Air usage Efficiency Calculator for PCPs ??? (another calculator added 9/7)

[lloyd-ss]

Tworr's thread about his new 308 Condor stirred up a lot of interest in calculating air usage efficiency numbers for PCPs, and in particular, for big bores.

Air efficiency is a calculation of how much air a gun uses to produce a certain amount of energy (FPE). It is commonly measured two ways:

FPE/cuin     the number of foot pounds of energy that can be produced per cubic inch of air at standard sea level pressure

bar-cc/FPE   number of ccs of air at sea level pressure that it will take to produce one foot pound of energy

Both calculations convert the amount of compressed air at any given pressure back to a level playing field of sea level pressure.  Basically, how big a volume does that burst of air that comes out behind the pellet expand into?  You might think of it as putting a plastic bag on the end of the barrel and seeing how big it blows up.  One is the inverse of the other, with the conversion of cc's to cubic inches thrown in.  For FPE/cuin, a larger number is more efficient, and for bar-cc/FPE, a smaller number is better.

For example, a fairly efficient gun like a Disco might produce  2 FPE/cuin, which is the same as it using 8.19 bar-cc/FPE.  So, if that Disco produces a 20FPE shot, it will puff that plastic bag up to 10 cuin.  But if its efficiency were only 1 FPE/cuin, it would puff the bag up to 20 cu in.  And if you had a big bore that produced 200 FPE at a 1 FPE/cuin rate, it would puff that bag up to 200 cuin.  

Has anyone seen an online calculator for this, or would you like to have one available?  I think I can get one up and running in a short while (or a little longer, seeing I have to talk my wife into doing the web programming part), a fairly simple one first, then one with more capability.

Bob added some mods to my initial suggestions of calculator inputs and outputs, but then he got carried away and got greedy, LOL.  That's ok Bob, all ideas are welcome!  Bob, you can add your other embellishments back in if you want.  
Lloyd
PS, I hope my explanations were clear and accurate.  


Here is what Bob has suggested, which could be a good starting point:

I would suggest entering the average velocity (the Chrony gives you that) instead of the average energy and let the program do the math....

Inputs:

1. Pressure before first shot
2. Pressure after the last shot
3. Tank volume (cc)
4. Pellet weight (grains)
4. Number of shots
5. Average velocity for all shots taken.

Outputs:

1. Tank Volume (CI)
2. Average energy in FPE
3. Efficiency in bar-cc/FPE
4. Efficiency in FPE/cuin  

[rsterne]

HI Lloyd....

This is a GREAT idea, and the way you have it currently set up is fine.... I know I was being a bit silly with the Metric to Imperial stuff.... Everybody on this website seems to work with the Imperial units, except that the reservoirs on the guns are mostly rated in cc.... so lets just stick with those units.... Pressure should be in psi instead of bar, and velocity should be in fps instead of m/s.... but I think that's what you had in mind anyway.... If somebody has different units, they can easily convert before inputting the numbers....

Bob

[melloroadman]

Can you include cf in the tank in put ? What about psi per shot average ? Marvin

[rsterne]

Can you include cf in the tank in put ?

Not sure what you mean by that.... The reservoir volume where you are reading the pressure drop is what is needed.... If the gun is tethered to a SCUBA tank then you would use the internal (water) volume of the tank in cc (not the CF content) and the pressure drop for the string.... 

What about psi per shot average ?

That could be included as an output, but it isn't a linear function.... The psi drop starts off less and increases in a non-regulated PCP.... In a regulated gun it should be linear, however.... I think including it as an output would be OK though, as long as you realize the limitations of the information....

Bob

[tworr]

There are online tools that allow you to build a excel spreadsheet on the web and people can enter their own data.
This isn't just saving the data to post as a html file, but actually a working database. I had found a free one yesterday, but I'm looking for it again.

[melloroadman]

If I am reading this right  none of this is liner. It is a average over a said amount of shots . Yes it is under stood . Then can a conversion from cf to cc be listed ? Marvin

[rsterne]

1 CF = 1728 CI = 28,317 cc.... However, I don't think that's what you mean.... You would be unlikely to use 1500 psi from a tank with an internal volume of even 1 CF as that would be 103 CF = 178,759 CI = 2,929,329 cc of air used....  

SCUBA and SCBA tanks are rated in CF because that is the amount of air they hold at their rated pressure.... As an example, a 3000 psi tank rated at 80 CF holds that much air at 1 bar.... Its internal (water) volume is NOT 80 CF.... Just think how big that is!.... A tank 4' x 4' x 5' inside would have a volume of 80 CF.... The internal volume of the tank depends on the rated pressure....

For an 80 CF, 3000 psi SCUBA tank, the internal volume would be 80 / (3000 / 14.5) = 80 / 207 = 0.386 CF.... That would be 668 CI or 10,944 cc.... However, an 88 CF SCBA tank (which contains more air) is actually smaller inside because it contains 88 CF when filled to 4500 psi.... The internal volume would be 88 / (4500 / 14.5) = 88 / 310 = 0.284 CF.... That would be 491 CI or 8038 cc....

What we need for the input for the calculator is EITHER the inside (water) volume of the reservoir and the pressure drop during the shot string.... OR the actual amount of air used at 1 bar pressure.... Since we don't have a flow meter that will measure how many CF have been used up, the easiest way is to use the volume and pressure.... If you are using a tethered setup, you would need to calculate the internal volume (in cc) of the tank (as shown above) and use that as the reservoir size.... If you do this for each tank you have and write it down on the tank, you can then use that for your input volume.... For best results, you would have to add the volume of the reservoir on the gun (if any) to the tank volume....

Note that small (paintball) tanks are rated in CI and that is the actual internal (water) volume of the tank so no conversion is necessary other than to multiply by 16.4 to convert to cc....

HTHs....

Bob

[tworr]

http://www.trirand.com/blog/

https://github.com/mleibman/SlickGrid/wiki

http://www.labnol.org/software/embed-tables-spreadsheet-data-in-websites/7435/

Maybe one of these will help in some way.

[melloroadman]

Bob please . I think you know what is needed to do the job of a tethered rifle . Can it be done .Marvin

[rsterne]

The best way is for you to calculate the internal volume of your tank.... I can help if you give me the CF and pressure rating.... Then you use that as the volume input for the calculator....

Personally, I would think that the pressure drop on a large tank (I assume you are talking an 80 CF SCUBA tank) would be so difficult to read for a single shot string that it would be almost impossible to get any accuracy.... Let's take a string from your Corsair as an example.... The reservoir is only 155 cc, and you used (as an example) 2000 psi for a shot string.... If you had the gun directly tethered to an 80 CF SCUBA tank (which is 10,994 cc) the pressure drop for the same string would only be 2000 x (155/10,994) = 28 psi.... Can you read the gauge on your SCUBA tank accurately to within 1 psi?.... 3 psi?.... An reading error of only 3 psi in opposite directions at each end of the string on the SCUBA tank would be 20% of the results of the efficiency calculator.... Most 3500 psi gauges I have seen only hae 100 psi divisions.... Anything less than that is an estimate.... I can only read such a gauge to ~50 psi.... 

I'm not trying to be an ^&* here.... perhaps Lloyd can help explain....

Bob

[rsterne]

OK now that you added the diagram I understand what you are trying to do.... and yes, you can do it that way for very large shot strings with a very powerful gun.... I would suggest that the simpler way (that would work for any size gun, tethered or not) would be to use the volume of your tank (668 CI) and the pressure drop (400 psi) in the calculator.... The results would be:

400 psi / 14.5 = 27.6 Bar x 668 CI = 18,428 CI / 1728 = 10.66 CF of air used....

Lloyd can chime in if he thinks there is a practical way to allow the CF of the tank (and the rated pressure) to be input.... and yet not REQUIRE that is the way the input is done as then it would be useless for short strings with a less powerful gun....

Bob

[lloyd-ss]

http://www.trirand.com/blog/

https://github.com/mleibman/SlickGrid/wiki

http://www.labnol.org/software/embed-tables-spreadsheet-data-in-websites/7435/

Maybe one of these will help in some way.

Todd,
I will take a look at some of those and see how they might work.  Getting a database of statistics going would be very neat.  I just hope it wouldn't turn us all into a bunch of liars, like fisherman!  Or gas mileage! 

The first one will be a pretty straight forward web-based calculator that anyone can use.  In the future it can get more involved, for sure.
Lloyd

[melloroadman]

Bob please understand that a shot string would not consist of 3-5 shots . It is more in line with what you see . I understand what you are saying and that is why the shot string is as well increased . One could as well start with 3000 psi and shoot till the psi is at 2950 and calculate that way as well with the number of shots taken .
 My tank sizes are 80 cf 2 are rated at 3300psi and 1 at 3000 psi .All 3 have the same out side measurements . But others may want to use this program as well .I am not the only one that tethers Marvin

[rsterne]

The problem with only using 50 psi for a string is are you really using 50 psi, or only 45?.... or 55?.... or 60?.... The difference works out to a HUGE percentage error and makes the whole point of doing an efficiency calculation pretty much useless.... Unless you are shooting tethered, most shot strings ARE 5 or 10 or 20 or 40 shots, not 120.... There is nothing wrong with shooting 120 shots or more when tethered, and in fact the advantage to it is that you have enough gauge movement to be able to get reasonably accurate results....

While your three 80 CF tanks may be the same dimensions on the outside, they will be different inside.... The 3300 psi tanks would be 80 / (3300 / 14.5) = 80 / 228 = 0.351 CF = 606 CI = 9,936 cc.... All you need to do is write 10,994 on the outside of your 3000 psi tank, and 9,936 on the outside of your 3300 psi tanks and you can use the calculator the same as someone shooting 10 shots through a Disco.... This calculator has to work for everybody to be useful, don't you agree?....

Perhaps a small calculator to change the CF and rated pressure of a SCUBA/SCBA tank into volume in cc which could then be used in the main calculator would be useful?....

Bob

[lloyd-ss]

..........................

Lloyd can chime in if he thinks there is a practical way to allow the CF of the tank (and the rated pressure) to be input.... and yet not REQUIRE that is the way the input is done as then it would be useless for short strings with a less powerful gun....

Bob

Bob and Marvin,
First, let's talk about the basic calculator which would be where you entered the cc volume of the tank on the gun and shot it un-tethered.  That will probably be the most accurate way to do the calcs, because the pressure changes will be large and errors in reading won't matter much.  Marvin, you are right, the air efficiency changes as the pressure in the tanks drops and is not necessarily linear.

If you just shoot one long string, everything will be averaged and it will look linear.  One way to get more accurate results if you were, say, shooting a gun from 3500 psi all the way down to 1000 psi, would be to break it into 2 or 3 strings.  Lets say after shot six you are at 2550 psi, then shot 12 puts you at 1450, and the shot 17 is 1000psi.
You'd have 3 different shot strings to enter, each with their own average velocity and own beginning and ending psi.
Then the output would show how the fpe and efficiency changed for each different string.  It can show where the most efficient fill pressure for a particular pellet might be.  Kinda like doing a dyno curve on an engine while monitoring the fuel consumption to find the most fuel efficient speed at that particular load.
Lloyd..... more coming

[rsterne]

I like the idea of breaking the string into sections so you can look at the efficiency in various sections of the curve.... All that is required is to take intermediate pressure readings after every "X" shots and then input each string into the calculator separately.... Like Butcher said, only take 3 shots as the 4th requires a lot more pumping.... The same thing would apply to a Disco.... Instead of shooting 25 shots, just take 10 or 15....

Bob

[melloroadman]

Thanks Bob . But as far as reading the gauge . I think that it would be just as accurate if not more so than the guess at the refill psi after the shot string at each refill . A flow meter is the only true means of removing this error . Marvin

[rsterne]

I usually record the pressure after each shot string (I know, I ALWAYS should).... I agree that the usual 1" gauge mounted on a PCP is pretty useless with it's 500 psi divisions.... That is why I usually replace them with a gauge with 100 psi increments.... If that isn't possible, and you have a good gauge on your fill setup you can tell what pressure you are at by filling slowly until you hear the check valve "click".... or with a pump, wherever the gauge stops moving 100-200 psi on a pump....

My point is that it's a lot more accurate even if you are out by 100 psi when you have a 1000-1500 psi range than trying to read a gauge to just a few psi when the difference between start and finish pressures is only 50-100 psi.... When all is said and done, the only REALLY accurate method is the way Lloyd does it.... with a strain gauge and a digital readout.... (lil' jealous look)....

Our methods may vary, and so, therefore, will the accuracy of our results.... In the end, all that matters is if you feel you are getting useful information....

Bob

[lloyd-ss]

Bob please understand that a shot string would not consist of 3-5 shots . It is more in line with what you see . I understand what you are saying and that is why the shot string is as well increased . One could as well start with 3000 psi and shoot till the psi is at 2950 and calculate that way as well with the number of shots taken .
 My tank sizes are 80 cf 2 are rated at 3300psi and 1 at 3000 psi .All 3 have the same out side measurements . But others may want to use this program as well .I am not the only one that tethers Marvin

Marvin,
Yup, I see what you mean.  Many folks like to tether their guns and shoot a butt load of shots and then want to do some calculations. If you can get an accurate measurement of the drop in tank pressure, good enough!

So if the gun has a 400cc tank, and you have an 80 cuft 4500 psi tank, how many cc's do you add to the 400cc in the gun to get the water volume of the entire set-up?  That's really all you need to now, the water volume of each tank.  I will add that to the list.

I was looking on the scuba forums and there is certainly a lot of confusion there on tank volumes, too.  We are not alone.

Lloyd
P.S.  Thanks everyone for chiming in on this.

[lloyd-ss]

I have a digital pressure gauge that i use with my prototypes.  If you look back at some of my excel spreadsheets you can see that I have the psi for each shot recorded.  I ended up making a gauge from a pressure sensor chip and and an instrument amp and an A to D convertor.  It has a resolution of 1 psi, and an accuracy of maybe 3 psi.  For me it works great.  One drawback is that because darn near everything I build has some sort of slow leak,  it is painful to sit there and watch the LCD display slowly tick the numbers off as the air slowly leaks.