Highscool Chemistry Experiment

[strangers]

It's been forever since I've been here, but anyway,
I've convinced my highschool chemistry teacher to let our class build a potato gun after we finish the final test for this semester and I was wondering if anyone has any advice on what kind to build, the scale, the materials and anything like that. The reason I'm asking for all this is if something goes wrong, it could be very bad.

[Fnord]

Well, pneumatics aren't really chemistry-related. A metered propane/mapp combustion would be a good choice. Golfballs/ tennis balls make good consistent ammo for experiments.

Pressure rated pvc will be fine if the weather is reasonably warm. If you are unsure of a component's rating just ask us.

[psycix]

Both types are completely safe if executed properly.
Though I would say combustion has a higher chance of failing (DWV used or wrong fuels like hydrogen or acetylene), while pneumatics fail more violently if they do.

I agree to fnord, for chemistry you should do combustion. Pneumatic is science.
If chemistry or science doesnt matter, I would go for pneumatic.
If you use a safety popoff valve and pressure rated pipe (AND FITTINGS) nothing can go wrong (unless you drop the gun from a large height).
Even while combustion is easier to build, pneumatics are less picky to fire. They are not depending on a precise amount of fuel.

Also, a well built pneumatic can be more powerful then a combustion one, and on top of it, more people (like teachers and parents) will accept the project. Many people do not know that a combustion gun is safe because they are set off when they hear something related to explosions (even if it is not an explosion as the fuel deflagrates instead of detonates.

[strangers]

Interesting, since we're no longer have our bunsen burner in any working condition we have several bottles of propane lying around in the chemistry supply room, problem is that I'm not sure how the administration would look at the idea, if they react the same way my parents did, convincing them that propane is only a little stronger than the butane found in hairspray only without the filler may be a little hard.

I was leaning more towards combustion as we just finished covering stoichometry and gas stoichometry for the fuel/air ratios, and that a combustion cannon made from sch80 pw pipe would be guaranteed not to fail.
For ignition I was thinking of a spark plug and an inductance coil http://www.sparkmuseum.com/INDUCT.HTM see the black and white schematic and possibly using a relay in place of the vibrator.
For loading probably a union so I can also use burst disks.

[Fnord]

Actually, I think butane burns hotter and it's heavier, so per volume I would imagine propane is weaker. The difference is flame propagation speed, which is propane's strength (which no one has to know about )

In my opinion, pvc pneumatics are still more dangerous, given the constant pressure load. Has there ever been a combustion failure that wasn't due to user error? I can't remember any.

Also, just buy your own propane. It's cheap, and your school rules probably have nothing against bringing a tank to school with you.

[starman]

If you go combustion, do an advanced metered propane model. A spray and pray is much too easy and won't offer near the learning opportunity at high school level. The metering will force you to do some real calculations and give you some cool formula graphics you can brazenly install in your report...

It will also be a much more reliable gun.

Actually, I think butane burns hotter and it's heavier, so per volume I would imagine propane is weaker.

According to HGDT, butane is slightly less energetic than propane. Also, Jimmy's fueling dataindicates that per mole, butane is heavier and needs a little more O2 but will offer roughly the same energy as propane.

[ALIHISGREAT]

you should try as hard as you can to find pressure rated ABS which doesn't suffer from the problems of PVC

[starman]

you should try as hard as you can to find pressure rated ABS which doesn't suffer from the problems of PVC

Other than not recommended to be used in cold weather, what problems would those be with a 1x advanced combustion or a pneumatic 100 psi or less?

[john bunsenburner]

I am not sure about you school but bringing in gas cylinders is complicated as hell in mine. I once needed a 30L compressed air cylinder for an experminet to see wether or not feeding air from the bottom of a compost would increase efficiency, it ended with three teachers trying to explain to my why not to do it while the principle agreed, i got pissed, told them i forgot about the project took the cylinder back to the place, and well everythign is back in order...Any way you see wher ei am going with this...high school teachers can be really naive and paranoid at times and so certain things just wont work out, i have learnt never to ge tmy hopes up about things in school, like that there is no disappointment in the end. But if you managed to convince your chem teacher then your on your way, this is great, and so i wish you all possible luck, just try and avoid bringing your own stuff, use their propane, or use another fuel alotgether, liquids are often less suspicious...

[starman]

Here in the States, there are some communities/schools/teachers who have fallen completely to nanny state-ism, and would veto an excellent project like this...it really depends on the community/school/teacher in question.

It sounds like his particular teacher is cool and supportive. It's unfortunate your situation isn't as accommodating john.

[jimmy101]

1. You've got propane bottle in stock so use propane.

2. Measure fuel with a syringe (you've probably got those as well). Cheaper, easier, simpler and just as effective as a meter.

3. Sched 40 pressure rated PVC is fine. Overbuilding with Sch 80 really won't accomplish anything at all since you'll still probably use a clean out plug which isn't pressure rated.

3. Use a piezo for ignition instead of the buzz coil. In the long run a piezo will be cheaper and easier than the buzz coil.

4. Calculate the proper fuel load and use that. If a high speed projectile is a problem at the school then just use something light kike a nerf football.

5. Under fuel to less than 3% and show it won't fire at all.

6. Over fuel to 10% propane and show it won't fire at all.

Chemistry in action!

[strangers]

For 'safety' reasons I'm going to use the sch80, keeps teachers from freaking about the fact that it's using propane, and instead of the cleanout cap I'll probably use a big 3" or 4" ball-valve, nearly $22 at Lowes, but it'll be easier and safer than a cleanout cap, and I seem to have bad luck with piezo lighters they always seem to break after only a few uses, but I might be able get a hold of a cheap stun gun I can re package so it's not technically a stun gun any more. And as for the syringes that's a no go, our chemistry lab doesn't get used that often and as such is under stocked. Like a alot of schools most of our schools budget goes to sports programs, mainly football, it's gotten so bad they charge for condiments now, no extra portions, or anything.
Our chemistry teacher is a little wary about it thus the reason for the extra 'safety' precautions. But I may build only a small advanced combustion launcher maybe a 2" chamber and a 3/4" barrel, that way I can shoot painballs.

[Fnord]

A 2" ball valve would be fine... most people use this size.
Also, You could reinforce a cleanout with bondo or something, or try to find a hexagonal threaded plug, though they are kinda rare around here.

Stunguns are overkill for a school project. Were you using those crappy long nosed lighters, or an actual BBQ grill ignitor that broke? I've never had one stop working.

Like a alot of schools most of our schools budget goes to sports programs, mainly football, it's gotten so bad they charge for condiments now, no extra portions, or anything.

Typical. Instead of providing quality food or funding a good science/history/math/english department, they throw money at training dumb jocks, 98% of whom will never go anywhere in terms of a sports career.

[psycix]

Typical. Instead of providing quality food or funding a good science/history/math/english department, they throw money at training dumb jocks, 98% of whom will never go anywhere in terms of a sports career.

99,9%
Its indeed very stupid. If they would fund science, math and chemistry the whole school could build football cannons instead of kicking it. Much more fun, and it doesnt make you tired and sweaty.
Science owns sports.

[inonickname]

Even if using a bv (and fan hopefully) still try to build it to have access to your chamber. In the how to section theres an advanced combustion guide which has good advice on fuel metering and an excel spreadsheet.