Pressure Instrumentation

[PeteS]

I'd like to be able to graph actual chamber pressure over time. Is there a reasonably easy and inexpensive way to do this?

[Gun Freak]

Pneumatic or combustion?

[Gippeto]

Would you care to better define "reasonably easy and inexpensive"?

Pressure transducers have come down in price (hugely so), but would still require electronics of some sort to tie them into a computer and then custom software written to make sense of the input.

How much can you do yourself?


Another route is strain gauges.

http://www.shootingsoftware.com/pressure.htm

[PeteS]

I have some wacky ideas for a two stage cannon that I won't go into right now that are combustion powered, but it is more complex than that. For practical purposes I think we can call it combustion.

Would I care to better define "reasonably easy and inexpensive"?
I am kind of a cheapskate so I probably don't want to spend hundreds of dollars. I am not too concerned about putting in quite a few hours of work.

How much can I do?
I am not too good with electronics, but am not afraid to solder. I spend some of my life as a programmer so I am reasonably proficient there. I do have and can use a mini lathe and mill, but am not an expert with them.

[dart guy]

You could try making a pressure guage fit in the tank and then take a vid of it.

[Gippeto]

If the goal is accurate data, I think you're looking at several hundred dollars at least.

Might be worth your while to check with Dewey or Poland Spud...they seem pretty handy with electronics and would likely be better able to advise you in that regard.

Perhaps an interface can be accomplished with some of the Arduino kits??

[PVC Arsenal 17]

I am sure there is a cheaper solution, but: http://www.ebay.com/itm/0-250-PSI-Press ... 0402277797

Or make your own version of that, there are numerous ways.

Your run-of-the-mill barometric pressure sensor won't do it.

[jackssmirkingrevenge]

I have some wacky ideas for a two stage cannon that I won't go into right now

Maybe you should, it might not be a new idea and others before you might have tried, so it would be good to learn from their experiences in that case

[PeteS]

I have some wacky ideas for a two stage cannon that I won't go into right now

Maybe you should, it might not be a new idea and others before you might have tried, so it would be good to learn from their experiences in that case

I might at some point before proceeding, but I want to mull it over a while first.

[jimmy101]

The challenge with any pressure sensor in a spud gun is response time and the overlaid acoustic signature. Sound happens in about the same time domain as does the pressure signal making the signal from a transducer a bit of a challenge.

A $1 piezo transducer could be used but calibrating it is pretty tricky. A piezo produces a voltage that is proportional to the rate of change of pressure so you need to integrate the signal. For qualitative work, of you just want to know the timing without need for the actual pressure, it might be OK. You can plug a piezo directly into the MIC input of a pc and record the signal directly. A voltage divider, or just a series resistor might be a good idea to keep from over-voltage into the MIC input.
http://www.inpharmix.com/jps/Piezo_Spud.html

You can try to use a standard dial pressure gauge but usually they (1) react much too slowly and (2) the rapid change in pressure may well trash the gauge even if the gauge's max pressure isn't exceeded.
See the sequence of images near the bottom of http://www.inpharmix.com/jps/Chamber_Temperature.html

You can use a $1 tire pressure pencil gauge but again the high rate of change of pressure makes the reading pretty inaccurate. In particular, the rapid pressure change gets the scale moving too fast, it then overshoots the actual peak pressure by a lot. In the image below there is a dial and a pencil gauge installed on a test combustion chamber.


In guns, the solutions it typically to use a strain gauge. They can be had for fairly cheap but will need a bit of electronics (but you can use the laptop's MIC input for the data logger and save some $). A strain gauge will need to be calibrated. A good page on strain gauges and guns is http://www.ktgunsmith.com/straingauge.htm

As an alternative you might consider measuring temperature instead of pressure since they are often related (though the relation might be pretty complex). A typical temperature sensor is much to slow, and requires much to much heat, to work inside a gun (combustion or pneumatic). But there are other ways to measure temperature, for example, the speed of sound is very sensitive to temperature (and pretty insensitive to pressure). So if you put a piezo inside the chamber, wire it into a laptop MIC input, record the sound of the gun operating, then extract the resonant frequency of the chamber as the gun goes off, you can extract out the average temperature in the chamber. For a gun you would also need to be able to correct for the increased length of the chamber as the ammo moves.
http://www.inpharmix.com/jps/Chamber_Temperature.html

A final way to measure the pressure might be to accurately measure the projectile position versus time (and the static and dynamic friction of the ammo to the barrel) as the gun fires. You could then calculate the pressure versus time data for the chamber.

[PeteS]

Interesting... I think maybe for my purposes temperature might be a good metric to use.

BTW, I think I might be satisfied with just peak pressure rather than pressure over time and figure that might simplify this whole thing quite a bit.

[PeteS]

It occurred to me that if I was satisfied with peak pressure only that having a small metering chamber connected with a check valve would be pretty accurate. It could have a gauge attached or just a schrader or presta valve. If using the valve a tire gauge would work up to the pressures that you can find one for. I have one that does 160 PSI and I think they probably make some for presta that go higher. The reading could be taken at your leisure after firing.

Am I missing any flaws with this idea?

[jimmy101]

It occurred to me that if I was satisfied with peak pressure only that having a small metering chamber connected with a check valve would be pretty accurate. It could have a gauge attached or just a schrader or presta valve. If using the valve a tire gauge would work up to the pressures that you can find one for. I have one that does 160 PSI and I think they probably make some for presta that go higher. The reading could be taken at your leisure after firing.

Am I missing any flaws with this idea?

Yes you are.

For a combustion the pressure drops very rapidly in a closed chamber. Figure within one or two seconds the pressure will have dropped back to atmospheric.

[PeteS]

It occurred to me that if I was satisfied with peak pressure only that having a small metering chamber connected with a check valve would be pretty accurate. It could have a gauge attached or just a schrader or presta valve. If using the valve a tire gauge would work up to the pressures that you can find one for. I have one that does 160 PSI and I think they probably make some for presta that go higher. The reading could be taken at your leisure after firing.

Am I missing any flaws with this idea?

Yes you are.

For a combustion the pressure drops very rapidly in a closed chamber. Figure within one or two seconds the pressure will have dropped back to atmospheric.

Due to temperature change? Or do you mean too quickly for the check valve to pass the pressure to the measuring chamber? Something else?

[ramses]

Due to temperature change? Or do you mean too quickly for the check valve to pass the pressure to the measuring chamber? Something else?

Temperature. My hybrid will actually pull down to a substantial vacuum after the piston valve closes from the cooling effects.