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PSI Calculations

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PSI Calculations

Unread postAuthor: cannon freak » Sun May 21, 2006 9:49 pm

How would you go about calculating the psi that is created in the chamber of a cannon for say propane, (C3H8), at a certain pressure and at a certain mixture. I would just like to know the equation or equations for it, I know a far amount about chemistry so Il probably be able to understand it.
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Unread postAuthor: Andrew52 » Sun May 21, 2006 9:56 pm

in like equation? I think it would be inacurate although if you used a check valve and a gauge that would work
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Unread postAuthor: sgort87 » Sun May 21, 2006 10:41 pm

It has been shown using electronic pressure sensors that the standard peak combustion pressure is about 40 psi. The problem with equations is that they are too in depth. There are just too many variables in the way the cannon is built to end up correct.

The problem with a check valve and guage is that not all the pressure stays in the check. After the peak pressure is hit the pressure is still allowed to flow out. The only good way to measure this is with an electronic pressure sensor that will record the pressure curve or at least the maximum pressure.
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Unread postAuthor: boilingleadbath » Sun May 21, 2006 11:06 pm

Yes, let's see here:
Calculate the heat of combustion
Thus calculate, using the average specific heat capacity of the gasses, the temperature rise.
Apply the combined gas law.
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Unread postAuthor: sgort87 » Sun May 21, 2006 11:29 pm

Then account for the ammount of spark points, the way the flame front is allowed to travel, chamber diameter and length, any turns in the chamber and barrel, the choke between the chamber and barrel, the ammount of friction between the projectile and barrel, the mass of the projectile, need I go on? There are just too many variables to get an accurate prediction.
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Unread postAuthor: CS » Mon May 22, 2006 12:27 am

I actually had a idea on how to physically measure the pressure within a given launcher. Basically as stated above the problem with a check valve and pressure gauge is that the gases will simply cool and pressure will be lost via heat, rather then "leakage".

So why not place a piston between the chamber and check vavle? This would compress the air on the other side of the piston then push threw check, and should remain at same pressure since pressure wasnt created by heat, rather compression.

If you know me Im not really high up on the "smarts" just use a rather refined sense of logic and common sense. So first off Im not sure if the above method would create the same pressure inside the combustion.

So if one were so inclined, one could simply pump air into the combustion chamber (which obviously needs to be sealed) to say ~40PSI and see what it reads, now if it reads something besides ~40PSI the method still isnt worthless. One could simply variate pressure within chamber to match the display on gauge, once reached the pressure of chamber would match that of the combustion. Rather simple, but appears complex due to my lack of "explainibilty".
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Unread postAuthor: Pete Zaria » Mon May 22, 2006 12:50 am

Just a thought... I have a tire gauge (round gauge style, not the pen style) that you just push onto the tire's schrader, and the needle stays at the highest pressure until you press the release button on the side of it. These gauges are $12 at the local auto parts store. Couldn't you modify one right into the chamber? It has a built-in check valve and pressure release...

I think I'll try it.

Peace,
Pete Zaria.
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Unread postAuthor: boilingleadbath » Mon May 22, 2006 4:01 pm

Sgort, if you are calculating chamber pressure for fuel camparison purposes only, none of those variables need to be accounted for.
Yes, the calculated pressure won't be the acctual force felt by the projectile, but meh.
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Unread postAuthor: sgort87 » Tue May 23, 2006 1:32 am

Tony and Pete, sorry but those wont work either. The piston will be too slow to catch the pressure and the schrader will not have the right amount of flow for even a relatively accurate reading.

BLB I was not aware that fuel comparisons were being performed by him.
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Unread postAuthor: Pete Zaria » Tue May 23, 2006 1:39 am

I was talking about modifying one of the gauges I mentioned above to tap right into the chamber, no schrader. The gauge is $12 and does the job perfectly. I'll buy one and install it on my combustion and let you know how it goes.

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Pete Zaria.
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Unread postAuthor: sgort87 » Tue May 23, 2006 2:27 am

Just having a guage on your chamber will do nothing. I garantee you will not see it do anything. (Yes, I have tried it. :( )
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Unread postAuthor: Benny » Tue May 23, 2006 5:23 am

so only the check valve thing works on hybrids?
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Unread postAuthor: sgort87 » Tue May 23, 2006 10:29 am

It's not even very accurate on those, but I think they are closer percentage-wise on the correct reading.
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Unread postAuthor: Benny » Tue May 23, 2006 5:06 pm

ok, thanks. :D
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