Air usage ????

[melloroadman]

Was their some one calculating air usage ? If so condor 480 cc , fill 3100psi , 30 shots, and refill 2000 psi . This one I will pay more attention to next time . 201 converted to 25 caliber . Teathered to a 80 cf tank that is regulated rifle tubes, 380 cc. Start psi was 3100psi , 120 or 130 shots , lost track. End psi 2700 . Marvin

[Ribbonstone]

Usually done by keeping track of the volume of air, the pressure used, the number of shots, and the total energy produced by those shots.

Usually worked out as follows:

air volume is cc's X BAR pressure used

The above result is divided by:

total energy of shots (figuring out each shots energy and adding them all up)

So lets say you have a 400cc air tank, starting at 200 bar, you take 40 shots. At the end of 40 shots, the tank has 150 BAR. 
 That gives us the first 1/2 of the equation: we know its 400cc x 50BAR = 20000

The second half requires you to figure the energy for each of the 40 shots taken and add up that energy.   LEts say that adds up to 1280 foot pounds (some shots got up to 38 foot ounds, some were only 29 foot pounds, but in total they all added up to 1280.

So divide 20000 by 1280 and you get 15.625. 

Using this system, the best tuned guns earn a 10 to 12. 

[rsterne]

Yep, you need the total FPE for the shot string.... or at the very least the average FPE and number of shots.... to do the calculation.... The results can either be in Barcc/FPE (values from 10 to 20 are common).... or in FPE/CI (where the cu.in. is the volume of air a 1 Bar).... I like the latter system because a bigger number is better.... and values of over 1.00 FPE/CI are reasonably efficient but values under that could use some help.... It depends to a large extent on caliber, however, and big bores tend to be more efficient.... 1.00 FPE/CI equals 16.4 Barcc/CI....

Bob

[melloroadman]

n/t

[melloroadman]

Thank you but I wanted to know how many cfa was being used . Tank is rated at 80 cf at 3000 psi . So there is 37.5 psi in a cf . I had 3100 psi so I had 116 cfa. to start with . Ended with 2700 psi or 101 cfa . So I used 15 cfa with say 120 shoots or .125 cfa per shot . This I can relate to better . Thanks Marvin

[rsterne]

There is no such relationship as "37.5 psi in a CF" as psi measures pressure and CF measures volume.... However, the pressure would drop by 37.5 psi for each cubic foot of air used from an 80 CF tank.... I assume that is what you meant.... 

What do you mean by the abbreviation "cfa".... I have never seen that before....

In your original post, you said:

Teathered to a 80 cf tank that is regulated rifle tubes, 380 cc. Start psi was 3100psi , 120 or 130 shots , lost track. End psi 2700 .

An 80 CF tank contains 80 cubic feet of air (at 1 Bar) when filled to 3000 psi.... If you overfilled it to 3100 psi, it would contain 3100/3000 x 80 = 82.67 CF.... The same tank at 2700 psi would contain 2700/3000 x 80 = 72 CF.... The amount of air you used would be 82.6 - 72 = 10.67 CF.... This can also be calculated by using the 400 psi drop divided by your figure of 37.5 psi drop per CF.... same answer, 10.67 CF.... This is 10.67 x 1728 = 18438 cubic inches at 1 Bar.... If you shot 120 shots, you used 18438 / 120 = 154 CI of air at 1 Bar per shot.... That is 154 / 1728 = 0.089 CF per shot....

If I understood what you wanted, I might be able to be of more assistance....

Bob

[melloroadman]

In no way did I say your way is not right or bad . There is more than one way to catch fish . I will not argue over this . Nor am I telling any one that this is the way to do it . But it tells me what I wanted to know . cfa means cubic feet of air . And I think you seen the relation ship of my method . You were able to calculate a answer and divers do as well . I see good in what you state but it does not work well for me .Marvin

[rsterne]

.125 cfa per shot

Sorry, I don't follow your math.... but if you're happy with your answer that's fine too.... I'm not here to argue, just to help.... and I'll be happy to stay out of your threads if you don't desire that....

Bob

[melloroadman]

This might help explain my math . But I could be wrong .If you would like to explain instead of putting me down I will listen . As to why you used your method .Marvin
http://en.wikipedia.org/wiki/Boyle%27s_law

[rsterne]

Surely not trying to put you down, and I do understand the basic Gas Laws.... If you can explain where you got this it would help....

I had 3100 psi so I had 116 cfa. to start with . Ended with 2700 psi or 101 cfa. So I used 15 cfa

What you said before that about 37.5 psi per CF is basically correct.... On an 80CF tank for each CF you use the pressure drops by 37.5 psi.... Since you were working on a tethered 80CF tank and you used 400 psi of air (3100-2700) then you used 400 / 37.5 = 10.67 CF, not 15 CF, right?....

Your method of dividing the amount of air used by the number of shots to get the CF / shot is right on.... but for whatever reason we are calculating a different amount of volume.... You are getting 15 CF and I'm getting 10.67 CF.... If you can explain where you are getting the 116 CF at 3100 psi and 101 CF at 2700 psi then maybe I could understand....

Bob

[melloroadman]

3000/80=37.5 psi 3100x37.5=116cf

[rsterne]

ummmmmm.... OK.... Look at the units....
3000 PSI  / 80 CF = 37.5 PSI/CF.... (pressure change per cu.ft. air used)
Then 3100 PSI x 37.5 PSI/CF = 116250 PSI^2/CF.... That's a pretty strange unit, and over 1000 times bigger than 116 CF....

If you take 3100 PSI  / 37.5 PSI/CF = 82.67 CF....
then 2700 PSI  / 37.5 PSI/CF = 72.0 CF....
82.6 CF - 72 CF = 10.67 CF....

Please look at it again....

Bob

[melloroadman]

You still have not really explained why you are dividing the 3100by 3000then multiplying it by 80 . I changed my glass and now see there is no decimal after the 116 . I was reading it as 116.250 . With out explanation there is no clarification . By your figures I am using less air so that looks good to me . You have to remember that when we communicate this way our choice of words are very important .Please explain Marvin

[rsterne]

Please look at the units.... that is always a good way to see if you are doing the math correctly....

When you arrived at your 37.5 figure, you divided 3000 PSI by 80 CF to arrive at that number.... 37.5 PSI/CF....

However, you then took that number (37.5) and mutiplied it by 3100 PSI instead of dividing 3100 by 37.5.... Look at it this way....

The tank is a set internal volume.... When the pressure is 3000 PSI it contains 80 CF.... If you increase the pressure (by adding more air) it will contain more CF.... You can calculate that as follows:

The increase in the amount of air inside the tank is directly proportional to the pressure.... If we increase the pressure from 3000 psi to 3100 psi you have put 3100 / 3000 = 1.033333 times as much air into the tank.... Since it held 80 CF at 3000 psi, it will hold 80 x 1.033333 = 82.6666 CF at 3100 psi (not 116 CF).... Likewise, at 2700 psi, it will hold (2700/3000) x 80 = 72.00 CF (not 101 CF)....

In order for you to put 116 CF into an 80 CF tank, you would have to increase the pressure to (116/80) x 3000 = 4350 PSI.... definitely not a good idea....

HTHs....

Bob

[melloroadman]

Thanks Marvin

[melloroadman]

Teathered the 201 again . 2700 psi start . Shot 120 shots regulated at 2000 psi . End pressure was 2400 psi . Used 300 psi which should be .06666 cfm per shot . At 2000 psi I am still over a 1000 fps on low power with a 31 grain bullet . Marvin

[rsterne]

Well your math is spot on.... In case you are interested, you have now supplied enough information to calculate the efficiency....

1000 fps with a 31 gr. bullet is about 69 FPE.... You said "over 1000" so let's call it an even 70 FPE per shot.... You used 0.06666 CF per shot which is 0.06666 x 1728 = 115 cu.in.... The efficiency is therefore 70 / 115 = 0.61 FPE/CI.... That works out to 16.4 / 0.61 = 27 Barcc/FPE if you prefer those units....

The math over the string works out the same.... 120 shots x 70 FPE = 8400 FPE total.... Total air used was 300 / 37.5 = 8 CF.... In cubic inches that is 8 x 1728 = 13824 CI total air used.... The efficiency is 8400 / 13824 = 0.61 FPE/CI....

Bob

[melloroadman]

This is a rifle I just swaped barrels on . What is considered normal . Here is what it was doing per fill . Only refill numbers I had were on low power at 1300 psi .Marvin

[rsterne]

I can figure out the FPE of the strings easy enough.... but I need the volume of the air reservoir to do an efficiency calculation.... I'll do the FPEs while you look for and post the reservoir volume....

Bob

[rsterne]

First string on low power.... Average velocity is 1043 fps.... with 31 gr. pellets that works out to 74.9 FPE x 10 shots = 749 FPE total....

For the heavier pellets.... average velocity is 870 fps.... with 52 gr. pellets that works out to 87.4 FPE x 10 shots = 874 FPE total....

In both cases, the pressure drop was 2300 - 1300  = 1000 psi.... In Bar, that is 1000 / 14.5 = 69 Bar.... Once I have the reservoir volume I can do the efficiency....

"Normal" efficiencies are all over the place.... I have measured them from 0.57 FPE/CI up to 1.92 FPE/CI in .22 cal.... but for the most part they run about 0.90 to 1.20 FPE/CI.... My .25 cal Disco runs 0.96 FPE/CI at 45 FPE average over 11 shots (all shots between 870 fps and 906 fps) with 25.4 gr. pellets....