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Change PSI at lower Tempratures... Use the Ideal Gas Law?

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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Change PSI at lower Tempratures... Use the Ideal Gas Law?

Unread postAuthor: bigbadbrad » Tue Jan 22, 2008 3:03 pm

In our second generation cannon we decided to convert from Axe Body Spray to a Propane fuel. We figured a better, cleaner burning, fuel would yield us greater distances. And so far we’ve had nothing but success!! We added another barrel to the existing blast chamber (to make it a .85 ratio), added a chamber fan, updated the electrical system (of our own design) and made the necessary modifications to take the propane gas. At 74°F we can get it to fire perfectly every time.

Right now we have a bit of a problem. In southeast Texas the weather here tends to change quickly and frequently. In the afternoon on the ranch we can be comfortable at 74°F but 5 hours later we’ll be down to 45°F. When the temperature drops the starting PSI on our propane tank drops as well. And the 69 PSI previously required for optimal firing of our cannon no longer works in our blast chamber. We know we need to adjust the amount of propane in our blast chamber to compensate for the temperature change but the problem is we don’t know by how much. And we are hesitant to just start experimenting for fear of blowing ourselves up. Is there a formula we should use?

We’ve been trying to use the Ideal Gas Law (PV=nRT) but frankly none of us are math or chemistry majors. Most of us went the liberal arts route. Can you guys help?

Our gun specs are as follows.

Blast Chamber
Diameter 4”
Length 41.5”

Barrel
Diameter 3”
Length 86.8”

At 74°F it works like a champ.

THESE PSI MEASUREMENTS ARE AT THE PROPANE TANK NOT AT THE BLAST CHAMBER. These are just to help us with a starting point to develop a custom spreadsheet for temperature adjustment. If we keep the mole constant we're seeing an unusable PSI come out of the equation (when we tested the cannon with the PSI using a constant mole in teh equation and just adjusting temprature it does not result in a successful firing). I ran the equations for you to see the output with all the values for the 2 time periods we're working from. At 74°F the PSI on the propane tank read 100PSI. At 45°F (5 hours later) it read 80PSI and there had not been any propane removed and no leaks. When the temperature returned to 74° the following day the PSI again read 100. We need to know how to adjust the PSI so we get the same result (explosive force) at 45°F as we get at 74°F. We know we have to adjust the PSI for temperature. But by how much? We do not want to blow ourselves up. That's the goal.


PV=nRT

100PSI at 74°
P = 6.8046 atmosphere

V = 36.1978 liter

T = 296.33 kelvin

n = 10.1292 mole

R = 0.08206

80PSI at 45°
P = 5.44368 atmosphere

V = 36.1978 liter

T = 280.222 kelvin

n = 8.5692 mole

R = 0.08206
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Unread postAuthor: Hubb » Tue Jan 22, 2008 3:14 pm

This works, too.

But, with knowledge like this, your math is just fine here. Welcome to Spudfiles.
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Unread postAuthor: bigbadbrad » Tue Jan 22, 2008 4:08 pm

right. I have a meter and an automoated spreadsheet with the formulas that calculate the PSI based on meter volume and blast chamber volume. And if the temprature is between 65 and 90 we're fine. but lower than 65 and the PSI in the tank starts to drop. and thus we need a higher PSI in the chamber to get it to fire.

http://www.mrduffy.com/hpc/HPCtool2.0.xls
The fuel meter is on the second sheet if you want to take a look.

On that other post none of that takes into consideration the temprature of the propane.

As the temprature decreases the PSI needed to fire the cannon increases. but at what rate? safety is the biggest concern with a 10 foot long cannon with a 3" barrel.
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Unread postAuthor: Ragnarok » Tue Jan 22, 2008 4:17 pm

Actually, you shouldn't need to do much maths at all.
Not only is the air in the chamber denser, but so is the propane you are injecting.

But, as you seem to be having problems with the pressure dropping too low, halve the regulator pressure and inject two half lots of propane.
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Does that thing kinda look like a big cat to you?
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Unread postAuthor: bigbadbrad » Tue Jan 22, 2008 5:20 pm

I can get 69 PSI in the chamber no problem. even if I charge the meter to 80, release half of the 69 PSI, charge the meter again to 80 and release the other half its still the same problem. it won’t fire. The problem isn't getting the PSI into the chamber.

the problem is that when a gas condenses you'll need less of it.

we figured it out. and we made a tool that calculates for the change in temprature.

http://www.mrduffy.com/hpc/
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