I've been working on a piloted QEV style paintball marker and am running into results that I do not understand:
- Paintball velocity is ~75% of predicted velocity
- Efficiency (shots per tank) of set-up is ~20% of predicted value
I'm hoping someone here might be able to help me understand my results.
Description of Set-up:
Before building an actual paintball marker, I've build a proof-of-concept test cannon. It uses all of the parts that the paintball marker will use, but mounts them in an easily accessible and exchangeable configuration.
- HPA tank to a low pressure regulator (Palmers). Then to a 3-way, 2-position pilot valve (McMaster). Then to a QEV (the chinese black box QE-04). The exhaust of a QEV leads to a 7 inch barrel.
- I've built a sear-based trigger mechanism to operate the pilot valve to remove the human element from triggering. A pull string releases a sear allowing a spring to quickly un-press the pin on the pilot valve.
Description of Operation:
- trigger mechanism is set, pressing the pin on the pilot valve which pressurizes an air chamber (0.56" ID x 12" long)
- paintball is muzzle-loaded into the barrel
- pull string is pulled, trigger mechanism releases pin on pilot valve, QEV opens and air chamber empties through barrel
For my test set up, I get the following results:
- Code: Select all
PSI Pred. Actual FPS
80 256 187
100 296 228
120 326 257
140 354 276
160 381 429
180 402 318
Note (10 shots were taken at each pressure, std dev are between 5 and 10 FPS)
3000 PSI, 48 in^3 tank should yield ~550 shots. I am getting about 110 from a tank.
Thoughts on the results:
Let's start with efficiency. When I turn the pressure to 140 or above, there is an audible hiss as air leaks from the system. I would expect this to result in some loss of efficiency, but it seems like that wouldn't explain the huge difference between my prediction and my results. I am really at a loss for what to make of this. I'll rebuild the system with better thread sealant to see if this makes a difference.
- Can anyone comment on how much a leaky system affects efficiency?
The velocity measurements are ~75% of my predictions, which is not bad, but I'm frustrated that they are not closer. I suspect the cause is one or more of the following (please let me know if any of these match your experience):
- Pressure loss as air must take a 90 degree turn to get from QEV to barrel
- slow to pilot QEV caused by low Cv (0.11) of pilot valve
- slow to pilot QEV caused by too small diameter of tubing from pilot valve to QEV.
- air leaking around paintball (most paintballs needed a tiny push to go in)
- friction between paintball and barrel (seems likely)
Notes on predictions:
- I built a spreadsheet to simulate the operation of the paintball marker. It is based on the model described in the link below. Of particular note:
- The model takes into account choked flow through valves.
- I have further modified the model to account for the temperature decrease as the gas expands into the barrel.
- I have verified my model against the test results in the paper, so I am confident that the model is valid at those scales.
- My spreadsheet agrees with the predictions produced by GGDT (Gas Gun Design Tool).
Model used for results predictions: http://www.iontrap.wabash.edu/publications/AJP_80_24_rohrbach_air_cannon.pdf.
Your thoughts and suggestions would be appreciated.