The Chamber Pot Experiment

[FuzzyGrub]

The Chamber Pot Experiment

A while ago I installed a depinger into my Mrod 25 cal.  I did a before and after string and found that the depinger changed the tune.  While it did lower the power some, it seemed to extend the shot count and improve the efficiency.  You can read about it here:
http://www.gatewaytoairguns.org/GTA/index.php?topic=53313.msg525431;topicseen#msg525431

Others piped in and said this “chamber pot” had been used in other guns to do the same. Since then, I have been wondering how to use this to best advantage.  What would be the optimum chamber size and optimum inlet hole size for a given power level?   Can this effect provide longer and/or flatter strings than a stock gun?   If so, it would be nice to have some means to calculate a good starting point for both variables, such to save time on depressurize and pumping the gun up, over and over again.

That data does not seem to exist, so the following experiment is to try to come up with a way to do so, for at least a stock 25 cal Mrod (version 1).   Who knows what else we can learn. 
 
Our resident expert (Bob) can hopefully provide some much needed guidance along the way.   

Initial Data:
Here is the initial before/after strings:


Depinger Data:
The depinger looks similiar to this, but is black with two equal size holes:



Outer Diameter   1.05”
Inner Diameter   0.825”
Overall length   1.383”
Inside Depth   1.121”
Hole Diameter #43 drill X2   0.088”
Wall Thickness   0.112”

Calculated Values:
Material Volume   1.197545ci   9.804335cc
Chamber Volume   0.599245ci   9.819828cc
Inlet Hole Area    0.012164sq”   

Gauge Block Volume: TBD
Valve to Gauge Block Volume: TBD
Valve Volume:  TBD
Total Chamber Volume: TBD + 9.8cc

Next up: Fill in the TBDs and select chamber sizes and holes sizes to test with.  Develop draft plan.

[FuzzyGrub]

Does anyone have the correct dimensions for the following:

- Gauge Block Thru hole Diameter and Length:  My WAG is 3/8"Dia X 1.25"L
- Space Between Gauge Block and Valve:  My WAG is 1"L X 1.0625"Dia
- Space in Valve:  My WAG is 3/8"Dia X 1.5"L

If I use my WAG values, I get 19.5cc, for a total chamber volume of 29.3cc.

I can drill a hole in the side of the depinger cup to get an even 10cc, and my thought would be to test at 5, 10, and 20cc, or total chamber test at 24.5, 29.5, and 39.5cc

Inlet Holes: Current is two #43 drill size.  Thought to try 1 hole, 2 hole, and 4 hole, or 0.0061, 0.0122, and 0.0243 sq in. 

For a cup spacer, tri-5-ron said he used some aquarium gravel filter tube.  I think I have some in cellar.  For the 5cc, depinger would be flipped. 

Comments on my size choices?

[FuzzyGrub]

Draft Test Plan:

Establish Baseline:
- Remove depinger, measure gauge block, etc
- Tune stock gun to a "standard" 16 shot 40fpe string
- Record Settings for baseline

Large Chamber Test (Medium Inlet):
- Shoot String and Compare to baseline
- Tune for longest 35fpe (4% Es) string

Medium Chamber Test (Medium Inlet):
- Shoot String and Compare to baseline
- Tune for longest 35fpe (4% Es) string

Small Chamber Test (Medium Inlet):
- Shoot String and Compare to baseline
- Tune for longest 35fpe (4% Es) string

Small Inlet Test (Medium Chamber):
- Shoot String and Compare to baseline
- Tune for longest 35fpe (4% Es) string

Large Inlet Test (Medium Chamber):
- Shoot String and Compare to baseline
- Tune for longest 35fpe (4% Es) string

Optimum Chamber, Optimum Inlet Test:
- Select and Install optimum chamber/inlet
- Tune for longest 35fpe (4% Es) string

Best Stock Tune Compare Test:
- Remove depinger
- Try to achieve equivelent optimum chamber & inlet tune
- Compare to above


All testing will be with JSB Kings.  All strings will use fill from 3.2K to 1.8Kpsi to capture "complete" strings. 

[rsterne]

My first thought, when looking at the raw data for your initial before and after strings was that I was looking at the same thing that happens when you change hammer strike.... On the "before" curve, you are missing the first part of the curve, and hence some of the useful shots.... The "after" curve is like what you would get if you backed down the hammer strike a bit, moving the curve to the right, losing a bit of power off the top, and gaining a shot or two (compared to what you would have had if the first part of the "before" curve was present.... The biggest gain in shot count, however, is because the front half of the curve is now present....

The reduction in PEAK velocity makes sense, the prechamber is reducing the AVERAGE pressure during the shot, although if you have 30cc total and (assuming you are using 25.4 gr. JSB Kings) since you are at about 40 FPE, you're not exactly starving for air, so the pressure drop isn't very much, maybe 10%.... However, there must be a secondary effect going on here, because if you JUST reduced the pressure, you would expect to keep the same "before" curve, but loose even more shots at the front end.... You LOST velocity from the peak of the curve, so not only are you losing average pressure INSIDE the valve, you are also losing flow THROUGH the valve.... Either that, or for some reason the valve is opening LESS, just as if you dialed back the hammer strike....

There is definitely more than one thing happening here....

Bob

[Motorhead]

Not sure where the "read into" it applies here or there ? ..... See a POT being somewhat like a regulator being between HP side and LP plenum side to the extent that with cup in between valve & HP air storage, valve will see an internal pressure drop during the shot.
Where if cup was not there the entire volume and pressure main storage it's under would bear on valve during the shot cycle.

So with this pressure drop within valve, the outflow drawing valve closed is reduced, tho at the same time energy potential driving pellet is reduced because of this pressure drop.  Now that said, With less outflow around poppet it would seem poppets hang time ( Dwell ) would be longer.

Makes for one sorta messed up puzzle of a picture we were having a good grasp on ... Chamber pot huh ?
thought they did away with those in the early 20th century 

[rsterne]

Not only is the "outflow" reduced, the pressure exerted on the stem is also reduced, and it is the MAJOR force trying to close the valve.... Therefore, as Scott says, the dwell should increase, but at a lower pressure.... Now the lower pressure is the reason for the velocity loss, that makes sense.... but why would INCREASING the dwell shift the curve to the right instead of the left?....

My brain hurts !!!.... LLoyd !!!.... HELP !!!

Bob

[FuzzyGrub]

The before string was suppose to be 16 shots, so mostly extra data points, but agree one or two more on the front.   

The after string was truncated on the front.  It actually started at 3.3Kpsi vs the 3.1K which is shown.  The missing frontend shots are: 792, 794, 800, 796, and 805.

If I knew what the after was gong to look like, I would have started the before at 3.3Kpsi.  That is part of the reason of re-doing a baseline.

Looking at my gun now, it don't think there is 1" between valve and gauge block, but very hard to tell. Also the amount in the valve was a complete WAG. 


Anything about my approach that should be changed before I start? 

[lloyd-ss]

I can feel the brains working, LOL.
Bob asked that I get my nose out of the lathe and join the fun.  Good suggestion!  John, I think you may have pegged it when you said you didn't think there is one inch between the valve and the gauge block.  The gauge block in the M-rod pretty much touches the valve, so that would eliminate the 1.065 dia x 1" long part of the chamber pot (who came up with that term, anyway?).  That volume is 14.6ccs, so instead of 29.3cc, I think you only have  14.7 ccs.  That's half as much, and plugging the info into my calculator, it looks like that is probably starving a 40 fpe shot.
My calculator shows that you could easily loose 40 fps by going from plenty of air to only 14cc.
I think that what your first string shows, does indeed make sense.
What do you think?
Lloyd

[lloyd-ss]

John, back to your original comment about using a chamber pot (still gotta laugh) to tune a shot string... or at least as part of the strategy to tune for a nice shot string, I agree. But darn, there are sooo many variables to deal with. I think you've already demonstrated how using too small of a chamber pot can reduce the overall velocity and possibly add a few more shots to the string.  The shrouding around the valve is a big factor, too, just like the transfer port.
There are choke points after the valve head, and there are volume and flow limitations before the valve head.  I certainly don't know which is most important, but I do know that they are very dependent upon each other.  I've seen where when  the before-valve and the after-valve flow all out of balance, it ruins efficiency and string length.  It's a fairly delicate balance.  Now where can I  find some finesse?

[rsterne]

FuzzyGrub.....

I think you methodology is good.... but I also think you should return the gun to your control settings and record a string over the original pressure range before tuning it for the best string in each configuration.... otherwise the tuning may well hide the data we need....

Bob

[FuzzyGrub]

Name: I have heard it called a Shot Pot, but thought this would be more memorable.     Yea, old enough to know what one is, but not old enough to have used one!   

Lloyd: From the before/after it would seem that the current size would be relatively close to what the optimum might be.  From an experiment standpoint, though it would be good to get a data set above and below that.  If anything, we should see the "small" chamber choke it off.   I didn't want to go any smaller, because the inlet holes are offset, and if very close to gauge block, may cause a restriction and alter results further.

Bob: The step "Shoot String and Compare to baseline"  is meant to do that.  That will be done with the established baseline preload, throw, and Vs.   I should add that to the "optimum" test though.  Because the tuning part will take me more time, was thinking of only doing it for the optimum, but thought we would miss something.


Question: When I do the small Inlet test, is there a chance to create too much of a pressure drop and break the delrin chamber?   Especially if I tried it on the small chamber? 

Testing: Not sure what my time allotments will be today, but hope to at least get the baseline done.  Large Chamber, (medium Inlet) maybe on Sunday.

[lloyd-ss]

John,
This is a good experiment that will have some real application.  I put a typical shot from your data into my internal ballistics model, one shot with the full M-rod tank of 230 cc ( 855 fps) and then one shot with the 15 cc Chamber Pot (827 fps).  Here are the theoretical graphs of what is happening inside the gun with each shot. The blue line is the pressure in the gun, behind the pellet.  The PSI scale is on the right vertical axis.  For each shot, I made the assumption that the dwell was identical and the starting PSI of 2500 was the same.

For the shot from the 230cc tank, the valve opens and the pellet sees almost full tank pressure, then the valve closes when the pellet is about 3" down the barrel (where the blue line drops off), but it keeps accelerating because of the pressure that is still trapped behind it.

The shot from the 15cc tank is very different.  You have to look carefully at the graph, but when the valve opens, the pressure drops immediately to about 2300psi as the limited air has to fill the exhaust port and the transfer port. This wasn't an issue with the big tank because you had a relatively unlimited air supply.  But with the 15cc shot, you get that initial loss of pressure, and then as the air expands behind the pellet, the pressure drops faster. Final pressure behind the pellet when the valve closes is 2065psi for the 15cc tank, and 2466psi for the big tank.  But also, I believe the air efficiency has gone up slightly with the smaller Chamber Pot, possibly an added bonus.

These graphs are theoretical and do not take into account all the variables, but the model does track closely with real world testing.

As far as the chamber pot cracking, that is a good question, and I hope you will give us the answer, LOL.

[FuzzyGrub]

Lloyd:  Thanks for the graphs. 

So, for the small chamber test I'd expect an even more drop of psi within that first 2.6" of barrel length, and for the large chamber, something closer to the 230cc chamber graph. 

Now, adding in the inlet hole, based on its size, the pressure drop will be even less in that first 2.6" length while the valve is open. 

Looking at the 15cc graph, does the slope of the ramp down change with the initial pressure (first 2.6")?  ie is there less downward slope at 3Kpsi?  I'd expect that the initial, near vertical drop would be less, is that correct?

Maybe this is just covering for a sloppy tune?    But, seems like there is something more to it.

[rsterne]

Lloyd's graph with the 15cc chamber will be based on NO inlet hole, if you notice, he is using that for the "tank cc".... ANY hole allowing air into the chamber will decrease the slope of the line between the initial drop (when it fills the transfer port) and when the valve closes.... I don't think the initial vertical drop would be much affected by the hole size, unless it gets quite large....

Bob

[lloyd-ss]

Lloyd:  Thanks for the graphs. 

So, for the small chamber test I'd expect an even more drop of psi within that first 2.6" of barrel length, and for the large chamber, something closer to the 230cc chamber graph. 

Now, adding in the inlet hole, based on its size, the pressure drop will be even less in that first 2.6" length while the valve is open. 

Looking at the 15cc graph, does the slope of the ramp down change with the initial pressure (first 2.6")?  ie is there less downward slope at 3Kpsi?  I'd expect that the initial, near vertical drop would be less, is that correct?

Maybe this is just covering for a sloppy tune?    But, seems like there is something more to it.

The initial vertical drop is strictly a ratio and proportion thing, P1xV1=P2xV2.  You start with the initial volume in the chamber pot, then as soon as the valve opens the volume increases to include what I call the "transfer port volume."  That transfer port volume in reality is the volume inside the valve (on the exhaust side of the valve head) plus the exhaust port + the transfer port + plus the barrel volume up to the pellet. The volume of the transfer port that I used in the spreadsheet is an estimate, and I have not measured all the different parts of the exhaust circuit.  Bob states it correctly that I do not take into account any re-filling of the chamber pot thru the bleed holes.

Regarding the slope of the curve, you are correct that a smaller chamber pot would have a steeper slope. Going to a larger chamber pot, it doesn't have to be too big where the additional volume becomes irrelevant.  Bob's rule of thumb of 1cc/fpe is pretty good.

Again, the slope of the curve is another ratio and proportion thing: you are filling the initial transfer port volume with the vertical drop in pressure, and then you are filling the barrel volume for the first few inches as the pellet accelerates until the valve closes.  If the valve dwell is the same, at a higher pressure, the valve will close when the pellet is farther down the barrel because the higher pressure will have accelerated the pellet faster in the same amount of time.

John B, thanks for the additional reference. (Were you at the Roanoke show this past fall?)
There are a lot of what-ifs and variables in this project and a very efficient tune is in there somewhere.

Lloyd-ss

[FuzzyGrub]

You never get as much time as you hope!

Took a while to fish the depinger out.  The hole is barely big enough to fit a coat hanger.  It deformed the hole so will use the spare that was measured.   Will need to add eyelet for easier fishing without damage. 

Recorded Settings: P=5T, T=0T, V=3.25T

Changed settings to: P=5T, T=0T, V=4T

Filled gun to pump = 3.3Kpsi Gun = 3.4Kpsi

32 shot string from 3.4K to 2.8K psi (gun)



Here is the 16 shot string from 3K to 2.2Kpsi:



I am sure I could tweak it more, but my thought is it is "good enough", and has easy settings to return to.  I would consider it in the "normal" range for a 25 cal Mrod tune, using 50psi a shot.   

[FuzzyGrub]

Had less time today.  Finally found the aquarium tube, but it was too old and brittle.  Found and cut some 1" aluminum tube.  Didn't find an eye hook, but used a screw, but it came loose after only 3-4 shots!   Anyway, here is the data for the large chamber, medium inlet, from 3.4K to 1.8K (gun gauge):



And a comparison of the best 16 strings:

[rsterne]

Might I suggest you add an efficiency calculation in your spreadsheet?....

Total FPE = (average FPE) x (# of shots).... for 16 shots that equals 16 x average FPE
Total CI air used = (pressure drop / 14.5) x reservoir size (CI, which is cc / 16.4), for an MRod 215 cc = 13.1 CI.... so Total CI = pressure drop (psi) x 0.903
FPE/CI = Total FPE / Total CI = (16 x average FPE) / (pressure drop x 0.903) = 17.7 x average FPE / pressure drop (psi)

It will help with the comparisons....

Bob

[FuzzyGrub]

Ok, here is medium chamber, medium inlet:





About equal to large chamber, but appears to be broading out a little.


Bob, will do. The gauge is not the best for getting accurate data, though.  I rechecked the largechamber ending pressure and it was at 1900 not 1800.  The medium chamber as just a hair below 2000. 

I am also going to start looking at the best 24 shots.  Small chamber, medium inlet next up. 

[rsterne]

For 24 shots, you will have to change the constant to 26.6 ....

Bob