# burn rate measurement method?

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Has anyone tried casting propellants into drinking straws to measure burn rates? It seems like that might be a good way to make consistently sized samples for burn rate measurements.

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That will give you a "simple" measurement of linear burn rate, but may not have much relevance to the actual burn rate inside a combustion chamber as most propellants have burn rates that vary with pressure. The best way to measure the actual burn rate is to cast either a pair of thin wires or optical fibers in the propellant and burn it in a combustion chamber under pressure. You know the distance between the two elements, and measure the time between when the first one breaks and the second one breaks. You can "implant" multiple elements in a single grain, across the radius or down the length. This is an inexpensive method and can be quite accurate. It's a standard methodology for measuring burn rates in propellants and explosives.

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Thanks Dan, that sounds like a good method. Do you know where I can find information about creating a timing circuit to make these measurements?

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What sort of accuracy is required? If within a couple of microseconds, the easiest way would be with a PIC processor.

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The DATAQ starter units will do digital measurements for about \$25. One could connect a 5v source to each digital input via a different element wire. When the wire breaks the signal drops to zero. The DATAQ starter units will time to 0.005 seconds or better (high-speed add-on will time to about 70 microseconds!)

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Terry, Can you recommend a DATAQ model number? They have so many choices it is hard to figure out the best one for this job. Thanks, Tom

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Depending on how small a measurement you want to make, the circuit can be quite simple. You can also us a PC to make the measurement (and start the burn). If you don't want to measure smaller than 1 millisecond (1/1000 second), you can buy oscillator modules where you just hook them up to power and they start running. The circuit requirements are: a time base (the oscillator), a counter (or counter circuit), and the start/stop logic.

As I mentioned, using a PC (an old laptop would work well), you can use the parallel port to start the igniter and measure the differences between the burn-through times. You will want to use DOS, as Windows is not a real-time operating system. Typically, a PC's clock will only measure down to 55 milliseconds (55/1000 sec). There are routines that will let you get millisecond resolution. The one I'm most familiar with was written by a guy names Robert Graves at the Univ. of Victoria in British Columbia. Can't find a google reference to him (the article was written about 1995 or so), but did find other references to millisecond timing on the PC.

If you don't know enough to design the circuit (it's very straightforward), perhaps you can get someone at the sci.electronics.design newsgroup to help. I don't have any way to post a circuit diagram or pictures. There are lots of references on the web for interfacing to a PC parallel port for measuring things like switch closure and turning things on/off.

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The DI-194RS is a serial port unit. I've got one of these, but no longer have a laptop with a serial port. It costs \$25. see:

DI-148U is USB and costs \$50.