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Combustion Curve Investigation Cannon

Post questions and info about combustion (flammable vapor) powered cannons here. This includes discussion about fuels, ratios, ignition systems, safety, and anything else relevant.
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Unread postAuthor: D_Hall » Sun Apr 06, 2008 1:56 pm

jimmy101 wrote:A 1KHz pressure transducer would need, for example, a 90% response time of less than 1mSec. That is pretty fast for a mechanical/electrical device. The transducers are going to be at least a couple hundred bucks each.

I believe this is why Lent was discussing using somebody else's gages? At least I hope that was a big reason. Suffice to say that what Jimmy says here is spot on. We use such transducers at the office. Our "workhorse" models are about $1200 each. The really fast response ones? I honestly don't know but I wouldn't be surprised if you didn't have to add a zero to the end of that price (we only use those when we absolutely have to).

I wouldn't thing temperature affects would be all that great.
Again, whether they are or aren't, it's trivial to protect the gages.

Instead of pressure sensors you might consider strain gauges glued to the outside of the barrel. Since they aren't in the combustion envelope then the temperature changes are not a problem. There are small piezo resistive elements made for real guns that are just glued to the barrel. You could use several glued to the barrel at various places. You might take a look at [...]

To calibrate the setup you'll probably need some way to plug the barrel and a way to pressurize the gun. Perhaps a threaded fitting on the muzzle and schrader valve on the chamber.

I actually HAVE done this with a 30 mm gun before. As you say, you need to calibrate. Another potential problem is that the barrel will ring like a friggin' bell. You WILL capture the harmonic frequency of the barrel....and I don't mean the acoustics of the gases inside the barrel, I mean the harmonics of the barrel itself. This is very important (read: desirable) to the extreme-accuracy firearms crowd, but if you're looking for pressures and such you'll need some decent signal processing (it can be post test, no need for real-time here) to strip the barrel harmonics out.
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Unread postAuthor: Lentamentalisk » Sun Apr 06, 2008 3:53 pm

Well thanks for all of the comments, I will see next Saturday what I can convince Cal to lend me.
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Unread postAuthor: Lentamentalisk » Mon Apr 14, 2008 8:09 pm

Ok, so I went to Cal day and talked to tons of people in the ME department, as well as one of the operators of the combustion lab, and saw all sorts of incredible stuff.
No luck so far on sensors, but the guy from the combustion lab who I spent an hour or two with had an ingenious idea for a cheap way to at least measure the maximum pressure at various places cheaply: stick a pressure gauge behind a check valve. The issue with this is that since we are dealing with hot gases, they will cool down after your shoot, so the pressure will drop. My idea was to put a piston before the check valve, so that the hot gases push the cold air through the check valve. This, I realize, is getting quite bulky and not necessarily very accurate, because it can only respond as fast as the piston can move.
Anyways, I have basically decided to give up on this for the time being, at least until I can get more help from Cal.
Sorry if I got your hopes up.
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