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My first advanced combustion is about 4" x 18" chamber with interchangeable barrels and a 1/2" metering pipe. It has a 13.5kV BBQ igniter and triple spark gap, each maybe 2 mm wide. It has a 4" brushless fan. Using propane or MAP-Pro gas, I can get it to fire well and repeatedly no problem. I'll post pics/vid once we go shoot it in the coming weeks.
I decided to make a second, smaller cannon with the idea of fueling/metering syringes. The goal was MAP-Pro gas, but for now I'm using propane to test. I for the life of me can't find consistent measurements for how much fuel to use.
The chamber is about 2" by 6", but it's more accurate to use between 7" and 8 " due to extra fitting space (I actually use 23ci for calculations). I decided to use a piezo igniter since it doesn't use batteries and a single spark gap due to the chamber size. I also have no fan in this one. I have two meters, both 1/2". One is about an 1" and the other about 6".
Using the burntlatke calculator, it tells me I need about 7 PSI for the 6" meter and 45 PSI for the 1" meter. However, using P1*V1 = P2*V2, I calculate I need about 3 PSI for the 6" meter and 17 PSI for the 1" meter. Inconsistent answers always confuse me (I assumed 4% of chamber volume needed for propane volume).
I should also mention that the fueling set up has about 1/2" by 6" of dead space, which I feel effects trying to gauge the small meter somehow.
I can't measure 7 psi with my 160 psi gauge so anything less than 10 seems impossible, but that doesn't seem to matter anyways. I haven't gotten anything except 30 PSI for the 6" meter to ignite, and it's a tiny puff followed by air hissing through a random leak in the cannon. The duct tape on the barrel doesn't peel at all. I tried 10, 15, 20, 25, 30, 35, 60 and only 30 has ignited, twice in a row at that. The theory doesn't apply to the practice.
For the 1" meter, I got it to ignite after several tries at 60-65ish PSI and it was extremely weak, barely peeling the duct tape. I tried again but not sure what the pressure was, didn't work. at 80 PSI it fire on the first click and actually pulled back half of the duct tape, but wasn't any louder than a fart.
Normally I lurk and don't ask questions because I can figure it out or find it through research. But this time I'm really stumped as to what the heck is going on. It only works at pressures that seem theoretically too high. Not having a fan makes me skeptical that that's the issue, but so does using a piezo. I know many good cannons exist that are much simpler than mine so I'd appreciate some help or suggestions.
(I'd prefer not to post pictures because I like showcasing finished products only. And if you somehow remember from my last pneumatic question in November, I haven't post those air gun pics yet...)
You can tell I've done extensive trouble shooting, but that's not good enough. I'll answer any specifics you might have about the cannon.
Thanks in advance
EDIT: Forgot to mention, GGDT tells me I need 11 PSI for the 6" meter and 66 PSI for the 1" meter. Again, every answer I get is different.
Let's say you have a 23ci chamber.
Your 6" by 0.5" meter has a volume of 1.18ci
The total volume of chamber + meter is 24.18ci
The volume of propane you need at atmospheric pressure is 4.2% of this total, so 1.02 ci
This is smaller than you meter volume, so let's use your 1" meter instead.
This has a volume of 0.2ci. Total volume is 23.2ci, so volume of propane needed at atmospheric pressure is 0.97ci.
This means that the meter needs to be pressurised to (0.97/0.2) x 14.7 = 71 psi.
One thing you have to know about your gauge is, does it measure absolute pressure, or pressure above atmospheric? Most air gauges read zero when the pressure is actually atmospheric (approx 15 psi).
This manometric malarkey is a load of to me though.
My solution would be one of these:
Just squirt 16cc of propane into a well ventilated chamber and you're good to go
I was wondering about the gauge myself, I'm not sure if it's absolute pressure or not. If it measures pressure above atmospheric, do I subtract 14 psi from the calculated pressure needed?
I figured I would be recommended syringe fueling, may or may not look into it for field use with this gun. I also don't have a 20ml syringe, only 10ml. I might just need to do more tests with the small meter, possibly the 80 psi was weak due to overfueling.
If I still get weird results I'll make a video as evidence that it's acting funny.
Thanks for the quick response, it also answered some of my unasked questions.
You should assume there is 15 psi in the meter when it reads zero, therefore if you need to fuel to 70 psi, the actual gauge reading you should aim for is 55 psi.
Are you flushing the meter with propane before taking it to pressure?
You can do multiple squirts from a small syringe, first put 10cc then 6cc. I would try this even if just to make sure that the issue is fuelling and not anything to do with the ignition system.
After a misfire, are you ventilating the chamber properly before trying again?
Actually, no, I've never read anything about flushing the meter but I did wonder if preexisting air was an issue. (of course it'll throw off the propane to air ratio heh) Not sure how that would be possible with an end cap on my meter, because I can't push/flush out the existing air, only add propane to it.
And yes, every misfire I unscrew the barrel and meter, blow in the chamber and let it sit out while I fuel up another test.
I'll try the syringe fueling tomorrow once I get a set up going and it's not 1am haha.
EDIT: One more thing, if my fueling port is 1/2", should I be worried about fuel escaping while syringe fueling? More so because I would have to do it twice.
Took my time, used 16ml of butane and injected it in. Duct tape over the barrel. Made a nice "fump" and worked first click! Best ignition yet.
Immediately tried to do a second test after blowing though the chamber and giving it a second to exhaust. Measured 16ml butane and it didn't work at all.
I'm thinking that venting the chamber might be the actual problem here. That would explain why none of the propane tests worked at all, because there was never the correct amount of air to propane mix.
Is there a way to get around not having a venting fan? I don't have one small enough and I was hoping I could do this without any batteries. If not, I have an idea for an external exhaust system, but I would prefer not. Also a hand pump seems a bit... strange for this concept gun.
Rather than make any more double posts, I'll edit this one to add any results from today.
Test 1: Ignited 16ml first try
Test 2: Didn't ignite, probably didn't vent the chamber long enough
Test 3: Gave the chamber plenty of time to vent. Took over 20 clicks to get the mixture to ignite. Launched a wooden slug with a loud bang. Looking promising but maybe there is a problem with the ignition after all... How can I improve it? 2-3mm spark gap in the center of the chamber, uses a BBQ piezo. Used to be maybe twice as wide, but I assumed a smaller gap would be better. Venting might not be the issue after all, or maybe it's both.
Test 4: 55psi in the 1" meter. No ignition. Doubting my trigger system.
Found this in another thread, I'll be taking apart my chamber again I suppose:
Turns out it's inconsistent past about 4-5mm, so I can't make it as large as I want. Darn. I've made it as large as it will go though.
Well. The bolts I use to conduct my sparks are apparently restricting it. Straight from the piezo I can get a massive spark larger than 1/4", but in my mini spark strip it's not making sparks large or consistent.
Further testing of the disassembled chamber shows that something is completely screwed up with my spark setup. The bolts I use to carry current into the chamber appear to impede the high voltage to the point that it can't jump a 5mm gap, even though it can jump 7mm easily without the bolts. Redesigning and assembling a new spark setup.
Okay, so now things just aren't working correctly. It'll jump a 4mm gap with just the wires, but trying to run current through and bolts causes extreme failure. It can't really make a spark jump further than a 1mm anymore.
I need a new ignition source......
This is the last atmospheric pressure combustion I made:
http://www.spudfiles.com/forums/el-caon ... 16467.html
That's using a small piezo ignitor from a cigarette lighter, nice big healthy gap and it worked every time.
How blunt are your bolts compared to the other spark setups? Sparks jump easiest, and most reliably, from sharp electrodes. The breakdown voltage of air is about 3000 Volts/mm between blunt electrodes and about 1000 Volts/mm between sharp electrodes. Notice that Jack's setup has at least one sharp electrode. An excellent screw to use for a spark gap is fine thread wood or drywall screws. They have sharp points and the high thread count gets a good grip on the chamber wall.
If you look at high voltage equipment you'll notices a fanatical avoidance of sharp points. Even in a van de graff generator the high voltage is stored on a large radius sphere to minimize spark formation. The part of a van de graff that must have low resistance across an air gap is often made with a large number of fine wires, each providing a sharp point to assist in conduction.
The sharp points on the brushes at D and E facilitate conduction. The high voltage is stored on the large radius sphere A.
For a spark gap you want the sharp points not the blunts.
Actually the sparks weren't jumping from the bolts, merely traveling through it. I thought the bolts were restricting the electricity, but it turns out the piezo was defective and became worse and worse with every click. It doesn't work anymore.
I completely replaced the trigger mechanism and reassembled the entire thing. I might have to do more testing, but I'm not sure if my set up to use meter syringes and propane on this small cannon is going to work at all. Worse comes to worse, I'll stick with butane fueling because it'll still work with the concept of this cannon, but I might have to rebuild the chamber.
Test 1: Fired second click with 16 ml of butane. Lost the projectile (wood, bounced into grass), made a large boom. I haven't really payed attention to the loudness of these guns I make, but it's probably louder than I should be using in my yard...
Test 2: 55psi of propane in the small meter. Didn't ignite. Going to leave plenty of time for it to vent before trying propane one more time.
Test 3: 55psi again, didn't ignite after multiple clicks. I think there must be something wrong with the metering set up, and that it's not precise enough for a smaller combustion. I suppose I'll switch to using butane. Don't have much money though, so not sure if I can go buy parts now.
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