Air use calculations, estimating shots out of a tank

[mafitzy32]

The Premise: I've been trying to figure out the ideal tank for my Phantom that will work for a decently long session in a big game, without being overkill. I currently use a 13/3k, which is fine for walk on games at my local field, and a 68/45, which is great for my Mag or Macdev, but is just too big on the Phantom. I get around 120-130 shots from the 13/3k before pressure starts to drop off.

The Question: I've used Boyle's Law to calculate the storage volume of a compressed gas, which is this: V_a = p_c V_c / p_a Where p_a= 14.7psia, V_a = volume of gas at atmospheric pressure in cubic feet, p_c= pressure after compression in psi, and V_c= the volume of gas after compression in cubic feet.

For my 13/3k this would come to V_a= (3014.7)(.0075)/14.7 = 1.53 cubic feet.
For a 17/45 this would come to V_a= (4514.7)(.0098)/14.7 = 3.01 cubic feet.

Do I have this right? Does a 17/45 hold twice as much air as a 13/3k, AND does that mean I will get roughly twice the number of shots out of it?

I found the table on zdspb but it doesn't supply the math and doesn't cover the smaller tanks.

[JeepDVLZ45]

Rule of wrist…..

10-12shots per ci @3000psi, so roughly 15-18shots per ci @ 4500psi. <—very general numbers….and from my experiences. Someone else should chime in and correct me.

so a 13/3000 yield 130-150 ish shots, a 15/4500 should yield 225-270

keep on mind the bigger the tank, the more air will be left in the tank…ex. A 70ci tank will get closer to 9 shots per couple due to its actual size. But this comes down to efficiency as well, your count will fluctuate based one a hundred other variables including, Temperature, paint match, the reg you’re using, your velocity etc.

your math is “correct”, but it only works with spherical chickens in vacuums…I spent enough years doing the physics thing (on both sides of the classroom) to know if you squint your eyes, cock your head and round enough…it works every time 😂

I’m very happy with my 15/4500. If you want to push your shot count, match your paint well and go with the 17ci.

[JeepDVLZ45]

Also, I run a joyride. So 150 rounds on my belt plus 12 in my top tube.

I’ve never come close to running out of air in my 15/4500 (I typically only get a 4k fill), it’s also nice and light so paired with my phantom, it’s a pleasure to run all day.

[Chuck E Ducky]

I can shoot my Joyride (150 shots + a 12round stick feed) empty on a 9ci 3k with a hot fill with my Phantom at 280fps with shots still left in the tank.

[zinger565]

Jeep is pretty much correct here. You could go with the classroom model, but you're making a ton of assumptions, and you also can't just go on total volume @ STP in the tank. At the very least you need to know at what pressure you'd consider the tank "too empty to shoot". Then volume you need to compare is (volume @ max pressure - volume @ cut-off pressure). That at least covers the "air left in the tank" issue.

Then there's all the inconsistencies around temperature, fill consistency, shot-to-shot air efficiency, etc etc. I guess the long story short is go with the "rule of wrist" and take it from there.

Also wanted to note that, love'em or hate'em, HK just released a 36ci/4.5k tank that's even smaller than the standard 48ci "peanut" tanks.

[JeeperCreeper]

I saw that little tank. Might pick one up

[uv_halo]

As best I can tell, your math works out.

[mafitzy32]

Thanks for the input guys. I've got a Palmers Stabilizer on it, and bore match pretty religiously. Definitely understand the spherical chickens thing. I'm a big squinter, polisher of turds, etc. The paint at my local field is OK, but not great, and I'm definitely not getting a full 4500 psi fill most of the time. I'm mostly looking to see if I would roughly see a decent increase in the number of shots, not just and extra 20. Good tip on the 36/45. That might be the ticket!

[Chuck E Ducky]

How much paint you trying to shoot? A 4.5k 15ci or 17ci will work perfectly on the phantom. If you like the L stock length got with the 17ci if you like the t stock collapsed go with the 15ci. But I run 6x 50 round pods and a JoLo on my Emek without running out of air with a hot 4.5k fill. The phantom is even more efficient.

I would go with a nice light setup for an open class phantom (15ci or 17ci 4.5k). Hell you could even run a the 13ci of the vertical ASA if you wanted it’s shorter then the aluminum ones.

I can pretty much play all day on a single fill with my stick fed CCM on a 17ci. I think it’s the perfect size tank for just about anything other than competition semi play. They are definitely the best pump tanks hands down. Only downside is they need to be re hydro. 2” OF 3AL tanks do not because they are exempt.

Also Stock Silver springs 825psi tank reg input pressure, dump the reg if you want efficiency numbers. 280fps should be about 1.5 turns in from flush on the TPC with a good paint to bore match.

[mafitzy32]

Thanks for the tips Chuck. I'd like to be getting around 300. I probably shoot more than I could, but here we are. I hadn't considered removing the reg, but I will see if I have the parts to skip it and keep the bottle in the sport setup where I've had it. I think I have a T stock around here somewhere too, so maybe the vert bottle would be the move provided I ditch the reg. I have been struggling to get 280fps recently, so maybe a new set of springs would be in order as well, or maybe the reg is hamstringing me. I have been enjoying not needing to worry about the rehydro on the 13/3k, but the local shop does it pretty painlessly.

[Chuck E Ducky]

You will get that on any one of the 4.5k tanks with ease.

[latches109]

For a 17/45 this would come to V_a= (4514.7)(.0098)/14.7 = 3.01 cubic feet.

as molecules get closer pressures go up faster than Boyles predicts, using boyles (in our use case) above 3000psi will be slightly off

[P + a(n/V)²] (V – nb) = nRT

The VanDerWaals equation would be more accurate - which accounts for molecules attraction 'a' and the molecules themselves taking up space 'b'

I found this article to help explain more clearly - scroll down to "Comparisons under High Pressure"
https://www.wondriumdaily.com/ideal-...aals-equation/

[Tarsun2]

this has me cracking up.

a few years back i was given a project in the lab to set up an air powered, ultra high speed, centrifuge. (100,000rpm+).

it was neat for sure. the carousel was elevated by a stream of air and also spun by air. Think back to the school experiment where a hair dryer kept a pingpong ball suspended in the air current. and then just add another hairdryer to spin the ball. thats kind of like what this does. it was completely driven and regulated by air pressure/flow.

ok, so it needed to use a high pressure tanks due to its location. i calculated how much air it needed to spin at a certain speed for a certain period of time based on pressure and flow. went over the formula twice to make sure i didnt miss anything. went over the numbers and the math checked out. i was set and proud of myself.

installed the unit and got everything set up. tested, in-serviced the staff, and we were off to the races. the tank was supposed to last a month. it lasted one day... (face palm). lol

reached out to some people smarter than me and although my math was correct, my formula used was not.

[Spider!]

When using equations on bottles, you have to calculate the difference in volume for each tank, at the full pressure and at the low shot pressure ( like 600 or so). Then you can scale that differential volume number of the small tank shots to the differential of the larger tank.

Remember your tank cools after filling, so you're top pressure is usually 5-10% lower than fill pressure.

Once or twice, I was at a field that filled over 4.5k, so that it cooled to 4.5k.