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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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hybrid peaks test.

Unread postAuthor: SpudFarm » Sun Jun 08, 2008 3:10 pm

i am planning on doing some peak readings on Larda up to 10x mix.

the question's i have is:

how do i get the pressure in a gauge but not out? there will be so little volume there that even the smallest loss of gasses will mess the readings up pretty bad.

will heat loss be a problem there?

how do i do this cheap?






do not ask about the progress on the hybrid since i will wait the posting out until 90% of you have forgotten it so it get replies. if i have posted it now all of you would have thought it was the progress thread and not care to open it.

remember the progress thread is dead of a reason
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Unread postAuthor: Carlman » Sun Jun 08, 2008 3:20 pm

check valveand gaugue or comp readouts via instraments
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Unread postAuthor: SpudFarm » Sun Jun 08, 2008 3:27 pm

a normal checkvalve would let to much air out and cost a huge amount of money for those pressures.

BTW Carl should let more time walk past on writing.
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Unread postAuthor: D_Hall » Sun Jun 08, 2008 3:52 pm

The smaller the system, the more significant heat loss is to it.
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Unread postAuthor: SpudFarm » Sun Jun 08, 2008 4:02 pm

so i would need a camera filming the gauge?
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Unread postAuthor: D_Hall » Sun Jun 08, 2008 4:18 pm

Realistically, when you said "cheap" I think you married yourself to crappy results. High speed events rarely lend themselves to inspection via cheap methods.

Were it me....

Do a number of tests using burst diaphrams, shear pins, or similar. Find a thickness/number/etc. where you can run 10 shots and have 5 diaphrams hold while 5 do not.

Now using a similar set up but simply an air pump, run 10 tests using the same diaphram thickness and record the pressure at which each diaphram fails.

Average the second set of numbers and you've got your answer.
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Unread postAuthor: jimmy101 » Sun Jun 08, 2008 5:01 pm

Like d_hall said, doing it on the cheap and doing it so it is accurate enough to be worth while are often mutually exclusive.

If you really want to do it on the cheap you could probably build a homemade tire pressure gauge. (Tire pressure gauges are peak pressure recorders.) Most tire pressure gauges only go up to about 100 PSIG which is what, an order of magnitude less than what you need? So, you would have to build one from scratch. Need a piece of tubing that'll take the pressure, a piston that seals well enough (but doesn't have to be absolutely perfect, just minimal gas loss in the time frame of a few tenths of a second), a stiff spring, a sliding scale as the recorder.

The gauge can be calibrated with a bathroom scale. Lets see, a 1/4" diameter piston at 1000 PSIG is 49.1 pounds of force. Well with the range of a bathroom scale.

Heat loss is only an issue to the extent that heat is lost within the time it takes the gauge to read since this is a peak recording gauge. A standard high pressure gauge would never work since the thermal half-life (hence the pressure half-life) is probably measured in tens of milliseconds for a small metal chamber. To record the motion of a standard gauge with a video camera you would probably need at least a couple hundred frames / second.

If you took a standard 100 psig tire pressure gauge and replaced the stock body and piston with a body and piston with 1/10th the area you could use the stock spring and sliding scale. Any ideas where you can get a ~0.1" diameter piston/cylinder assembly that'll handle 1000 PSIG? Perhaps a very small hydraulic cylinder?
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Unread postAuthor: Carlman » Sun Jun 08, 2008 11:31 pm

spudfarm wrote:BTW Carl should let more time walk past on writing.


Sorry man was PUI then :oops:
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Unread postAuthor: Killjoy » Mon Jun 09, 2008 12:45 am

Mcmaster has peak pressure gauges for a 100 bucks in you pressure range. I think this would be your best bet for accuracy and simplicity.
check page 566 under the general pressure gauges.
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Unread postAuthor: D_Hall » Mon Jun 09, 2008 9:31 am

Killjoy wrote:Mcmaster has peak pressure gauges for a 100 bucks in you pressure range. I think this would be your best bet for accuracy and simplicity.
check page 566 under the general pressure gauges.

Only problem is something like that wasn't designed for a highly dynamic environment. IE, results should be taken not with a grain of salt, but rather, with an entire salt lick.
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Unread postAuthor: SpudFarm » Mon Jun 09, 2008 9:43 am

hmm seems like peak pressures is harder to messure than a gauge and a check valve as i wanted :(
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Unread postAuthor: jimmy101 » Mon Jun 09, 2008 10:28 am

D_Hall wrote:
Killjoy wrote:Mcmaster has peak pressure gauges for a 100 bucks in you pressure range. I think this would be your best bet for accuracy and simplicity.
check page 566 under the general pressure gauges.

Only problem is something like that wasn't designed for a highly dynamic environment. IE, results should be taken not with a grain of salt, but rather, with an entire salt lick.

The gauge might be OK for closed chamber studies. The environment is dynamic but you may well have a few tens of milliseconds to get the peak reading before heat loss sets in to any great extent.

The fluid filled gauges might either help or hurt the accuracy of the reading. The fluid will dampen the needle's movement and make it much less likely to overshoot (which would hose the accuracy of the peak recording needle). On the other hand, the slow movement of the needle might keep the needle from reaching the maximum pressure reading before heat loss starts to affect the pressure.

spudfarm wrote:hmm seems like peak pressures is harder to messure than a gauge and a check valve as i wanted Sad

Yep. Which is why there is so little data on the pressure in combustion and hybrid guns. It isn't that nobody has thought of the idea. Instead, nobody has been able to come up with a simple and cheap way of doing it. It certainly can be done, gun and ammo manufacturers (engine manufacturers, rocket designers, ...) routinely measure chamber pressure versus time data in situations with much higher pressures and much shorter time domains. But they don't do it with a $10 (or $100) worth of equipment.
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Unread postAuthor: SpudFarm » Mon Jun 09, 2008 10:39 am

i really want to do some tests on peaks but it wont happend. :(

if i did get it to work i may have got HGDT more accurate to.
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Unread postAuthor: D_Hall » Mon Jun 09, 2008 6:19 pm

jimmy101 wrote:But they don't do it with a $10 (or $100) worth of equipment.

Just to emphasize this.... At the office? Yeah, this would be a trivial task. We even have a laptop computer that is set up to do high speed stuff like this.... That laptop? Is it something special? Oh, you bet. It has a pricetag of over $200k.
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Unread postAuthor: Lentamentalisk » Mon Jun 09, 2008 6:39 pm

the solution I came up with to do this, back before I realized how incredibly hard it would be is this:

Issue 1: the pressure peaks quickly, then declines
Solution: check valve with a pressure gauge

issue 2: The gases involved are very hot, and will cool down quickly
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.

Yes it is bulky as hell, and yes it probably won't work, but that is the only way under 200k that you can accurately do it.

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
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