features of a tank are buoyancy when full and when empty. the whole goal is to be neutrally buoyant at the end of a dive with 500 psi left in the tabk, no air in the bc.

10-12-2007, 14:27
Correct! The pressure a tank is filled to does not equate to its volume - so...a larger tank will still hold more air at the same pressure (psi).

Yep on the weighting as well. As you start out, you end up playing with your total weight quite a bit. As you become more comfortable with new gear and diving in general, you will find that you tend to naturally shed a few pounds.

Start by shedding the six or seven and see how the dive goes. Ask your buddy if you look trim in the water (good weight), head-up/feet-down (too much weight), or head-down/feet-up (too little weight) when you swim along. Then, at the end of the dive, see if you tend to float up and have to fight a bit to stay down (at the safety stop) or if you need lots of air in the BC to stay there. A lot of air means too much weight, fighting to stay there means too little. Ajust for the next dive and do again.

10-12-2007, 19:13
Thanks guys. That makes so much more sense now. I think what I'm going to do is get my breathing under control, so that I can shed some weight and then go for the steel tank.