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help with electronic propane metering system(onboard)

Post questions and info about combustion (flammable vapor) powered cannons here. This includes discussion about fuels, ratios, ignition systems, safety, and anything else relevant.
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help with electronic propane metering system(onboard)

Unread postAuthor: geniusbomberman » Wed Jul 29, 2009 3:57 pm

first of all, My idea would work better if I could find a digital(computer or other digital output) pressure gauge, but I haven't found a source for under 400 dollars. also, would some of the combustion from a propane cannon go partway into the meter pipe? and if the valves on the meter pipe were sprinkler valves, would the momentary combustion damage them? would the be sufficient valves for a metering system? Thanks in advance.
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Unread postAuthor: jrrdw » Wed Jul 29, 2009 4:09 pm

I think it was sgort87 that entered a combustion cannon in the 1st contest that had on board eletronic metering.

No, sprinkler valves are not good enough for metering, use brass ball valves.

Edit, the home page has been changed there for the 1st contest can't be found there. I will post a link as soon as I find it.

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Last edited by jrrdw on Wed Jul 29, 2009 4:33 pm, edited 1 time in total.
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Unread postAuthor: TurboSuper » Wed Jul 29, 2009 4:22 pm

Look up "pressure transducer" on www.digikey.com. They're much cheaper, albeit they'd likely need proper instrumentation circuitry around them.
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Unread postAuthor: jrrdw » Wed Jul 29, 2009 4:55 pm

OK, found sgort87's entry from the 1st contest. Not exactly what your thinking of (I think), but close. Here is the the link.
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Unread postAuthor: jimmy101 » Wed Jul 29, 2009 5:00 pm

You can get electronic pressure sensors for pretty cheap, certainly less than $20. The only time the pressure sensors start to get real expensive is whan you get to greater than about 100 PSI and need fast response times, say down in the millisecond or less range.

You would need to build the needed circuitry if you want a digital output instead of a continous voltage. That could be done with a cheapo DVM chip as long as you don't need to take readings all that fast.

Assuming this is for a meter and the max pressure is say 100 PSI or so and you only need a response time of less than 1 second, and you want some kind of a digital output that can be fed into a control circuit ...

Get a cheapo tire pressure gauge and hack it. If you are using a single chip computer for the controller then hook the LCD display of the gauge up to the input ports of the computer. Convert the 7-segment pattern back into real numbers in software.

I got this tire pressure gauge at RadioShack for something like $2.
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Works OK, even talks (in English and Spanish).

Harbor Freight has a "regulator" with a digital readout for $12.
Part number 98426-1VGA,
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Rip the case open and connect the display lines to your uP. It isn't a real regulator, just a flow regulator.

EDIT:
Did a quick google search trying to find a circuit to do the 7-segment decode. It is almost certainly out there somewhere but I can't figure out the correct query. Can't use "decode 7-segment" because the standard driver for a 7-segment display is already called a "7-segment decoder".

Here's a great question for a CS class: "How many of the 7-segments do you actually need to decode the display? Do you really need to monitor all 7 segments or is there sufficient info in a just a few of the segments? It would be trivial to do the decoding with a PLU but you would still like to monitor as few segments as possible.

How many logic gates would it take to do a 7-segment to binary conversion in discreete TTL? Is there a trick that can done with a mux/demux? Hmmmm, a 4-bit mux and no more than 4 AND gates should be able to do the 1:10 decoding and activate one of ten output lines. That's not exactly digital but getting close. That would be for a single digit of the display.
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