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hybrid peaks test.

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: rp181 » Mon Jun 09, 2008 7:34 pm

Ever heard of pressure transducers? its pretty much a digital gauge that puts out a electrical signal, with the current or voltage varrying depending on how high the pressure is. Ive seen ones designed for high tempetures, and accurate ones, but im not sure what the response time is. To get around the tempeture problem, you could have a piston with one side to the hybrid, the other COMPLEATLY filled with hydrulic fluid, with the gauge in that chamber.

I looked at the shooting thing, you could just buy the strain guage, and find someone that has some programming and electrical knowlege to make a simple program, the program just detects resistance changes.
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Unread postAuthor: D_Hall » Tue Jun 10, 2008 8:23 am

Lentamentalisk wrote:Solution: rather than moving the hot air from the barrel through the check valve, you put a piston in between, so the hot air pushes the piston, which in turn compresses your cool gases, behind a check valve.

Once again dynamics kill you. In this case, you're probably going to get readings that are artifically high as you've essentiall built the pump stage of a 2-stage light gas gun.

oh ya, there is also this stress gauge that they use for rifles, lemi find the link real quick
Check this out, its fairly cheap (comparatively) and with it you can measure the pressure very accurately and quickly
http://www.shootingsoftware.com/pressure.htm

Totally adequate assuming you know how to calibrate 'em and such. Problem is that's not always trivial. Also it's very easy to pick up barrel harmonics using strain gages. If you DO pick 'em up, that too can affect the accuracy of your results.
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Last edited by D_Hall on Tue Jun 10, 2008 8:39 am, edited 1 time in total.
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Unread postAuthor: D_Hall » Tue Jun 10, 2008 8:26 am

rp181 wrote:Ever heard of pressure transducers?

High accuracy, high frequency response pressure transducers tend to have four digit price tags. And then there's the issue of recording their output at sufficiently high data rates to actually make use of their output....and that's even more expensive than the gauge.

I looked at the shooting thing, you could just buy the strain guage, and find someone that has some programming and electrical knowlege to make a simple program, the program just detects resistance changes.

LOL.... You are aware that the resistance changes involved are *MINUTE*, right? You'll need a precision bridge circuit and some good low noise amplifiers. Probably the cheapest way to attack this problem, but still not cheap in the sense that the OP means cheap.
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Unread postAuthor: SpudFarm » Tue Jun 10, 2008 8:54 am

Larda saw this thread and sent me link to a digital gauge that recorded 5000 freakin pressure drops/rises a second!

the bad thing was the $1176 tag
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Unread postAuthor: Mitchza89 » Tue Jun 10, 2008 9:02 am

Just a quick idea that just ran through my head. If you have a digital camera that can record video in a good quality, set it up so it's looking at the guage and record. Do the test and then stop the camera. Play back the video on your computer and slow it right down. You should be able to see what the guage is roughly pointing at during the peak.

That's about the only way I can really think of...

Best of luck mate.
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Unread postAuthor: SpudFarm » Tue Jun 10, 2008 9:05 am

i said that earlier and i got a response "you need atleast a few 100 frames per second"
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Unread postAuthor: Mitchza89 » Tue Jun 10, 2008 9:08 am

Oh sorry mate. I read all the posts but I must have missed that. The latest digital camera have quite a good resolution with a very high frame rate. What sort of camera do you have?
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Unread postAuthor: SpudFarm » Tue Jun 10, 2008 9:09 am

i have a really bad digital cam.. :oops:

7.2megapixels and 25fps
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Unread postAuthor: Mitchza89 » Tue Jun 10, 2008 9:12 am

25? Hmmm that might pose to be a bit of a problem... That's really surprising. My mum has an 18 month old Cannon camera that's got quite a decent frame rate. I can usually see the frames where the ammo has left the barrel, hanged in the air and then hit the target...

I'd honestly give you're camera a go. It couldn't hurt and won't cost ya anything. Dispite it having a crappy frame rate, one frame might capture the peak just at the right moment. Something similar happened to me :wink: .
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Unread postAuthor: D_Hall » Tue Jun 10, 2008 9:18 am

Mitchza89 wrote:25? Hmmm that might pose to be a bit of a problem... That's really surprising. My mum has an 18 month old Cannon camera that's got quite a decent frame rate. I can usually see the frames where the ammo has left the barrel, hanged in the air and then hit the target...

Never confuse frame rate and shutter speed.

I'd honestly give you're camera a go. It couldn't hurt and won't cost ya anything. Dispite it having a crappy frame rate, one frame might capture the peak just at the right moment. Something similar happened to me :wink: .
If you were looking at a mechanical gauge, you were very lucky indeed if that number really means anything.
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Unread postAuthor: rp181 » Tue Jun 10, 2008 1:44 pm

All in all, thers no cheap way of doing it :wink:
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Unread postAuthor: SpudFarm » Tue Jun 10, 2008 1:49 pm

i got a nice idea! you know those gauges that goes up and bring another needle with them to show how high it went?

that is the selution. a small chamber (if not Larda) and a meter (wich will be the meter i use) and a pressure gauge with two needles.
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Unread postAuthor: Lentamentalisk » Tue Jun 10, 2008 4:59 pm

spudfarm wrote:i got a nice idea! you know those gauges that goes up and bring another needle with them to show how high it went?

that is the selution. a small chamber (if not Larda) and a meter (wich will be the meter i use) and a pressure gauge with two needles.


D_Hall wrote:If you were looking at a mechanical gauge, you were very lucky indeed if that number really means anything.


Like he said, a mechanical gauge probably will not move in that very short time, and if it does, it will have momentum, and read something absolutely wrong.
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Unread postAuthor: jimmy101 » Tue Jun 10, 2008 7:02 pm

D_Hall wrote:Never confuse frame rate and shutter speed.

Yep, and the thing that matters is the shutter speed, not the frame rate. At least if you are trying to capture a transient event.

If you very brightly illuminate the scene the camera will crank down the apeture and shutter speed. The 30 FPS video then becomes a frame every 1/30 second but the actual time duration of the image is much less. Figure the fastest shutter is 1/1000 second for a generic camera.

Get a six-pack and a couple friends and video the gauge on many different gun firings. Eventually you'll luck out and get a frame that was exposed at the instant the gauge had max'd out. You should be able to tell that the guage is actually maxed out based on how bad the image of the needle is blurred. At the peak pressure the gauge stops moving briefly giving a clear image.

Now the only question is how accurate is the gauge in measuring a transient pressure? It almost scertainly was not designed for this type of measurement. Did the needle undershoot or overshoot or is it reasonably accurate?
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Unread postAuthor: jimmy101 » Tue Jun 10, 2008 7:08 pm

D_Hall wrote:And then there's the issue of recording their output at sufficiently high data rates to actually make use of their output....and that's even more expensive than the gauge.

Naaah. Dig up a retired PC, like an old Win98 box. You now have all the hardware, and most of the software, to record the transducer output at more than adequate sample rates. Two channels, 16-bit resolution per channel, ~44,000 samples/second.

Of course, the PC soundcard's inputs are all DC uncoupled, so you might have to get creative with a little signal conditioning ... AM, FM, PWM, ... if you want to record events that are of longer duration than typical sounds.
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