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Method for capturing combustion pressure peak

Post questions and info about hybrid (compressed gas with fuel) powered cannons here. This includes discussion about fuels, ratios, ignition systems, build types, safety, and anything else relevant.
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Unread postAuthor: D_Hall » Wed Oct 07, 2009 11:50 am

To be fair, in a case like that nobody really cares much about magnitude. They'd likely be viewing such a gage as a yes/no type of indicator, not for accurage overpressure numbers.
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Unread postAuthor: D_Hall » Wed Oct 07, 2009 11:56 am

psycix wrote:Oil vapor in an environment with oxygen can diesel. Liquid oil won't.

In theory.

But oil is volatile. Not horribly so, but it *IS* volatile.

Put it this way: A few years back we had an explosion here at the office that was traced to a combination of adiabatic compression of air in the core of a ball valve and the thin film of oil that was left over from manufacture/machining of the ball valve.

Situation:
1) Ball valve exposed to 3000 psi air on the upstream side.
2) Small bypass valve is used to expose valve to 3000 psi air on the downstream side as well.
3) Valve is opened.
4) When 3000 psi air from both upstream and downstream rush into ball, air in the ball was adiabatically compressed (call it 200:1 compression).
5) The air in the ball had oil fumes in it due to film of oil left over from manufacture.
6) BOOM.
7) One $12,000 valve only used once is sent to the recycle bin.
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Unread postAuthor: Brian the brain » Wed Oct 07, 2009 12:47 pm

You could use two checkvalves.
In between is air.The ignited mix compresses the gasses between the two checkvalves and captures it behind the last one wich can be read from the gauge.

If that doesn't work replace the first checkvalve by a piston..
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Unread postAuthor: jimmy101 » Wed Oct 07, 2009 1:04 pm

D_Hall wrote:To be fair, in a case like that nobody really cares much about magnitude. They'd likely be viewing such a gage as a yes/no type of indicator, not for accurage overpressure numbers.

True. But "yes/no" is the same as quantitative measurement at very low precision and accuracy. The reading is probably good to 1 sig fig. :D

Though at lowish pressures (~20 PSI) the reading appears to be pretty darn good, see the calibration chart in the PDF.

But oil is volatile. Not horribly so, but it *IS* volatile.

At sufficiently high pressure damn near everything is combustible. Especially if it has a fairly low heat capacity and is not in contact with a large heat sink. At high enough pressures even steel is combustible. Though steel usually just forms an oxide layer.

Motor type oil is generally considered to be "non-flammable", even OSHA and the MSDS lists it as such. What it does at a compression ration that is ~10X higher than in a diesel engine not withstanding.
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Unread postAuthor: psycix » Wed Oct 07, 2009 1:18 pm

D_Hall wrote:Put it this way: A few years back we had an explosion here at the office that was traced to a combination of adiabatic compression of air in the core of a ball valve and the thin film of oil that was left over from manufacture/machining of the ball valve.
<snip>
$12,000 valve

:shock: What diameter was that valve?
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Unread postAuthor: D_Hall » Wed Oct 07, 2009 1:22 pm

psycix wrote::shock: What diameter was that valve?


6 inch. But there were other special aspects to it. Pneumatically actuated. Nickel plated (anti-corrosion).
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Unread postAuthor: kenbo0422 » Tue Oct 27, 2009 7:04 am

Here's a fix for your check valve and gage:

Place them on a small length of pipe/tubing. The hot gases will never reach beyond the check valve so there won't be any cooling effect to mess up your reading.
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Unread postAuthor: jimmy101 » Tue Oct 27, 2009 10:39 am

kenbo0422 wrote:Here's a fix for your check valve and gage:

Place them on a small length of pipe/tubing. The hot gases will never reach beyond the check valve so there won't be any cooling effect to mess up your reading.

Won't work like you think it will.

Compression of the air in the pipe/tubing and gauge will heat it. That raises the pressure. The check ball seals off the gauge. The air in the gauge and pipe cools and the pressure drops.

The usual ghetto solution for this type of pressure gauge setup is to use an oil or water filled gauge and piping. The problem with that is the inertia and kinetic energy of the moving liquid. Think "water hammer" on steroids.

To get an accurate pressure you are going to need to use either a very expensive (~$100) pressure sensor or do the work to calibrate a ~$20 strain gauge.

It might be easier to do the pressure readings in a closed chamber. A tire pressure type pencil gauge, adequately dampened, should work OK. As would a suitably dampened peak recording gauge ("lazy needle" gauge).
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Unread postAuthor: kenbo0422 » Tue Oct 27, 2009 10:51 am

The 'hot' gases don't get to the check valve, therefore there is no cooling of gases between the check valve and the gage. That is the reason for having the tubing connected between the chamber and the check valve. There is little space between the valve and gage so there will be little gas traveling up the tubing.

Nice web page, BTW.
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Unread postAuthor: Technician1002 » Tue Oct 27, 2009 11:55 am

In some combustion pressure testing I worked with once used dimple disks. They worked on the same principal as the bit blocks used to test a great white sharks bite strength. A series of crushable disks are bolted to a metal block with a small cavity behind it. The peak pressure is recorded by the amount of denting the lid received. Due to the size of the cavity (shallow) this was designed to not burst the disks. After the test some of the thinner disks would be dented and the thicker ones were not dented. The yield strength of each disk is known. This gave us a go no go test. Did we exceed 2500, but not 2700 PSI for example. Calibration for yield pressures that would record a dent was done by hydrostatic testing. This type of yield testing is used to test the pressure wave of explosives.
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Unread postAuthor: kenbo0422 » Tue Oct 27, 2009 12:11 pm

Or... you could get the oil filled gage with the peak pressure needle on the base of a tee. The water hammer effect can be rerouted by a straight line across the top of the tee and into a cushion. :idea:

Actually I like the strain gauge idea. Calibrate with a set of weights. But, I guess, seriously, nobody is going to go that far on here. Probably the best bet is the gas eq program I just downloaded to give a fair idea of where its going to be.
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Unread postAuthor: D_Hall » Tue Oct 27, 2009 3:55 pm

kenbo0422 wrote:The 'hot' gases don't get to the check valve,

Once you understand the concept of "adiabatic compression" you'll find that yes, there ARE hot gases in the system you describe. The previously cold gas molecules become HOT when you compress them suddenly (no combustion required).

Example: 10:1 adiabatic compression that starts at room temp will give you (roughly speaking) a 10:1 jump in temperature. So your "cold" gasses are suddenly HOT!

Or... you could get the oil filled gage with the peak pressure needle on the base of a tee. The water hammer effect can be rerouted by a straight line across the top of the tee and into a cushion

And while you're googling for adiabatic compression, you might want to look into something called "Pascal's Law." Basically it states that your "tee" idea won't work.

Actually I like the strain gauge idea. Calibrate with a set of weights.

Huh? While strain gages are neato toys that have obvious application here, I'm not sure how/where weights are going to help you calibrate them.

But, I guess, seriously, nobody is going to go that far on here. Probably the best bet is the gas eq program I just downloaded to give a fair idea of where its going to be.

Admittedly I'm the exception for obvious reasons, but as I type this, Vera has 36 strain gages and $5000 worth of pressure transducers installed (ie, very high quality). Note that I have the ability to record 100,000 data points per second. I *will* be getting that data.
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Unread postAuthor: jimmy101 » Tue Oct 27, 2009 4:45 pm

Actually I like the strain gauge idea. Calibrate with a set of weights.

To calibrate you would just measure the strain gauge's signal as a function of static pressurize. Obviously that means you need a way to seal the gun up at the expected maximum operating pressure, and a suitable source of compressed air. I believe you can pick up a piezo resistive strain gauge for a few tens of dollars, maybe even less. A PC soundcard and a battery (a voltage regulator would be nice) .. ta da, a pressure gauge. Always meant to give it a try...

D_Hall wrote:Admittedly I'm the exception for obvious reasons, but as I type this, Vera has 36 strain gages and $5000 worth of pressure transducers installed (ie, very high quality). Note that I have the ability to record 100,000 data points per second. I *will* be getting that data.

100,000 data points /second ... {monty python voice}that's easy{end monty} ... so you are just using a single PC soundcard? :P

I suppose you mean 100K sample/sec for each of the 36 strain gauges.
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Unread postAuthor: kenbo0422 » Tue Oct 27, 2009 4:47 pm

The strain gauges we used were made by us in our production facility. We used them to test 'high' pressure indentations into various materials. By placing a series of weights on it, the calibration would be made. It was placed into a machine that had its own strain gauge and the machine would then push on the inserted gauge apparatus. The machine would get its calibration through the comparison to the inserted gauge. High loading testing could then be done using the testing machine.

I figured that a known material, like a flat metal diaphragm, having a liquid and piston on one side, a strain gauge on the other, would be a way to test. The strain gauge bends with the diaphragm from a known pressure on the piston. Anyway...

What about the loops of tubing you see on high pressure systems that are connected to pressure gauges? What are those used for??
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Unread postAuthor: kenbo0422 » Tue Oct 27, 2009 4:51 pm

After reading Jimmy's post it occurred to me... Well, a question came to mind: How the heck do they measure the pressure in a real gun?? :shock:
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