pressure vs. volume

[ClumsyCuttlefish]

I think my brain is technically challanged, ) so forgive me for using elementary-school-level-words.
I always thought that SPG shows you how much air you have in your tank (well, for me it was always around 3000psi). I rent tanks. At one dive shop I rented the steal tank. It looked smaller for me than the aluminium ones I usually used. My SPG showed me 2 400 psi. I asked the employee of the dive shop why there is not much air and he answered me that SPG shows not the volume of air but the air pressure and that I have the same amount of air but it is more compressed.
I tried to revew my diving books and could not find anything on this topioc. Could somebody, please, explain me how does it work?

 

[Dyno Bill]

SPG, submersible pressure gauge. It reports the pressure in the tank. The tanks volume is set by dimension and material design. The steel tank you mentioned was likely an older steel 72 tank. The numeric value is an indication of the tanks volume at the filled pressure rating. Steel tanks come in varying volumes depending on the tanks intended design. Steel tanks vary from old 50 Cu Ft to 130 cu ft high pressure tanks. You need to do a search here on this board, I'm sure there is tons of information about tank volume here.

 

[Scott]

This may or may not help
Scuba Cylinder Specification Chart from Huron Scuba, Ann Arbor Michigan

 

[bleeb]

Pressure gauges only indirectly tell you how much gas is in a tank. To convert a pressure reading to a volume, you also need to know how much the tank holds when 'full' i.e. filled to its maximum rated operating pressure. You're in a part of the world that follows the North American convention of categorising tanks based on how much gas they hold when filled to their rated pressure. The aluminum tank was almost definitely an Al80, which holds 80 cu ft when filled to 3000 psi. (Just to make things more confusing, the so-called standard Al80 doesn't really hold 80 cu ft. It really only holds 77.4 cu ft.)

For the steel tank, you'd have to know its rated volume. Either the shop would have to tell you, or you can try to decode it from the stamped markings on the neck. Your dive books may or may not explain in enough detail to figure this out. If not, if you still have access to the tanks, try posting the neck markings here, and someone can help you figure them out. You may also want to browse the Tanks forum, since people occasionally post there trying to decode particularly cryptic markings.

As for the smaller size, appearances can be deceiving. Because of the higher strength of steel, the walls may be thinner and the internal volume larger than an equivalent aluminum tank, even though the external dimensions are actually the same or smaller.

Note: Must type faster in the future.

 

[TimKy]

Assume you have two scuba tanks that are exactly the same size. Now, assume you fill the first until the SPG shows 3,000 psi and the second shows 2,400 psi. There is a greater volume of air in the tank showing 3,000 psi. That is why the pressure is higher. You had to comPRESS the air more to get that larger volume in the same space.

Now assume you have two tanks that are not the same size. Say the first is an Low Pressure Aluminum 80 cubic foot cylinder with a fill pressure of 3,000 psi and the second is a slightly smaller High Pressure Steel 100 cubic foot cylinder with a fill pressure of 3,442. If you filled the Aluminum 80 to 3,000 psi, you would have 80 cubic feet of air. If you filled the Steel 100 to 3,442, you would have 100 cubic feet of air. So, at the rated pressures, the smaller Steel tank actually holds more air.

Now the complicated part is a partial fill of the HP Steel 100 versus a full fill of the Aluminum 80. To get that we use the formula: Volume of Gas = (PSI in Tank) * (Tank's Rated Volume)/(Tank's Rated Pressure). So the volume in the partially filled Steel tank would = 2,400*100/3442 or 69.73 cubic feet of air versus 80 cubic feet in the fully filled Aluminum tank.

So, if the Steel tank was an HP 100 (just guessing but these are slightly smaller than an Aluminum 80) and the pressure was 2,400 psi, then you did NOT have as much air as you would have had in a fully filled Aluminum 80 at 3,000 psi.

p.s. Take this with a grain of salt as I have had some Kentucky Bourbon tonight which can sometimes affect calculations.

 

[ClumsyCuttlefish]

Assume you have two scuba tanks that are exactly the same size. Now, assume you fill the first until the SPG shows 3,000 psi and the second shows 2,400 psi. There is a greater volume of air in the tank showing 3,000 psi. That is why the pressure is higher. You had to comPRESS the air more to get that larger volume in the same space.

Now assume you have two tanks that are not the same size. Say the first is an Low Pressure Aluminum 80 cubic foot cylinder with a fill pressure of 3,000 psi and the second is a slightly smaller High Pressure Steel 100 cubic foot cylinder with a fill pressure of 3,442. If you filled the Aluminum 80 to 3,000 psi, you would have 80 cubic feet of air. If you filled the Steel 100 to 3,442, you would have 100 cubic feet of air. So, at the rated pressures, the smaller Steel tank actually holds more air.

Now the complicated part is a partial fill of the HP Steel 100 versus a full fill of the Aluminum 80. To get that we use the formula: Volume of Gas = (PSI in Tank) * (Tank's Rated Volume)/(Tank's Rated Pressure). So the volume in the partially filled Steel tank would = 2,400*100/3442 or 69.73 cubic feet of air versus 80 cubic feet in the fully filled Aluminum tank.

So, if the Steel tank was an HP 100 (just guessing but these are slightly smaller than an Aluminum 80) and the pressure was 2,400 psi, then you did NOT have as much air as you would have had in a fully filled Aluminum 80 at 3,000 psi.

p.s. Take this with a grain of salt as I have had some Kentucky Bourbon tonight which can sometimes affect calculations.

This is the part I can't understand. When I consume air during my dive, which factor becomes more important: the initial SPG reading or the volume of the tank?
I other words, if I have e.g. 3000psi in my tank before the dive, would I use it with the same consumption rate from every type of tank?

 

[BabyDuck]

your rate of use of cubic feet of gas would be the same, all other factors like depth and the work you're doing also being the same, but 3000psi in a 6cuft tank is way way way less than 2000psi in an 80 cuft tank no matter what.

 

[doctormike]

Assume you have two scuba tanks that are exactly the same size. Now, assume you fill the first until the SPG shows 3,000 psi and the second shows 2,400 psi. There is a greater volume of air in the tank showing 3,000 psi. That is why the pressure is higher. You had to comPRESS the air more to get that larger volume in the same space.

I'm not an expert on this by any means, and I may be wrong, but I think that this terminology is confusing.

If you have two tanks of identical size, they will always hold an identical volume no matter what the pressure (that is the definition of volume, it is a spatial measurement). However, the higher the pressure, the more air molecules will be squeezed into the same volume. So from the point of view of actual usable gas, you will get more breaths at a given ambient pressure from a tank with a higher internal pressure than from one with a lower internal pressure, if the tank volumes are the same (remember, your regulator delivers gas at ambient pressure no matter what the tank pressure).

What I believe TimKy is referring to is the fact that we conventionally use volume to refer to gas capacity. So he is correct that there is a greater surface equivalent volume in the higher pressure tank. But the actual tank volumes are the same.

An aluminum 80 filled to service capacity holds enough gas molecules so that if they were expanded to one atmosphere of pressure, they would fill 80 cubic feet of space. If it is filled to a pressure of one half of it's rated service pressure, the gas it contains would fill 40 cubic feet of space.

When I consume air during my dive, which factor becomes more important: the initial SPG reading or the volume of the tank?
I other words, if I have e.g. 3000psi in my tank before the dive, would I use it with the same consumption rate from every type of tank?

The amount of gas you have (and therefore, the allowable length of your dive at any given depth) is proportional to the product of the volume and the pressure. They are both important and not really separable for this purpose. See Boyle's law for more on this...

So whether you have a big tank filled at a lower pressure or a small tank filled at a higher pressure, you will be using gas at the same rate (assuming all other factors are the same - work load, anxiety, conditioning, etc..).

This rate, your SAC, or surface air consumption rate, will determine how long you can dive at a given depth with a given tank (forgetting about NDL for the moment). Your SAC will drop with training, conditioning, experience, relaxation, etc... Most active divers develop a sense about this and know how much air is left in the tank intuitively during the dive (but you should still carry an SPG!).

 

[TSandM]

The simple answer is that each tank has a working pressure. That pressure is the pressure at which the tank holds its rated volume. So, if you have an Al80, it holds 77 cubic feet of gas at 3000 psi. If you have an HP80, it holds 80 cubic feet of gas at 3442 psi. They both hold essentially the same volume, but at different pressures -- so, as you might expect, the external measurements of the tank (diameter and length) are different.

To know whether you have a full tank, you need to know its working pressure. Some are full at 2400, and some need to be at nearly 3500 to be full.

 

[TSandM]

The simple answer is that each tank has a working pressure. That pressure is the pressure at which the tank holds its rated volume. So, if you have an Al80, it holds 77 cubic feet of gas at 3000 psi. If you have an HP80, it holds 80 cubic feet of gas at 3442 psi. They both hold essentially the same volume, but at different pressures -- so, as you might expect, the external measurements of the tank (diameter and length) are different.

To know whether you have a full tank, you need to know its working pressure. Some are full at 2400, and some need to be at nearly 3500 to be full.