Analog burst-disk

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Analog burst-disk

Postby Metiz » February 13th, 2013, 1:22 pm

For the past year I've been working on a 500 pound valveless pulse-jet engine. When completed, it should be the most powerful valveless pulse-jet ever.

If this engine runs it's going to be realy dangerous. Because of its shape, I need to add ejectors. Without them, it won't make the 500 pound mark. Because of these ejectors, the zone directly around the intakes is going to be extremely dangerous with pressure pulses that might be fatal, however, that's just speculation, I need to test this without putting myself at harm.

I was thinking about using burst disks. These things will burst if a certain pressure is reached but they're hard to find and expensive. Does anyone know how I can build some sort of analog?
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Re: Analog burst-disk

Postby leadacid » February 15th, 2013, 7:08 pm

Okay, so I am not a lawyer, but do be careful.

I guess I'm not sure what pressure you are attempting to measure. Open air pressure of the waves around the engine? Wouldn't a series of load cells around the engine also tell you the current pressure wave ... well, pressure? The rupture disk would only tell you that you reached a certain threshold, not necessarily what that was or is until it does break.

I've seen enough of these things used on MythBusters to know that it appears they use suspended PVC contraptions that look like they have an aluminum burst disk at the end. My thought is that you could build an equivalent using a consistent-density metal, perhaps aluminum foil or other sheet metal like that. I suspect that the variances in the metal would make it an unreliable tester though.

I see that eBay has a lot of industrial surplus for "rupture disk" and "rupture disc". Perhaps you can find a bulk lot?

http://en.wikipedia.org/wiki/Rupture_disc

Either way, good luck!
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Re: Analog burst-disk

Postby Metiz » February 16th, 2013, 9:53 am

Hey Leadacid,

I'm trying to measure pressure waves outside of the engine, up to about 1 metre from the the intake event horizon. I consider 400kPa lethal and, say, 300kPa "highly uncomfortable". Would this work with a loadcell? it would have to be airtight or it wouldn't record any load at all.
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Re: Analog burst-disk

Postby k-ww » February 16th, 2013, 10:56 am

Metiz: You need a absolute [as opposed to gauge] pressure sensor. Since atmospheric pressure [15PSI] is about 100kPa, one that measures 100PSI [about 700kPA] would do you nicely.

A typical one is: http://www.digikey.com/product-detail/e ... ND/3475899

You would need to build an amplifier circuit for it and calibrate the output signal.

[ hmm - guess what type of stuff I work with ;-) ]

what speed in measurements are you looking for [samples/sec & resolution]?
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Re: Analog burst-disk

Postby Metiz » February 16th, 2013, 5:59 pm

I'd like to keep this as simple as possible. I need 2 datapoints: injury and lethal. No need for exact data about the pressure realy. The engine will cycle maybe 10-15 times a second. If your solution is cheap and easy to do then I'm al game of course.

You suggest a 700kPa sensor, do you mean it goes up to 700? This sensor is calibrated for pressure >100kPa I asume, so if you turn it on outside it will just register pressure as zero. A digital scale won't measure the aircolomn above it either
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Re: Analog burst-disk

Postby k-ww » February 16th, 2013, 10:36 pm

an absolute sensor measures relative to zero presure [vacuum], so it would read 101.? kPa at atmospheric - a gauge one reads relative to local atmospheric pressure.
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Re: Analog burst-disk

Postby Metiz » February 17th, 2013, 9:17 am

yes I know, but either will work fine, as long as delta P =>400kPa
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Re: Analog burst-disk

Postby k-ww » February 17th, 2013, 10:22 am

I beg to differ - the gauge one is measuring relative to the local air pressure at the sensor, which will be disturbed by the pressure waves from the pulse jet. The absolute sensor is measuring relative to its internal pressure reference [vacuum], so you could place the 'raw' sensor wherever you wished to measure pressure. Gauge sensors are best used when one side is exposed to pressure in a container or pipe, and the other to 'quiet' ambient air [manofold vacuum gauge in a car]. I don't think that you could consider the air near your pulse jet 'quiet'.
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Re: Analog burst-disk

Postby Metiz » February 17th, 2013, 12:04 pm

I son't see the problem, unless I'm misunderstanding you. When using a gage, at ambient pressure, the meter will read zero. If 400kPa is fatal then the reader will indicate =>4 bar. If I use an absolute sensor, the fatal value will be 500kPa.

Of course the air is disturbed by the pressure pulses because a pressure pulse needs a medium to travel through. Pressure might be as low as 50kPa because of the air breathing by the engine, depending on where I'd place the sensor but I'm not interested in the low range of the pulses, only the high range. Reading 50kPa on an absolute sensor or "minus" 50 (under the zero mark) is of no relevance.
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Re: Analog burst-disk

Postby k-ww » February 17th, 2013, 1:02 pm

The problem as I see it is that you wish to measure pressures near the pulse jet in open air, as opposed to inside the jet. Near it's 'mouth' or exhaust. Air has a relativly 'low' impeadance to pressure differences, so that a gauge sensor in that situation will see the same pressure at both sides of the sensor, since it is masuring the difference between two points [call them the 'front' and 'rear' of the sensor. An absolute sensor has it's 'rear' pressure held at zero [vacuum].

Perhaps you supplying a block diagram of the pulse jet and where you wish to measure the pressures would help clear up our mutial misunderstanding. At least then I can reference my explainations to the diagram.
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