Actual air in the tank

[Far_X]

I am putting this in here as I think I must be dumb not to understand this. It is the result of a conversation I have had recently.

If you have a LP steel 80 and a HP steel 80, both holding the same volume of air but the HP is of course holding air at a higher pressure. What seemed logical to me was that the high pressure steel would give you more time in the water, forget decos and nitrogen for the moment; that the HP holds more molecules of air and so you have more to breathe. The actual breath you take would contain the same amount of air molecules regardless of which tank it came from due to the first and second stages. But I was told that they were both the same volume and that they would give you the same amount of time underwater. If this was true, then the breath from the HP must contain more molecules if it is to last the same amount of time as the LP. This just does not seem right to me!! Can anyone explain where I am going wrong?

 

[Quarrior]

What you need to understand is the physical dimensions of the two tanks. The HP 80 is physically smaller than the LP80, therefore the HP is required in order to pack in the same CUFT of air. As for usage, you'll get the same out of both tanks.

 

[pete340]

I am putting this in here as I think I must be dumb not to understand this. It is the result of a conversation I have had recently.

If you have a LP steel 80 and a HP steel 80, both holding the same volume of air but the HP is of course holding air at a higher pressure. What seemed logical to me was that the high pressure steel would give you more time in the water, forget decos and nitrogen for the moment; that the HP holds more molecules of air and so you have more to breathe. The actual breath you take would contain the same amount of air molecules regardless of which tank it came from due to the first and second stages. But I was told that they were both the same volume and that they would give you the same amount of time underwater. If this was true, then the breath from the HP must contain more molecules if it is to last the same amount of time as the LP. This just does not seem right to me!! Can anyone explain where I am going wrong?

The 80 in the description of the tank is the volume that the gas in the tank would occupy at one atmosphere. Both tanks hold the same number of gas molecules at their rated pressure. If the rated pressure is higher the tank's internal volume is less (P1*V1 = P2*V2).

 

[jonnythan]

An LP 80 and HP 80 both have exactly the same number of molecules (in theory) when full.

The HP 80 will have them all packed into a smaller space - hence a higher pressure.

So, they should both give you the same bottom time.. the only difference will be that when you reach a pressure low enough that the reg stops delivering air, the LP tank may have very slighly more air than the HP tank.

If both hold 80 cubic feet, the 80 cubic feet is really a measure of how many air molecules are in there. It's a way of saying "there are enough molecules stuffed into this tank to take up 80 cubic feet of space at atmospheric pressure." Since the internal volume of the HP tank is smaller, you need more pressure to pack them all in.

Make sense?

 

[MSilvia]

I'm not sure I follow your logic Far_X. Regardless of their rated pressures, if two tanks contain equal volumes of air (80 cubic feet) I'm not sure what makes you think one contains more molecules than the other. The molecules may be more or less densely packed, but that's not the same thing.

 

[Far_X]

Ok, I think I have it. I was thinking that if you filled an 80 LP and an 80 HP with water, it would take the same amount of water, without pressurizing the water. When people say cubic feet, you kind of think that LxWxH gets you there So because the 80 HP is smaller in dimension, I can see now whey they would be the same. Thanks for that. Boy do I feel as if I have crossed a big obstacle today...LOL.

 

[jonnythan]

Imagine how big 80 cubic feet is. That's a box almost 4.5 feet to a side.

Mighty big air tank

 

[Nemrod]

Most tanks are not actually able to hold the 80cf or 100 cf or whatever. They are usually rounded up for convienece. It is easier to say 80 or 110 or 72 than 78.96 cf.

Your first stage regulates down to a specified intermediate pressure from the tank pressure (regardless of what that is) and will maintain that pressure with some drop when you inhale until the tank pressure falls below intermediate which is usually somewhere like about 135 psi (but it varies). The simple answer, 80cf is 80cf regardless of the pressure but the tanks will vary in dimension. N

 

[jonnythan]

BTW, the internal volume of an HP 80 is around 1/3 of a cubic foot and an LP 80 is between 2/5 and 1/2 of a cubic foot (quick n dirty calculation.. real life numbers are probably pretty close).

 

[WJL]

Ok, I think I have it. I was thinking that if you filled an 80 LP and an 80 HP with water, it would take the same amount of water, without pressurizing the water. When people say cubic feet, you kind of think that LxWxH gets you there ...

This is the difference between how scuba tank capacity is measured U.S. style (e.g an "Al 80" cylinder) and "metric" style (e.g. a "12L" cylinder).

The U.S. style measurement refers to how many cubic feet of gas the tank holds at its rated pressure. The metric style refers to the actual internal volume of the tank - how many liters of water it holds, without paying any attention to pressure. So your original thinking was correct if you were dealing with metric tanks