pressure vs. volume

[TimKy]

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).

It is confusing. The term "volume" as used in this type of case typically implies "at standard temperature and pressure". That is why you can have different volumes of gas in two identical cylinders that have different pressures. If you let the air out of identical cylinders into balloons in a room at standard temperature and pressure, the tank with the higher pressure would have the bigger balloon because there was actually more gas compressed into the same physical size of the tank.

To really answer ClumsyCuttlefish: If you had 70 cubic feet of air in the two cylinders I described earlier - one a Steel HP100 and one an Aluminum 80, that 70 cubic feet of air would last you exactly the same time assuming you breathed exactly the same from each tank. When you first attached the SPG to each, the Aluminum 80 would read roughly 2,625 psi and the Steel HP100 would read 2,410. The same volume of gas would have different pressures because you are cramming it into different sized containers.

 

[tracydr]

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?

Volume of the tank. I dive a short tank on easy dives 63cu ft, to my husband's 80cu ft. We both start with 3,000 psi but he has about 17 more cu ft of air. This ratio continues as we use air throughout the dive. Works great as our SAC is almost the same ratio so our dive plan can include turning back when we are both at about the same reading on our SPG.

 

[Walter]

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?

No.

Look at two 80 cu ft tanks, one is full at 2400 PSI, the other is full at 3000 PSI.

The 2400 PSI tank holds .0333 cu ft for every 1 PSI while the 2400 PSI tank holds .0267 cu ft for every 1 PSI.

 

[vladimir]

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?

You could move to Europe. In addition to having a 32-hour work week :joke: they'd describe your tank's volume using units of volume (liters) :lightbulb:. So your French tank would hold 12 liters, perhaps. If it were compressed to 200 bar, say, you could compute that you have 2400 liters of air on the surface (1 Bar), 1200 liters at 10 meters (2 bar), 800 liters at 20 meters (3 bar), etc.

 

[Karl_H]

This is where Metric Units, Bars, come in much more useful than PSI Although Imperial Atmosphere Units are even better!

Basically 1 Atmosphere (roughly 1 Bar) is the usual 'outside pressure', a tank containing gas to 10 atmospheres will have 10 time as much volume as a tank at 1 atmosphere ('outside pressure').

The tank size is pretty easy to visualise so if you multiply the tank size by the pressure in Atmospheres or Bar you can 'see' how much gas is in the tank.

Gauges are always provided in pressure as firstly this is what they directly measure and secondly because to know the volume of the tank contents you need to know both the size and the pressure.

For reference 1 Atmosphere is roughly 14.5 PSI, Atmospheres are just easier to understand for the example I gave above.

There's a few 'cheat sheet' tables available for different tank sizes at different pressures, a laminated copy in the dive bag can help in making quick calculations without needed in calculator - Google should give you a few leads on this

Hope that helps!

Karl

Post-Posting Note! - Vladimir beat me to it!

 

[doctormike]

It is confusing. The term "volume" as used in this type of case typically implies "at standard temperature and pressure". That is why you can have different volumes of gas in two identical cylinders that have different pressures.

Right, and I didn't mean to be too pedantic about this - your description of the physics here was a good one, and helpful. I just wanted to be clear that the standard terminology uses the term "volume" in a technically incorrect fashion, and that can be a bit confusing later on when you try to intuitively understand how smaller tanks can hold more air.

Specifically, the standard terminology uses "volume" as shorthand for "equivalent volume at sea level ambient pressure and temperature". The volume of a cylindrical tank that has a radius of 3 inches and a length of two feet (close to an AL80, ignoring the tapered neck) will always be about 0.39 cubic feet, at the bottom of the ocean or in outer space:

volume of a cylinder = cross sectional area x length = Pi x (0.25 feet)(0.25 feet) x 2 feet = 0.39 CUF

If that 0.39 CUF of space is holding air at 3000 psi, that means that it is 204 times as compressed as air at 14.7 psi (surface pressure). Assuming that the ideal gas law still holds at this pressure, if you expanded the 0.39 CUF 204 times (Boyle's law, P1V1=P2V2), you would get 79.56 CUF..

 

[fnfalman]

To reiterate of what other posters have said:

A. Volumes of tanks - there's actually the volume of tank that is described by the tank's dimension and if you were to fill it up with water, it'd hold a certain amount of water (let's say 20-gallons of water). HOWEVER, the air volume of the tank is different because assuming that you have two tanks (A & B), both have the same dimensions and both hold 20-gallons of water. BUT Tank A is low pressure (LP, 2400-psi) and Tank B is high pressure (HP, 3500-psi), AND both these tanks are filled to their respective pressures, then Tank B has more air available in it. While the water volume may be 20-gallons, the air volume of Tank A may be 80-cubicfeet while the air volume of Tank B may be 120-cubicfeet. Since that we don't breath water, we don't care what the actual volume of the tank is but the air volume of the tank and that depends on both the dimension of the tank AND the pressure rating of the tank.

B. The pressure readings you have from one tank doesn't translate over to another just because of the difference in sizes & pressure ratings (see A). HOWEVER, if you were to figure out your air volume consumption rate then that will translate to other tank sizes. If you were to figure out that you'd use 50-psi/minute then it doesn't do you any good unless you use that same tank all the time. If that 50-psi/minute were to be converted over to, say, 1-liter/minute or 5-cubicfeet/minute then you can use these numbers for any tank of any size and any pressure rating. So, to recap, the pressure consumption rate is only good for the same tank while the volume consumption rate is good for any tank. 50-psi/minute is very different between a 3500-psi tank and a 2400-psi tank. 1-liter/minute or 5-cubicfeet/minute is the same regardless of which tank you use.

C. Most people measure the pressure consumption rate from a tank that they use and then convert it to volume consumption rate so that they can use it with any other tank.

D. There's a series of formulas to all of these conversions and calculations. If you want to know, I or others would be more than glad to post you to a link.

 

[doctormike]

To reiterate of what other posters have said:

A. Volumes of tanks - there's actually the volume of tank that is described by the tank's dimension and if you were to fill it up with water, it'd hold a certain amount of water (let's say 20-gallons of water). HOWEVER, the air volume of the tank is different because assuming that you have two tanks (A & B), both have the same dimensions and both hold 20-gallons of water. BUT Tank A is low pressure (LP, 2400-psi) and Tank B is high pressure (HP, 3500-psi), AND both these tanks are filled to their respective pressures, then Tank B has more air available in it. While the water volume may be 20-gallons, the air volume of Tank A may be 80-cubicfeet while the air volume of Tank B may be 120-cubicfeet. Since that we don't breath water, we don't care what the actual volume of the tank is but the air volume of the tank and that depends on both the dimension of the tank AND the pressure rating of the tank.

Thanks for the helpful post. I agree with your points B, C, and D, but the language in point A bit confusing (although I see the concept that you are describing, and it is also useful and correct).

Volume is a physical measurement of a given object, it is independent of other factors. What you are referring to as "air volume" is actually the equivalent volume at the surface, a shorthand way of describing how many air molecules have been packed into any give container. This is a concept similar to the "equivalent air depth" that we use for Nitrox calculations - it's not that we are actually diving to the EAD, but we are using that depth as an approximation so that we can make calculations more easily using air tables.

The equivalent surface volume is useful since it allows you to compare actual air content of containers that have different pressures and volumes. However, an Aluminum 80 contains approximately 0.39 cubic feet of space (or 11.1 liters), whether it is filled with water, air or a hard vacuum in space.

I know that I keep stressing this point, and it may seem annoying and pedantic. But when you are doing calculations later on, it becomes more confusing if you don't remember that the tank "volumes" are derived from a combination of actual physical volume and service pressure.

 

[fnfalman]

Thanks for the helpful post. I agree with your points B, C, and D, but the language in point A bit confusing (although I see the concept that you are describing, and it is also useful and correct).

Volume is a physical measurement of a given object, it is independent of other factors. What you are referring to as "air volume" is actually the equivalent volume at the surface, a shorthand way of describing how many air molecules have been packed into any give container. This is a concept similar to the "equivalent air depth" that we use for Nitrox calculations - it's not that we are actually diving to the EAD, but we are using that depth as an approximation so that we can make calculations more easily using air tables.

I have a Master in Chemical Engineering. I can get pretty fancy on Gas Laws, but I don't think that the OP would care for it.

The equivalent surface volume is useful since it allows you to compare actual air content of containers that have different pressures and volumes. However, an Aluminum 80 contains approximately 0.39 cubic feet of space (or 11.1 liters), whether it is filled with water, air or a hard vacuum in space.

I know that I keep stressing this point, and it may seem annoying and pedantic. But when you are doing calculations later on, it becomes more confusing if you don't remember that the tank "volumes" are derived from a combination of actual physical volume and service pressure.

All the OP has to know is the manufacturer's air volume at the manufacturer's recommended fill pressures. We're not trying to make somebody into an engineer here.

The calculations are also simple arithmetics/algebras.

 

[ClumsyCuttlefish]

Thanks a lot everybody for detailed explanations! I beleive I've got the idea. My physics teacher could have learned a lot from you!