which tank gives me more bottome time?

[lee3]

I was at my dive shop this weekend and got a confusing answer about the air in a tank. I was looking at 2 tanks, both steel, one was 80 @ 2500 + 10%, the other was a steel 72 @ 3000 + 10%. I figured I could multiply the size of the tank by the pressure in the tank to get some total to help me compare the amount of breathable air, but was told this is not so. The owner said the 80 would give me more bottom time, despite having a lower pressure fill. I just need some help learning the mathmatics to compare these if possible. There was also a steel 100 @ 3500 which would easily fit on my back and let me drop lots of weight and the owner commented that with that tank he can stay underwater for 75-80 minutes. thanks in advance for your help.

 

[Big-t-2538]

Essentially, tanks are rated with a certain volume and a certain pressure. This means that the volume spec. on the tank is the volume in cu. ft. when and only when the tank is filled to the specified pressure.

Your personal consumption rate is directly related to volume, so regardless of fill pressure, it is the volume in the tank that counts and limits your bottom time once you figure out a necessary reserve.

For instance, you are talking about 3 different tanks that have different volumes and different pressure ratings. You need to think about that if you cannot get a 3500 psi fill somewhere, which will affect the volume of the 100cu. ft. steel tank. It should not affect the volume of the 80 cu. ft. tank b/c no one should fill that tank higher than the specified working pressure on the tank. However, the larger Steel 100 will also be the most negatively bouyant and allow you to drop the most weight off of your belt.

Hope that helps
-T

 

[rmediver2002]

Most tanks (steel) are rated for capacity at the 10% over pressure.

The 80 cubic foot tank is 80 cubic feet at 2750 psi the 72 is 72 cubic feet at 3300 psi

The cubic feet cpacity is the breathable air, you can use your surface air consumption rate to calculate the amount of air you will use at a given depth. Do you know what your SAC is?

 

[Bob3]

Forget about the pressure, it has already been used in the equation. No double dipping allowed!

 

[scubafanatic]

lee3,
here's the equation to use:

the actual air pressure inside the tank
( measured with your SPG ) in PSI
------------------------------------------------------- X the internal tank volume in cubic feet = cubic feet of
the air pressure rating stamped on the stamped on the side of the tank air in the tank
side of the tank in PSI

Sample: how much air is in a HP 120 steel tank filled to 2900 PSI?

Answer: 2900 psi in tank as measured by your SPG
-------------------------------------------------------------- = .828
3500 psi stamped into the steel shoulder of
the tank

then, multiply .828 by the cubic foot capacity of the tank, which is also stamped
into the tank next to the place where the 'psi rating' was stamped into the tank.

so, in this case it would be .828 X 120 cu. ft. = 99.36 cu. ft.

thus, your tank would contain 99.36 cubic feet of air.


Hope this helps,

Karl

 

[scubafanatic]

lee3,
here's the equation to use:

--the actual air pressure inside the tank
( measured with your SPG ) in PSI--
------------------------------------------------------- X the internal tank volume in cubic feet = cubic feet of
--the air pressure rating stamped on the stamped on the side of the tank air in the tank
side of the tank in PSI--

Sample: how much air is in a HP 120 steel tank filled to 2900 PSI?

Answer: 2900 psi in tank as measured by your SPG
-------------------------------------------------------------- = .828
3500 psi stamped into the steel shoulder of
the tank

then, multiply .828 by the cubic foot capacity of the tank, which is also stamped
into the tank next to the place where the 'psi rating' was stamped into the tank.

so, in this case it would be .828 X 120 cu. ft. = 99.36 cu. ft.

thus, your tank would contain 99.36 cubic feet of air.


Hope this helps,

Karl

 

[scubafanatic]

lee3,
here's the equation to use:

--the actual air pressure inside the tank
( measured with your SPG ) in PSI--
------------------------------------------------------- X the internal tank volume in cubic feet
--the air pressure rating stamped on the stamped on the side of the tank
side of the tank in PSI--

Sample: how much air is in a HP 120 steel tank filled to 2900 PSI?

Answer: 2900 psi in tank as measured by your SPG
-------------------------------------------------------------- = .828
3500 psi stamped into the steel shoulder of
the tank

then, multiply .828 by the cubic foot capacity of the tank, which is also stamped
into the tank next to the place where the 'psi rating' was stamped into the tank.

so, in this case it would be .828 X 120 cu. ft. = 99.36 cu. ft.

thus, your tank would contain 99.36 cubic feet of air.


Hope this helps,

Karl

 

[scubafanatic]

lee3,

here's the equation to use:

--the actual air pressure inside the tank
( measured with your SPG ) in PSI--
------------------------------------------------------- = " Z "
--the air pressure rating stamped on the
side of the tank in PSI--

then, multiply "Z" by the number of cubic feet that's stamped into the shoulder of the tank.

Sample: how much air is in a HP 120 steel tank filled to 2900 PSI?

Answer:

2900 psi in tank as measured by your SPG
-------------------------------------------------------------- = .828
3500 psi stamped into the steel shoulder of
the tank

then, multiply .828 by the cubic foot capacity of the tank, which is also stamped into the tank next to the place where the 'psi rating' was stamped into the tank.

so, in this case it would be .828 X 120 cu. ft. = 99.36 cu. ft.

thus, your tank would contain 99.36 cubic feet of air.


Hope this helps,

Karl