Smith Rev3 build

[rsterne]

445cc = 27.1 CI.... 500 psi pressure drop = 34.5 bar.... total air used = 27.1 x 34.5 = 935 CI

30 shots @ 34 FPE = 1020 FPE....

Efficiency is 1020 / 935 = 1.09 FPE/CI.... which is typical for a rifle of that caliber and power level....

Bob

[michaelthomas]

Bob beat me to it.

http://www.calc.sikes.us/1/index.php

This is one of Lloyds calculators.......works nice.

Good job on your first match, Chip.  I still haven't really shot outdoors......lol.

Mike

[TimmyMac1]

You did great. You can be proud of your creation and your performance. Nice to finally put a face to the guy.
I Really appreciate you coming and I hope you are coming back for more. Joes place is awesome and it puts a smile on his face to see his benches all get used. After doubling the size of his World Class Range his efforts are being appreciated. He wants to do all he can to help raise the AirGun bar, like most of the guys you met.
Airgun Efficiency shows on the Target that you got your act together. As long as you can get thru a card you have enough air. When it does go horrid you can spend as much time on your sighters as you do the record shots and getting thru a card will be tougher sometimes.
Well Done

TimmyMac1

[csdilligaf]

Thanks guys.  If figures Lloyd has a calculator for everything.  How awesome is it that he lets us use them.  And Tim it was great to see you up there also.  Now I can put the finishing touches on the gun and stock.  I think it will be clear anodize over polished aluminum.  Comes out a nice gray color.  I have to many Black guns.

[SeanMP]

Congratulations Chip

A second is a stellar finish with a brand new gun on your first time out. Booyah!

I was just contemplating your efficiency number. As Bob says that okay but it's not stellar by any means

What would you estimate the size of the pocket (plenum) to be directly in front of the valve. That is with the valve in place and the cap nut screwed down. I did read in you previous thread that the supply port is .375"

What I am feeling is your getting too large of a pressure drop in the firing cycle. The port size is large enough to contribute a bit of plenum volume but not large enough to compensate for the lack of plenum.

[csdilligaf]

  The two chambers between the valve head and the back end of the Regulator are connected with a .375" dia transfer port.  With 1 cubic inch of volume at 1600 psi can that .375 hole be any kind of restriction?  Most of the guys just make the 30 shoots before they go be low reg pressure.  I have 900 psi above reg pressure so that is a good comfort level.  I wonder if the low volume Plenum contributes to me getting less than 5fps spread over the 30 shoots?  It might sort of be some kind of regulation on its own.

[rsterne]

If your total volume of plenum downstream of the regulator is 1 cu.in. (16.4 cc) that is only about 1/2 cc per FPE at your power level.... If you're running 1600 psi setpoint, here is what is happening during the shot:

You are starting with 1.00 CI of air at 1600 psi.... At 1.09 FPE/CI, you are using 34 / 1.09 = 31.2 CI of air at 1 bar (14.5 psi).... Therefore, you are using (14.5 / 1600) x 31.2 = 0.283 CI of air out of the 1.00 that you started with to make the shot happen.... That means that at the end of the shot, before the regulator can cycle and top up the plenum, you have (1.0 - 0.283) = 0.717 CI of air expanding to take up 1.00 CI of room.... This drops the pressure at the end of the shot to 0.717 x 1600 = 1147 psi, and therefore the average pressure DURING the shot is only 1374 psi.... Basically, that is the pressure you are using to make the shot, so you are generating 34 FPE using that much pressure.... That results in OK efficiency, but not stellar....

If you doubled the plenum volume to 2 CI (and asssuming no restriction anywhere up to the valve seat), then your pressure at the end would be (2.0 - 0.283) / 2.0 times the 1600 you started with, or 1374 psi, and the average would be 1487 psi.... That extra 113 psi average during the shot would allow you to dial back the hammer strike and sip a smaller quantity of air, increasing the efficiency.... If you got it up to 1.20 FPE/CI (a reasonable assumption), you would only be using 0.257 CI of air at 1600 psi (instead ot 0.283), put that back into the equation, and now your ending pressure is ((2.0 - 0.257) / 2.0) x 1600 = 1394 psi and the average during the shot 1497 psi.... so you get a snowball effect.... The higher pressure allows you to use less air, which decreases the pressure drop even further.... The net result is that instead of using air with an average pressure of 1374 psi, you are using air at nealy 1500, and that is where you are gaining efficiency....

When you were tuning your gun, did you shoot at various preloads and plot the velocity vs. the preload to get a curve like this?....



What that graph is showing, is that beyond a given point (the"knee" of the curve), adding hammer preload only wastes air (and makes the gun louder).... while too little preload puts you on the downslope on the right (not a good place to be, as the velocity will actually INCREASE when you shoot below setpoint).... If you tune to where the velocity JUST drops (eg. 5 turns out on that graph), you are effectively tuning right at what would be the top of the sweet spot with an unregulated PCP.... That is not only good for efficiency, but tends to have a very low ES as the output pressure of the regulator drifts around, as it always does.... because small pressure changes make virtually no difference to velocity....

Bob

[michaelthomas]

Hi Chip......I'll throw this out there, too. 

I have found that a pellet traveling at a given velocity can be quite different in accuracy depending on the way you get there.  If you have a high pressure charge and a small amount of air, or a low pressure charge using more air (like the scenarios that Bob has described above).......you can have very different results at the target. 
For benchrest , it's something that is worth playing with.....although it can be very time consuming.

I wish I could tell you that one way or the other works the best ......but I believe the answer is probably dependent on way too many factors.

Velocity is not velocity when it comes to accuracy......at least that is what I have found.

You need to shoot indoors to be able to see this, unfortunately. 

Mike

[rsterne]

It is possible to see than kind of effect with a PCP, much more common with a springer.... I've always thought it more likely that it is being caused by difference harmonics as a result of the sudden, sharp push of high pressure compared to a more gentle, slower shove with lower pressure....

Bob

[michaelthomas]

Bob, you could be right about the harmonics. 

I don't know what causes the difference, I just know it exists.  Maybe someday......

Mike

[csdilligaf]

Well, I guess Rev4 will have more plenum.  The Sumo actions I did had about twice and the did very well.  I moved in this direction to gain some features like the balanced valve and barrel clamps direction.  As I build things there is always something that comes to mind for the next time.
   Man Bob, that sure is some fine mathwork you did there.  I will study it and learn.

thanks,

[SeanMP]

I think your ES speaks very highly of the precision to which you have machined the components rather than the plenum starvation acting as a regulator.

I agree with all Bob's equations. However I see a slightly more complex sequence of event occurring which may indicate your pressure drop at the first part of the shot cycle(the critical part for efficiency) is much worse than originally calculated.

From what I've heard you say is you have two volumes of air connected by a port that is .375ID. The volume of a 1" long port of that size .1 cu in. Which by assumption, means that there are two volume of air each having .450cu in of air.

So using Bob's number of volume of air used .283cu in / the volume that it can immediately act upon I get .283/.450=.63. So 63% of the air immediately available is moving through the valve leaving 37% of the original volume left behind. or 1600psi x 37%=592psi. I say this because the pressure drop travels through the space like a wave. The port volume itself is not enough to replace the loss so the wave has to travel all the way to the end of the volume then travel all the way back. I've calculated this to be .3milliseconds. I say this because the other .450cu in of air is not immediately available to the initial shot cycle. So for a third of what we consider the critical part of the shot cycle your getting a huge pressure drop.
So your seeing efficiency similar to setup where the valve has a small volume of air which is directly connected to a larger volume of air but not immediately accessible to the shot. In fact the volume of air immediately accessible to the shot is very similar to a QB valve and your efficiency is also very similar.

[rsterne]

What you say may be true, Sean, but his valve is not unlike a Disco valve, in that the valve volume is small with only a 1/4" hole into it from the reservoir.... Then Crosman stick in a gauge block with only a 1/8" hole in it to further restrict the flow.... Get rid of the gauge block and the power and efficiency goes up immediately, even though the valve still has to breathe through that 1/4" (OK, so you can open it up to 17/64") hole.... Would it be better if the entire reservoir was open to the poppet, yes.... but just having that 115cc available for the valve to draw from makes a big difference from having the gauge block in the way....

Would it be even more efficient without the 3/8" restriction, probably.... but doubling the plenum from 1 CI to 2 CI will drastically improve things, IMO.... After all, the valve is open for 4-5 times the 0.3 msec it takes for the primary chamber to refill from the secondary.... That leaves the difference between two chambers with a 3/8" connecting hole and one larger chamber only influencing the initial (albeit most important) part of the shot cycle.... Without adding that extra volume to the plenum, the whole cycle occurs at lower average pressure....

Bob