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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: spudtyrrant » Tue Oct 27, 2009 4:59 pm

kenbo0422 wrote: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:
math, you can figure pressure using expansion rates
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Unread postAuthor: D_Hall » Tue Oct 27, 2009 5:13 pm

kenbo0422 wrote: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:

3 different ways...

1) Very expensive, very fast response pressure transducers.

2) Strain gages on the outside of the barrel. Note that this method is subject to barrel harmonics.

3) For cartridge loaded guns, sometimes very thin wires are wrapped around the brass. You shoot the gun and then you see how "mashed" the wires are. Based on the wire's deformation, you can make statements about the pressures in the chamber.
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Last edited by D_Hall on Tue Oct 27, 2009 8:58 pm, edited 1 time in total.
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Unread postAuthor: Technician1002 » Tue Oct 27, 2009 8:13 pm

For fuel air combustion, one of these would work fine.

http://www-mat.ee.tu-berlin.de/research/sic_sens/sic_sen3.htm
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Unread postAuthor: kenbo0422 » Wed Oct 28, 2009 5:14 am

Yep, that would be a perfect candidate 8) I especially liked the graph of the stroke at ignition: looks like a perfect parabola. The heating from compression was rather impressive. In a longer chamber, even with multiple sparks, a series of these would be interesting to graph just to see if a compression wave really does anything inside the tube that would be beneficial/detrimental to the cause... Might even give info on where to better place your ignition. :idea:
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Unread postAuthor: jimmy101 » Wed Oct 28, 2009 2:53 pm

Technician1002 wrote:For fuel air combustion, one of these would work fine.
http://www-mat.ee.tu-berlin.de/research/sic_sens/sic_sen3.htm

Yep, that's what I posted earlier, a piezo resistive element, usually wired up as a Wheatstone bridge.

The same type of sensor can be used both inside the chamber as a pressure gauge, or outside the chamber as a strain gauge, which can be calibrated to pressure.

Figure the sensor will be $100 and up.
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Unread postAuthor: btrettel » Wed Nov 04, 2009 1:16 pm

D_Hall, when using strain sensors, is something like a half or full bridge circuit adequate to remove the bending stress that would be problematic? To get only longitudinal stress multiple strain gauges can be used, one on the top and the other on the bottom of a beam. The strains will (or should) cancel each other out.

As for post processing, will determining the fundamental frequency and filtering out multiples of it be adequate? I can do that without too much difficulty. I could see this being problematic for stiffer barrels though.
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Unread postAuthor: Technician1002 » Wed Nov 04, 2009 3:54 pm

btrettel wrote:D_Hall, when using strain sensors, is something like a half or full bridge circuit adequate to remove the bending stress that would be problematic? To get only longitudinal stress multiple strain gauges can be used, one on the top and the other on the bottom of a beam. The strains will (or should) cancel each other out.

As for post processing, will determining the fundamental frequency and filtering out multiples of it be adequate? I can do that without too much difficulty. I could see this being problematic for stiffer barrels though.


In strain gauge scales, the common practice is to place the gauge on fulcrum points with the load supported on pivot beams so the only direction of force on the gauge is directly into and out of the gauge. In the case of a large object, the bias voltages are all wired in parallel and all the outputs are summed in a resistive adding network. I learned this servicing truck scales at a cement plant.

The scale has 6 sensors. Each sensor was a short rod with supports on both ends and one in the middle. The scale was tied down on one end so trucks stopping and starting could not shift the scale bed, but the truck weight was full rested on the 6 sensors. There is one in each corner and one on the middle of each side. The scale was used to measure sand, gravel, water, and cement to mix a batch in cement trucks. The loading of the recipe was automated and was set for the desired finished yardage.

One sensor went bad. I got to figure out which one. For that I used a small heavy load, a steam roller and weighed it in the 4 corners and on each side as well as the middle. The faulty sensor was quickly located.

I know it's too much info for this thread, but I figured someone may want to learn something about big scales.

The ones I serviced looked like these. They have two support points on one side and one on the other in the middle. They measured strain of bending the bar.

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Unread postAuthor: D_Hall » Wed Nov 04, 2009 6:46 pm

btrettel wrote:D_Hall, when using strain sensors, is something like a half or full bridge circuit adequate to remove the bending stress that would be problematic? To get only longitudinal stress multiple strain gauges can be used, one on the top and the other on the bottom of a beam. The strains will (or should) cancel each other out.

As for post processing, will determining the fundamental frequency and filtering out multiples of it be adequate? I can do that without too much difficulty. I could see this being problematic for stiffer barrels though.


Well, to be fair the few times I've been involved we've wanted both pressure and harmonic data so the barrel vibrations were seen as useful data, not something to be filtered out.

As for number of strain gages, we typically use full rosettes for each longitudinal location we're interested in: one on the top, one on the bottom, and one on the side. That allows us to see not just what pressure is doing, but what bending moments are doing and if there are vibrations in the horizontal axis as well.

How do we wire them? Beats me. I let my EE handle that side of the equation.
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