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Hybrid test results.

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: chartreusesnot » Mon Sep 17, 2007 11:42 pm

if you put a gauge behind a check valve, then you might be able to get the peak pressure with out having to spend all that much money on a fancy electronic pressure gauge with peak pressure setting things.
I am not sure if that is what ^^^^he^^^ just said, but oh well.
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Unread postAuthor: dongfang » Tue Sep 18, 2007 4:29 pm

Hi,

I´m looking into those electronic pressure gauges now. A complete gauge is really costly, but has anyone seem those sensor components at a reasonable price? I think the circuitry to go after them could be made quite simply - something like a sound card oscilloscope, that it calibrated against an ordinary gauge first. Many of those gauges have a quite acceptable 5% error margin.

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Unread postAuthor: jimmy101 » Wed Sep 19, 2007 12:35 pm

Dongfang:

I've been looking for suitable cheap pressure transducers for a couple years and haven't had much luck finding anything.

You can get piezo resistive transducers, good for up to ~100 PSI, for $20. I have a 0-99 PSIG talking tire pressure gauge that I got from RadioShack for $2.

For pressures greater than 100 PSI, and for measuring short duration events (tens of milliseconds long) the transducers start at ~$100 and go into the thousands of dollars.

I've experimented with just using a piezo transducer ($2 from RadioShack) and a PC sound card as the data logging system. Unfortunately, the frequency response of a piezo crystal gives a really confusing signal. A EE-PE tells me it would be next to impossible to extract the pressure versus time data without spending a lot of effort determining the exact frequency response characteristics of the piezo element.

I'm beginning to think the best way to do it (assuming cost is a significant issue) is via some kind of a mechanical transducer. Perhaps along the lines of a cheapo tire pressure gauge. You could let the scale of the gauge cast a shadow on a solar cell to convert the position of the scale into an electrical signal the soundcard could record.
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Unread postAuthor: dongfang » Thu Sep 20, 2007 4:07 am

Hi Jimmy

Thanks for a very informative answer. Good research.

In spudding, we abuse materials and all beyond its specs and purposes anyway -- I wonder if those $20 100 psi transducers can be, hmm, pressed into going up to 150? But Ok, there is still the response time problem.

What I would love to see was a pressure / time graph right before and right after a piston valve, and some place along the barrel (ideally, in the projectile). In fact, a friend of mine showed me some cheap but still quite good tiny accelerometers he had. The problem with them is that they only go to 500 g.. could still be fun to launch one of those, with a data logger connected.

I would suspect just as complex frequency characteristics of the pressure gauge readout thing you suggest..

Do you have a link to the manufacturer / a dealer of those $20 transducers? I think I can afford that just fine, and I might try play a little with them.

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Unread postAuthor: jimmy101 » Thu Sep 20, 2007 11:47 am

Dongfang:

For the ~100 PSIG transducers I'm not so worried about over pressurizing them as I am that they won't read above 100PSIG. Kind of defeats the purpose if you peg the sensor. But, I think you are right in that it really should be tried. Besides, an operating combustion gun really shouldn't get to 100 PSIG and a compressed air gun could be operated at 100 PSIG just to get data. I would think the piezo resistive transducer would respond pretty quick with flat frequency response.

Mouser Electronics:
http://www.mouser.com/catalog/631/1632.pdf
Several 0-100 PSI models with prices ranging from $24 to $94.

Digikey:
http://dkc3.digikey.com/PDF/T073/P2239.pdf
part number 287-1074-ND for $35 (and others on the same page)
http://dkc3.digikey.com/PDF/T073/P2240.pdf
part number 480-1927-ND for $24 (and others on the same page)
http://search.digikey.com/scripts/DkSea ... 100G-3L-ND
non stock for $19.20

The talking RadioShack gage would be a great thing to rip apart for a sensor since the whole thing only costs $2 and goes to 100 PSIG. Unfortunately, I don't think RadioShack sells them anymore. I also don't know what kind of sensor it is. No part number or anything on the part of the device I can see.
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In terms of accelerometers, I believe the Water Rocket hobbyists have onboard systems that will record pretty high accelerations. G's in the range that a spud gun gets to. Might be worth looking into and seeing if there are any designs that are affordable.
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Unread postAuthor: CpTn_lAw » Thu Sep 27, 2007 2:02 am

This topic is very interesting...I did trust steel but now i'm even more safe...
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Unread postAuthor: ALIHISGREAT » Sat Dec 08, 2007 9:16 am

Hmmm the black gunk you found in your chamber could be soot, this would be as a result of incomplete combustion.

when an alkane (propane, butane etc.) burns it turns into carbon dioxide and water, but if not enough oxygen is present then it makes carbon monoxide and water and if even less oxygen is present it makes carbon aswell. An example of this is bunsen burners in chemistry labs, when you close the air hole there is not enough oxygen and the flame turns yellow. this is because of incomplete combustion and the yellow colour of the flame is very hot carbon glowing. then when the hole is opened again there is no yellow colour in the flame because all the carbon is used up making co2 and water and if you look at the ceiling in chemstry labs they usually have darker spots above where the bunsens have been used which is the excess carbon from incomplete combustion.

so maybe you fuel measuremet was off?
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Unread postAuthor: Fnord » Sat Dec 08, 2007 9:46 am

Based on your post, I think I figured this out.
The fumes from the pipe dope may have been igniting when I started testing, and causing the mix to be a little too rich. I imagine it would have burned off eventually, so I don't think it affected the final tests.

I'm fairly sure my fuel measurements were accurate (within 5-10%, anyway).
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Unread postAuthor: ALIHISGREAT » Sat Dec 08, 2007 11:52 am

_Fnord wrote:Based on your post, I think I figured this out.
The fumes from the pipe dope may have been igniting when I started testing, and causing the mix to be a little too rich. I imagine it would have burned off eventually, so I don't think it affected the final tests.

I'm fairly sure my fuel measurements were accurate (within 5-10%, anyway).


ahh cool glad to help :) who would have thought listening in chemistry was useful :lol: (ok maybe i only listen becuase we are doing about oil and its fractions, specifically alkanes at the moment which are useful in spudding)
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Unread postAuthor: SpudFarm » Sun Dec 09, 2007 10:47 am

i dident read all the replies but is there a seam on that chamber?
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Unread postAuthor: Fnord » Sun Dec 09, 2007 7:46 pm

Yes. It's barely noticeable though.
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Unread postAuthor: jimmy101 » Tue Dec 11, 2007 1:57 pm

I wonder if the black gunk is because the fuel ratio is way off? Not because of an error in the math but because of the thermodynamics of the pressurization step.

It is actually pretty tricky to accurately pressurize a chamber to this kind of pressure when what you really want is a particular compression ratio.

When you compress air it heats up. GasEq says compressing a sample of air to 11 ATM heats the air up to 589K (600F). But you don't have anywhere near 11x as much air in the chamber like you expect, the rising temp also raises the pressure and less air is pumped into the chamber. GasEq puts the air at about 5.6 as much as at atmospheric pressure, the pressure is 11x higher but there isn't anywhere near that much air in the chamber since a big part of the pressure rise is due to heating. If you let the chamber sit a while it (and the contained gases) cool back off to ambient temperature and the pressure will drop. Lets see; T2/T1=P2/P1 at constant Volume. T1=589K, T2=300K, therefore P2/P1~1/2. So the pressure drops from 11 ATM to about 5.5 ATM and you only have half as much air in the chamber as you thought you did. To get to a true "11x" mix you then need to re-pressurize the system to 11 ATM, and continue to fill/cool/fill/cool until you are no longer adding any air to the chamber when it is equilibrated to ambient temperature.

So, it makes a difference how (and when) you fill the chamber. If the compressor head is plumbed directly to the chamber then the air is getting heated as it is compressed, as described in the paragraph above.

If you are filling from a reservoir at >11x, and the reservoir and contained gas is at ambient temperature, then when you fill the chamber the temperature of the gas drops. Now, if you let the chamber sit it'll warm up the gases and the pressure in the chamber will rise. You end up with more air in the chamber than you thought you had.

Either way, the fuel ratio is going to be off. You'll have either too much or too little air in the chamber for the fuel load.

Too much fuel in the chamber will produce the black gunk, basically it is soot with the water created by the combustion mixed in.

Of course, if your air / fuel ratio is off, then the peak pressure in the chamber is going to be less than you expect. It is possible that you aren't getting peak pressures anywhere near what you expect. GasEq puts the peak pressure at 1274 PSIA for an 11x stoichiometric mix. If the amount of oxygen is low by a factor of two (see the T2/T1=P2/P1 discussion above) then the peak pressure, according to GasEq, is 880 PSIA. Roughly 70% of what is expected.

(This entire discussion also applies to pneumatic guns. When someone says they pressurized their gun to 120 PSIG it makes a difference when and how the chamber was pressurized!

This also explains why, when you fire up a shop compressor, it will run for a few minutes, then shut off, then cycle back on again several minutes later after the compressed air in the storage tank has cooled off.)
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Unread postAuthor: ALIHISGREAT » Tue Dec 11, 2007 2:09 pm

i already commented on how the black gunk was because of incomplete combustion because his fuel was off... and then Fnord said it was probably some fumes from pipe dope that were saturating the chamber...
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Unread postAuthor: Fnord » Tue Dec 11, 2007 2:53 pm

I was filling from a 30 gallon compressor tank, which was room temp at the time. I let the chamber cool down to the point where it was just warm after each shot, which would have probably helped correct the cooling effect.

I'd like to try the test again to clear this up, but it's too cold outside. It would probably throw off my calculations in some way, and the water and mud don't have a good effect on my HV circuit.
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Unread postAuthor: jimmy101 » Tue Dec 11, 2007 3:38 pm

ALIHISGREAT wrote:i already commented on how the black gunk was because of incomplete combustion because his fuel was off... and then Fnord said it was probably some fumes from pipe dope that were saturating the chamber...

I doubt there is enough fumes in the chamber to leave noticeable residue. Besides, after the first shot or so the fumes should have all been burned, or flushed out of the chamber when the gun was flushed with fresh air.
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