Formula for figuring out volume from pressure

[underwasser bolt]

My thoughts were: Volume is constant since the tank don't change size. So as pressure increases. You would need put more air. Afterall it is why harder for the compressor to pump those last few psi.

Are you 100% sure it is linear??

The volume of the tank is constant as any gas will take on the volume of the tank. I THINK what we mean when we say a 120CF tank will hold 120 CF of gas at atmospheric pressure and we can pack it into a small tank because we have a handy air compressor that will pack it in there for us. If you were to fill a 120 tank with water, I assure you it will not hold 120CF, so that is NOT the actual volume. So, if you are not packing air up to the advertised pressure, you are not packing in the 120CF of gas at atmospheric pressure, but some amount less. And no, the process is not truly linear as the ideal gas laws used by most are just that, ideal, but they are quit accurate for general work. More accurate formulas are available if needed, but they are certainly not needed for scuba purposes. I doubt our SPG's are that accurate.

 

[ronbeau]

Searching for a compressor so until that happens.

I been having a tough time finding a dive shop that can successfully fill my HP120 tank to the rated spec 3442psi.

Almost all the time I get ~3000psi. How much air does this translate too? How do I converted pressure (psi) to volume (cf).

In addition to what everyone else has posted there is also a temperature component. You might say the the room temperature is pretty constant. However the rate at which the cylinder is filled can affect the temperature of the gas. As the temperature of the Gauss rises the pressure increases.

What this means is that if the fill station operator is filling the cylinder at a faster rate (thus generating excess heat and high her pressure) and shuts off the fill at 3442 PSI you will not really have 3442 PSI by the time you use the cylinder to dive. As the heated cylinder cools the pressure will decrease as will the volume of the gas. The result is that when you check the pressure in the cylinder before diving the next day you will find you do not have 3342 PSI in the cylinder.

Some people refer to this as a "hot fill". Id drop the cylinders at my LDS and let them fill them and pick them up the next day. They let them cool and then top them off to make sure I get a good fill. They do this for any customer who is willing to leave the cylinder as opposed to needing filled while they wait.

Hopefully I didn't add confusion here but this could also be a reason why you feel you are getting a short fill.

If you wish to look this up in your Scuba text you will find it under "Charles Law"

V₁/T₁ =- V₂/T₂

OR

(P₁ * V₁) / T₁ = (P₂ * V₂) / T₂

where V = Volume of the gas (air in this case) and T = Temperature and P = Pressure

This is effectively the formula for the general gas laws

Boyle's law and Charles law should be in Open Water textbook. The combination of Boyle's law and Charles' law may not be.

 

[spectrum]

If you wish to look this up in your Scuba text you will find it under "Charles Law"

V₁/T₁ =- V₂/T₂

OR

(P₁ * V₁) / T₁ = (P₂ * V₂) / T₂

where V = Volume of the gas (air in this case) and T = Temperature and P = Pressure

Where T is expressed in Kelvin.

 

[spectrum]

I been having a tough time finding a dive shop that can successfully fill my HP120 tank to the rated spec 3442psi.

Almost all the time I get ~3000psi.

There may be something else going on here.....

Ending up at 3000 PSI makes me wonder if you have more than one thing happening. I dive mostly HP and even when a knucklehead did a 20 second fill (I timed him) the drop wasn't that bad.

* Does the shop have the capacity to pump to 3442? Not all can get there to begin with.

* Shop panels are notorious for having bogus gauges. I have seen it time and time again. Often they have a special mark established based on heaven knows what!

* If they run it to 3442 on the fill station do you check with your gauge before you head home? It's possible that your gauge is reading low. If they fill to 3442 then you should see the same when you carry them right out to the car and hook up. I have been known to bring my tank checker right into the shop.

* After 4 hours that gas ball has cooled and it is what it is. What is the temperature at which you get the ~3000 PSI readings? Standard pressure should be provided at 70F. A rule of thumb is a 50 PSI drop per 10F. If it's 50F you should expect to see 3342 on a perfect fill as an example.

* We won't get into bribing the fill person to go slow or suggesting a water bath.

I have used 3442 since that is what we are talking about. Unless you are using calibrated digital instruments you are dealing with about 2 significant decimal places at best.

Generally a 10 minute fill, even of hot gas will settle around 3250 for me. I can live with that given my usage and the convenience of diving the lighter configuration the HP cylinder affords. I'm running 80 and 100 CF cylinders.

We are not alone... Even the AL80 divers generally loose 200- 300 PSI on cool down. For some reason as HP owner divers we have it in our heads that we need and want a full perfect fill. Without time and patience (or an over fill ) on the part of the shop and the diver it's not going to happen with any sort of cylinder.

Pete

 

[bleeb]

If you wish to look this up in your Scuba text you will find it under "Charles Law"

V₁/T₁ =- V₂/T₂

OR

(P₁ * V₁) / T₁ = (P₂ * V₂) / T₂

where V = Volume of the gas (air in this case) and T = Temperature and P = Pressure

Where T is expressed in Kelvin.

Or Rankine.