TargetTalk

How many refills of a 0.002825 cubic foot cylinder filled to 2900 psi will I get out of a 80 cubic ft air tank with a starting pressure of 3300 psi before the pressure goes below 2900 psi in the 80 cubic foot tank?

Its a Morini 162EI cylinder in case anyone was curious.

Can anybody with more math knowledge than I(which is just about anyone) help me on this one?

Thanks all!

TomF wrote:How many refills of a 0.002825 cubic foot cylinder filled to 2900 psi will I get out of a 80 cubic ft air tank with a starting pressure of 3300 psi before the pressure goes below 2900 psi in the 80 cubic foot tank?

Its a Morini 162EI cylinder in case anyone was curious.

Can anybody with more math knowledge than I(which is just about anyone) help me on this one?

Thanks all!

Hmm I got only 19 if your 80 cu.ft cylinder volume is 0.393 cu.ft. and small cylinder conains zero molecules of the gas before each refill...

It's a little more complicated as you have to provide the amount of air left in your cylinder when you go "off reg".

My best guess is your gun runs at about 1450 PSI so below that in round numbers would be 1400 On your first fill you would have an empty cylinder, so a full 2900 PSI fill, after that you would fill and only add 1500 PSI for slightly half of the volume.
If done right you are looking at 30 fils or so total before you would be below the amount to fill at 2900 PSI in your SCUBA tank. However you could still keep filling to the point that you would be so close to your "off reg" pressure that you would get one shot and then have to refil...not practical but say you continue to fill to 2100 PSI, you are looking at near trippling the number of fill you get with an ever decreasing amount of shots.
That being said you can really extend the life of your fills if you "string" two tanks. Meaning fill to any volume and PSI from tank A and topw with Tank B.

I have a 80 CF SCUBA tank for 3300 PSI and a Carbon fiber tank at 88 CF for 4500 PSI, I fill from the the SCUBA and top with the 4500 PSI tank.

I shoot a lot and can go nearly a year without having to top off the CF tank.

Len that sounds good but how did you figure that out. What formula did you use?

Thanks, Tom

Converted your measurement to cubic feet, then caclulated the decrease in pressure against loss of volume in tank 1 and how that would effect the pressure and volume of the second container and it's lower volume.

Things not caclulated that may have an impact; teperature of container 1 (SCUBA), final temperature of container 2 (gun), ambiant temperature, atmosphere of where you are, volume lost due to fill system (I would imagine not much because most seem to "screw right into" the DIN valve.

Thanks Len.

The parameters I gave are in cubic feet.

Now do I get more fills???

Thanks! Tom

No I mean into a ration of Cubic feet, still the same amount of fills.

Dumb question number 2.

The air cylinder has a manometer on it that goes to 300 bar. The specs say the gun is rated at 200 bar.

What is the pressure rating on the cylinder? I remeber the fellow at the dive shop said that air pressure vessels must be capable of containing twice their rated pressure, or something like that.

Will the gun operate at a higher pressure than 200 bar without damage. Anyone tried it?

(This is like looking for a gas leak with a match. Rest easy, I am not going to waste a $1200 gun, which is why I asked.)

Thanks!

Typically your dive friend is right, the body will hold 2-3x the rated without bursting...some more exotic materials will hold up to 5x.
The danger in going down this path is that not all the other parts can hold to that pressure or abuse.

Certainly the body may rate to 300 BAR or 4500 PSI, but the end components or the regulator may not hold to it, or those pesky seals.

Your reg is set to function with a feed pressure of X vaule to X-n value, feed it 2x value and all bets are off. It's not worth the hassle nor will it be entirely reliable, plus it's not worth the danger.

It seems many/most air pistol cylinders are rated to 300 bar but are to be filled only to 200 bar ... except for some (all?) Walther air pistol cylinders which are meant to be filled to 300 bar. The regulator of most match air pistols bring the pressure down to 70 bar ... which is the point when one must usually "reload" the cylinder ...

A lot of tube can easily hold 300 bar as you said but the problem is reg. If you could install a feeder reg that will provide the expected pressure you owuld be in fine shape. The downside is that the feeder would take up more volume than you would get in the exchange.

Mako wrote:It seems many/most air pistol cylinders are rated to 300 bar but are to be filled only to 200 bar ... except for some (all?) Walther air pistol cylinders which are meant to be filled to 300 bar. The regulator of most match air pistols bring the pressure down to 70 bar ... which is the point when one must usually "reload" the cylinder ...

I'm just curious whether your calculations considered the air in the tank as an ideal gas or not? At pressures such as 200 or 300 bar it begins to make a difference.

Just curious?

Paul

p1*v1=p2*v2

Solves lots of compressed air gun questions. I calculated the approximate number of fills using a spread sheet.

Wish I had this formula before. LOL!

Thanks all!

It's a bit more complicated than that as you have Boyle's, Charles', and Avogadro's laws as well as the dynamics of fill and temperature (artificial due to fill rate) and ambiant temperature outside the bottle.
As well well looking at volume and pressure, the pressure in your bottle is at a given volume, decrease the volume, you decrease the pressure which again works p1v1=p2v2 but at ballance not at fill capacity.

to get to a simple understanding of an appoximate number of fills, the calculation is very simple. The volume is constant in the storage bottles, but when you fill the pistol cylinder the pressure goes down in the storage bottle. A simple spreadsheet calculation takes the amount of pressure drop to fill the cylinder and subract the pressure from the storage bottle. Keep doing that and you will eventually get to where the pressure is at an unacceptable level in the storage tank. Time to refill!

I ignore temp and all the other variables because it makes so little difference and my refilling is done in the same place at appoximately the same temp.

It can be as complicated or simple as you want to make it. I was just looking for an approximation so I will know how often to refill my scuba tank.

TomF wrote:It can be as complicated or simple as you want to make it. I was just looking for an approximation so I will know how often to refill my scuba tank.

I really must be missing something on this thread. Isn't it much simpler to just use a gauge, either fixed to the diving bottle or to screw into the gun's cylinder. The gauges are typically not particularly accurate but they are fairly consistent. I know that at a displayed 170 bar I am still going to get at least 90 shots out of the gun, so I get the dive bottles topped up when they get below that figure. It actually takes such a small amount of air to boost the pressure back up that the shop doesn't even normally charge me.

It's funny how I gave you an answer, and yet now you are trying to tell me how it's done.
P1V1=P2V2 is good but it's not as accurate as it could be, plus again you are asking how many fills you get before you can't fill anymore. So the volume of loss in your fill system is of interest, the speed at which you fill is important. the ratio of nitrogen to oxygen is of interest, simply put my calculation still stands unless you have more information to offer as to volume loss, fill speed, where you are in relation to sea level, and most importantly when you go off your reg (the most critical of numbers) in hopes of calculating how many fills you will get .
Simply put you will get about 30 fills based on pressure needed before going off regulation (working pressure) with a "guestimate of losing 3 fills" based on volume loss in your fill system.
Again with the addition of a second bottle or a bottle at 4500 PSI your fills will go up quite significantly as you will lose less volume for the second topper bottle.

Shin got 19 but that's because he was filling each to time the max pressure from zero.

I guess at the heart of it, why bother asking in the first place if you already had the answer? or assert that you know more about the math than I do?
Seems like a waste of time for you and me, no?

TomF wrote:to get to a simple understanding of an appoximate number of fills, the calculation is very simple. The volume is constant in the storage bottles, but when you fill the pistol cylinder the pressure goes down in the storage bottle. A simple spreadsheet calculation takes the amount of pressure drop to fill the cylinder and subract the pressure from the storage bottle. Keep doing that and you will eventually get to where the pressure is at an unacceptable level in the storage tank. Time to refill!

I ignore temp and all the other variables because it makes so little difference and my refilling is done in the same place at appoximately the same temp.

It can be as complicated or simple as you want to make it. I was just looking for an approximation so I will know how often to refill my scuba tank.

Len, dont get so defensive. I figured about the same as you but I wanted to know more about the formula also. I was not trying to challenge your estimate. You gave good info. I just didnt communicate my intentions and needs well enough.

Anyway the fellow at the dive shop today has a program that does these calculations taking into consideration all the variables. And he got about the same but his estimate was higher, about 42. But since I never fill an empty cylinder, it will get lots more.

Anyway the important part is that my inspection sticker runs out at the end of this month so I needed to get the tank in and filled before I had to pay up for the annual visual inspection.

Thanks for the help, it is appreciated.

Tom

I am not sure you will get more as the calculation that I did works with the minimum needed pressure of about 1450 PSI existing to get shot one (1451 actually).
The issue being you never fill off of an empty but below 1451 PSI, you can't really shoot effectively.
If at all possible find out when your gun "shuts down" or goes off reg, then do the calculation based on the minimun fill to be useable, also calculate the loss of the air in the system, if you screw in to the DIN connector you will not have very much loss, if you use a short or microbore fill system with a quick fill, you may have a little more loss.

I wasn't really getting defensive, I was just saying that if you knew or had a methodology..then why ask, more of a question than anything else.