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EMT Electrical Conduit?

Post questions and info about hybrid (compressed gas with fuel) powered cannons here. This includes discussion about fuels, ratios, ignition systems, build types, safety, and anything else relevant.
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EMT Electrical Conduit?

Unread postAuthor: PeteS » Wed Oct 30, 2013 12:14 pm

I have seen some mention of this stuff being used as a barrel. Any idea of what kind of barrel pressure 1" EMT would be suitable for? The fact that it is seamed makes me a little leery of it. HGDT predicts 260.4 psi at 4X and 287 psi at 5X for the dimensions I am considering. I would want to plan for the pressure possibly being higher.
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Re: EMT Electrical Conduit?

Unread postAuthor: Labtecpower » Wed Oct 30, 2013 1:43 pm

I would trust that tube :)
Does it have a ridge on the inside? most welded tubes i've found have this welding ridge that makes it unsuitable for use as a barrel.
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Re: EMT Electrical Conduit?

Unread postAuthor: PeteS » Wed Oct 30, 2013 2:04 pm

Labtecpower wrote:Does it have a ridge on the inside? most welded tubes i've found have this welding ridge that makes it unsuitable for use as a barrel.

I don't have it on hand yet.

Many years ago I was an electrician and installed many miles of this stuff. My recollection is that most of it has some slight ridge at the seam, but if it hasn't changed it is pretty variable from batch to batch. It can't be too rough inside because it has to be able to have electrical wires pulled through it without damaging the insulation on the wires. It also has to bend smoothly so the seams are pretty decent.

I figure I can hand pick the length I purchase.
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Re: EMT Electrical Conduit?

Unread postAuthor: ramses » Wed Oct 30, 2013 7:29 pm

I have used 2" EMT on my hybrid up to about 8x. I believe the stuff I have is fairly smooth. There is definitely a visible discontinuity, but I don't think you can feel it much.
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Re: EMT Electrical Conduit?

Unread postAuthor: PeteS » Thu Oct 31, 2013 10:25 am

ramses wrote:I have used 2" EMT on my hybrid up to about 8x. I believe the stuff I have is fairly smooth. There is definitely a visible discontinuity, but I don't think you can feel it much.

Since I am planning to only go 5x that is good news.
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Re: EMT Electrical Conduit?

Unread postAuthor: Fnord » Sat Nov 02, 2013 8:04 am

Can also verify emt conduit seems suitable for hybrids. I've used the 1.5" stuff up to 9x as a barrel material. I would advise strategically positioning the weld seam so that it is facing away from you. There is not a huge safety factor on this stuff, but it is not likely to fail catastrophically.
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Re: EMT Electrical Conduit?

Unread postAuthor: PeteS » Sun Nov 10, 2013 3:51 pm

Things are progressing well with this project.

The tank is one of the squatty propane cylinders, the barrel is 6' of 1" emt, and a cast iron union holds the burst disk.

I test fired it as a compression only gun with a 150 psi burst disk. It chrono'ed at 799 fps with a light projectile.

I have since installed a spark plug, mounted a 30 KV grill lighter (the kind with a 9V battery), and set up a 100 ml syringe to easily fill from a propane tank and to screw onto the presta valve on the gun. I test fired it indoors with 150 psi burst disk and a 2X charge of propane (100 ml) and air. It made a big bang, but I didn't use any projectile so no chrono reading. I was in the basement and my wife said it was loud on the second floor.

I am looking forward to taking it outside and test firing it with a projectile and a hybrid mix. The goal is to break the sound barrier which according to HGDT it should do at 3X. I will probably fire it at 2X first and go from there.
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Re: EMT Electrical Conduit?

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Re: EMT Electrical Conduit?

Unread postAuthor: SpudBlaster15 » Sat Nov 16, 2013 1:08 am

Suitable operating pressure will depend on the wall thickness, grade of steel, quality of the welded seam, among other things. 5X should be fine; you're only going to be looking at about 500PSI max, which is hardly enough to reliably burst thinwall PVC, never mind mild steel. I wouldn't recommend it, but I've hip-fired my hybrid at 10X using a 1.5" EMT conduit barrel and a 150g projectile, and had no issues. Air/propane at less than a few tens of atmospheres preignition isn't likely to fragment a steel barrel in any circumstances, but a failure could result in severe burns, so if you decide to take it to higher pressures, consider wearing welding gloves and a thick cotton jacket when firing.
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Re: EMT Electrical Conduit?

Unread postAuthor: PeteS » Sat Nov 16, 2013 9:21 am

It has been working fine and has exceeded my expectations. The thing is crazy loud though. At 4x my ears were ringing for an hour or more. After that experience, I will wear ear protection when firing this gun. It was like an M-80 going off.

UPDATE:
The pooch and I just got back from playing with this cannon. My intention was to check the muzzle velocity with a potato projectile, but I left the chrono turned on last time and the battery was dead.

The gun was much quieter in the woods and using a potato plug as a projectile. The new smaller chamber also helps.

The damage to the tree was from a 1" diameter by 2" long plug of potato at 20'. I didn't mean to damage the tree. It was a surprise that a small chunk of potato would do that much damage.

I will have to find a dump where I can perforate appliances with metal projectiles.

A few pictures:
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