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20psi metered propane

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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20psi metered propane

Unread postAuthor: MortarArmy » Fri Sep 28, 2007 6:31 pm

Ive got a 9" long x .5" propane meter on my current gun. Using standard propane tanks, im getting 20psi of pressure in there, which is fine for my current design, gives me a near perfect 4% fuel mixture, but my question is, is my pressure gauge correct? with the valves fully open, the most i can get out of it is 20psi, it hits 20 and just stops, nothing more in there.

what im worried about is, what happens when i want to make a bigger gun. If i lengthen the pipe, will it remain 20psi? and if so, what can i do to get more pressure into it? I dont want this huge propane meter.
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Unread postAuthor: Pete Zaria » Fri Sep 28, 2007 6:36 pm

Your gauge is probably incorrect.

I'd like to see pictures (or just a more detailed explanation) of your setup, but assuming your meter pipe and gauge are connected directly to the propane tank (or through a torch needle valve), you should be seeing 60-90 psi depending on temperature.

Are you positive the tank is full?

Another possibility is a leak preventing more than 20psi from building up in your meter.

Show me a pic, or give me a more detailed explanation of your setup.

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Pete Zaria.
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Unread postAuthor: MortarArmy » Fri Sep 28, 2007 6:50 pm

you know what...now that im thinking about it, i think i know what my problem is. I used a propane valve from a camp stove to connect to the tank, i bet that thing is regulated.

but anyways

heres my setup - tank - valve - 1/8th npt fitting to bushing to 1/2" barb to pressure rated hose, to 1/2" barb to ball valve to pipe to t-fitting with gauge to pipe to ball valve to barb and then another peice of hose connecting to a barb screwed into the chamber.

its gotta be that propane tank valve i used, its gotta be a regulator. Which really sucks cause it was the only thing i could find that had a propane fitting and a threaded connector that i could get something out of.
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Unread postAuthor: Pete Zaria » Fri Sep 28, 2007 7:00 pm

Yep, I think you've found your problem.

I just hacksaw the tip off a propane torch and hose clamp a hose directly onto it. Works fine. A cheap propane torch should be less than $7....

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Unread postAuthor: noname » Fri Sep 28, 2007 7:06 pm

Or you can grap the needle valve in a vice, and unscrew the whole top section, leaving female 1/4" compression threads on the inside of the torch valve.
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Unread postAuthor: Pete Zaria » Fri Sep 28, 2007 7:07 pm

noname wrote:Or you can grap the needle valve in a vice, and unscrew the whole top section, leaving female 1/4" compression threads on the inside of the torch valve.


Holy crap, does that really work? I'm going to go try it right now.

Edit - Damn, there really are 1/4" threads in there! Can I stick an NPT 1/4" fitting in there? If so, you could just stick a hose barb in there, or even pipe it straight to the meter. Awesome. Thanks for the tip!

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Unread postAuthor: MortarArmy » Fri Sep 28, 2007 7:32 pm

right now it doesnt bother me. I turn the valve on, it fills quick, i shut the valves off, dump and fire. barely even have to look at the thing and i get a perfect mix. Im just worried about going to a bigger chamber soon and having to have this huge meter.
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Unread postAuthor: Novacastrian » Fri Sep 28, 2007 7:55 pm

Can you increase the size (diameter) of your meter pipe maybe?
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Unread postAuthor: noname » Fri Sep 28, 2007 10:50 pm

Pete, it is a very cool trick, and has been very helpful to me.
The threads are 1/4", but it's compression thread, not NPT. You'll need a 1/4" NPT x 1/4" compression fitting to adapt it. Simply unscrew the compression nut, take out the olive, and screw the male threads into the torch valve.
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Unread postAuthor: Dornep » Fri Sep 28, 2007 11:32 pm

MortarArmy wrote:you know what...now that im thinking about it, i think i know what my problem is. I used a propane valve from a camp stove to connect to the tank, i bet that thing is regulated.


Thats funny I had the exact same problem with my first cannon. I used a regulator off of a camping stove that was regulated to.. guess what.... 20PSI. :wink:

If you want to use a smaller meter pipe you can always increase the amount of PSI you inject into it. Keep in mind that the pressure in the propane bottle is affected by temperature which means if for example you have to inject 90PSI of propane into your chamber for a 4.3% mix, on a cool day you might not be able to get 90PSI out of the tank.

@Noname.. That is a really great tip dude.. I thought those things were one solid piece of brass!
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Unread postAuthor: Pete Zaria » Fri Sep 28, 2007 11:43 pm

You know, you guys could probably open up those 20psi camping stove regulators, pull out the diaphragm, put it back together, and run it at tank pressure... I don't have one in front of me but unless it's welded shut, I'm pretty sure it could be modified....

Just a thought.

I'll drop by a hardware store after work Monday and look for a 1/4" compression to 1/4" NPT fitting, and get a 1/4" npt to 3/8ths" hose barb. That'll look a lot cleaner than just having hose clamped onto a sawed-off propane torch nozzle.

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Unread postAuthor: Dornep » Sat Sep 29, 2007 2:43 pm

Pete Zaria wrote:I'll drop by a hardware store after work Monday and look for a 1/4" compression to 1/4" NPT fitting, and get a 1/4" npt to 3/8ths" hose barb. That'll look a lot cleaner than just having hose clamped onto a sawed-off propane torch nozzle.


Yea it will look a lot cleaner and it will be a lot safer too. I've had my meter pipe hose blow off the propane tank fitting twice since I built it. The first time I had the hose and tank lying in my lap tweaking one of my cannons and kapow! the hose shot off and put a nice dent in the drywall in the dining room. It sounded like a gun shot and I'm just glad it wasn't pointed at my gut when it flew off.

About the compression fittings... Are they the type with the fine straight threads compared to the large tapered NPT threads? I'm just asking because I have an 1/4" adapter that I bought a few weeks back that I can't use because of the fine threads on the male end.
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Unread postAuthor: Canadian Spudder » Tue Oct 09, 2007 7:18 pm

Darn, so the camping regulators done work? :( I am about to build my 1st propane meter and I was ready to use a salvaged one. Looks like I'm gonna have to buy a torch head.
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Unread postAuthor: Pete Zaria » Tue Oct 09, 2007 7:20 pm

As I said, if you open it up and pop out the diaphragm, it'll probably dump unregulated propane.... so you'd just need a simple meter setup with two ball valves, some brass fittings, and a gauge.... But a torch head is probably easier, and less than $8, so....

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Unread postAuthor: jimmy101 » Wed Oct 10, 2007 12:36 pm

The typical Bernzomatic torch head has two places where it can be unscrewed.

The mixer / nozzle unscrews from the pipe, usually pretty easily. This leaves the tiny orifice as a flow regulator. 1/4 or 3/8" hose will fit over the pipe after the mixer has been removed. You can remove the hose and reinstall the mixer and get back to a working torch.
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The main pipe (the bent pipe in the photo) unscrews from the valve body. I believe this joint is usually sealed with something like pipe dope. It can be hard to get it to unscrew without damaging the pipe. If you don't bung things up too badly, you can rebuild the torch. Be careful that the pipe seals well in the valve body before you ignite the torch. :D
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