Tank pressure

[Sideband]

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!

How hot is your car? Even an increase of 50 degrees F only amounts to an additional 250 - 300psi. I have 4000psi burst disks in my tanks. The temperature would have to increase ~200 degrees before my burst disks even blew.


Joe

 

[Kaiser442]

How hot is your car? Even an increase of 50 degrees F only amounts to an additional 250 - 300psi. I have 4000psi burst disks in my tanks. The temperature would have to increase ~200 degrees before my burst disks even blew.


Joe

Well shame on me for not doing the math then... It's not uncommon for the temp in a closed car to go 50 degrees+ over ambient - but not so much 200 degrees.

:blush:

 

[SeanQ]

Assuming the tank gas can be considered ideal:

P = nRT/V

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?...

There's a good reason we use the ideal gas law; working in the conversion factors takes too much time.

While we have the neat ideal gas equation, PV=nRT the real gas equation (van der Waals equation) is more complex.

(P(real) + ((n^2)a)/V(real)^2) x (V(real) - nb) = nRT

Where a is a constant for gas reflecting the intermolecular forces and b is constant actual volume of the gas molecules.

Now remember air has two main components, oxygen gas and nitrogen gas, so we would have to do the equation twice for each partial pressure. Yeck.


Let me get some paper...

 

[Blox]

Let me get some paper...

ARRGGHHHHH

 

[markfm]

Nah, do it with a slide rule

 

[WaterDawg]

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!

Thats not really the case, or Id be in real trouble! But yes press does goup, just not that much.

 

[JRScuba]

I agree that the physics discussion does have some merit. I had often wondered the same thing until I watched my pressure gauge on a few dives..
These were my observations:
Started with 3000psi in my garage. Went to the boat, put my gear on. Checked pressure: 3000psi. Standing on back of boat, put air in my BC. Inhaled on my reg a few times to make sure my valve was open. Pressure: 3000psi, jumped in water, decended to the bottom ( 45 ft ), added air to my BC. Pressure: 2900-2950. Started finning and in 6 minutes. Pressure dropped to 2500psi. Immediately thought " Wow I using gas up pretty fast.

After awhile my breathing got into steady rhythm and my gas consumption slowed down quite a bit.

So everybody is correct, only IMHO breathing is the cause the fast consumption. Not theory-- Practical

If there are any doubts, do what I did and Left me know how you made out. I'm always willing to learn something new.

 

[WaterDawg]

I had a tank that after riding in my friends hatchback for 2 hours read 3500psi I put it in the spring (70F) and wet to get more gear after 10 mins it was back down to 3000.

 

[dannobee]

Pressure: 3000psi, jumped in water, decended to the bottom ( 45 ft ), added air to my BC. Pressure: 2900-2950. Started finning and in 6 minutes. Pressure dropped to 2500psi. Immediately thought " Wow I using gas up pretty fast.

After awhile my breathing got into steady rhythm and my gas consumption slowed down quite a bit.

So everybody is correct, only IMHO breathing is the cause the fast consumption. Not theory-- Practical

If there are any doubts, do what I did and Left me know how you made out. I'm always willing to learn something new.

This is still consistent with the gas cooling off. The air in the tank will not cool off instantly after contacting the water, but will take a few minutes because of the differing heat transfer properties of gas. After the tank wall starts to cool, it still takes a while for the heat to come out of the gas. Adiabatic heat transfer and all that stuff.
Try a different experiment. Check your tank pressure first, then put your kit in the water for a five minutes before you get in. I'll bet the pressure will drop by nearly the same amount.

 

[Blackwood]

Assuming the tank gas can be considered ideal:

Which everyone does since they use Dalton's law (an ideal gas assumption).