Tank pressure

[dlent]

I've noticed that my Aluminum 80 goes from 3000psi to 2500psi very quickly, but it goes from say 1500 to 1000 a heck of a lot slower. Why is that? I looked at an old physics book and I can't seem to find a formula to explain it.

 

[jham2081]

Tank cools off as it hits the water, dropping the pressure. (or so i've been told, i flunked my first try at college physics).

 

[Blox]

I've noticed that my Aluminum 80 goes from 3000psi to 2500psi very quickly, but it goes from say 1500 to 1000 a heck of a lot slower. Why is that? I looked at an old physics book and I can't seem to find a formula to explain it.

I think jham2081 is right on the money.
If I remember correctly, temperature change in the gas of 1 Degree C will result in a change of pressure by (roughly) 0.6 bar (obviously increase in temperature of the gas = increase in pressure and vv) or, in imperial, 1 Degr F temperature change will result in abt 5 psi pressure change.

 

[ToddMH]

I've noticed that my Aluminum 80 goes from 3000psi to 2500psi very quickly, but it goes from say 1500 to 1000 a heck of a lot slower. Why is that? I looked at an old physics book and I can't seem to find a formula to explain it.

Assuming the tank gas can be considered ideal:

P = nRT/V

That is, tank pressure is directly proportional to temperature. So if your tank is 3000 psi at 30 C (303 Kelvin) sitting in the sun and it drops to 10 C (283 Kelvin) in the water we'd expect the pressure to drop to:

3000 psi * 283 K / 303 K = ~2800 psi

I'm not sure how the non-idealities of gasses at high pressures figures in, though. Does somebody more schooled in physics care to weigh in?...

But based on that calculation, you lose a couple hundred psi just from cooling alone. Plus perhaps the average diver's air consumption is a bit higher at the beginning of a dive before they settle into a breathing rhythm (pure speculation).

-Todd

 

[dlent]

Thanks for the quick replies everyone!

 

[mtsidford]

I agree with the cooling down process, but I'd also put an additional comfort level halfway through the dive being a conservation factor. I am always comfortable thoughout my dive but I still get that rush at the beginning of the dive due to not knowing what I may encounter on the next dive even if I've dove the spot many times before. The decent to me is exilerating even hundreds of dives later. Midway through a dive I've got my breathing and fin kicking dialed in so my air consumption is at a minimum. Sid

 

[Lightning Fish]

At the beggining of a dive you have an empty BC and possibly an empty drysuit. As you descend you add gas to them. During the dive (at least for me) gas is not added, thus, the amount of gas consumed is less during the middle portion of the dive.

Bill.

 

[Kaiser442]

At the beggining of a dive you have an empty BC and possibly an empty drysuit. As you descend you add gas to them. During the dive (at least for me) gas is not added, thus, the amount of gas consumed is less during the middle portion of the dive.

Bill.

A good thought - but the amount of air you add to get neutrally bouyant should only be a few breaths' worth. I can't imagine it would make a noticeable difference.

I agree with the tank-cooling idea. That was my assumption when I noticed the same phenomenon. It's a good reminder that it works the same in the opposite direction - so don't ever leave a full tank in your car in the summer!!! BOOM!

 

[Lightning Fish]

A good thought - but the amount of air you add to get neutrally bouyant should only be a few breaths' worth. I can't imagine it would make a noticeable difference.

Its been my experience that it does. Though I think the effect is more pronounced with inexperienced divers. About 20 dives ago, my tank pressure would go from 3000 psi to 2000 psi during the descent. I think the greatest gas consumption during the descent was my BC and drysuit. (And breathing when I'm not quite relaxed as mentioned by mtsidford.) Now the drop is about 700 psi. As a diver I'm more relaxed and I think I use my BC and drysuit more efficiently.

Bill.

 

[cdennyb]

If you start using nitrox or trimix you'll hate the idea of shelling out the big bucks for a fill and then blowing it into your drysuit or bc, so, add a small pony bottle to your kit, use a spare regulator with a hose and PRV and use it to blow comp air in your bc and suit. On cold dives you'll fill it with argon gas and it'll keep you warmer. That saves your expensive breathing stuff for breathing. JUst what I use.