Wednesday, September 17, 2008

Cola Deployment Improvements

Finally managed to make time to do more testing of the Mark II Cola Deployment Device. Following up on a suggestion that I consider using a small bike pump as the pressure source, I started playing around and came up with this test apparatus.

The basic idea is that the pump is used to pressurize an empty 1-liter PET bottle that contains a couple of balloons that serve as a quick-n-dirty pressure indicator. These bottles can take quite a bit of pressure before they go bang, certainly much more than you can get into them with a hand-pump.

Then a valve is cracked to blow up the balloon inside the cola bottle and expel the cola.  It's pretty controllable but I'm probably going to put in some sort of flow restrictor to make it more controllable.

All of the tubing and fittings are available at Lowes or Home Depot. The tubing is (iirc) translucent semi-rigid polyethylene -- the other option, clear vinyl, doesn't work as well with the fittings.

The fittings themselves are simple $2-4 plastic push-on, quick-release fittings. Amazingly, these are almost totally gas tight!  I pumped the reservoir bottle up to 4 or 5 bar and left it overnight and the leakage was negligible. These things are like legos for low-pressure pneumatics! On the other hand, the brass compression fittings you'll find in the same aisle at the store are a pain to work with -- very hard to get gas-tight.

Tip: some of the fittings have threads on them, which can be very handy (I added such a fitting to the cap of the 1-liter bottle, for example). However, these are pipe fittings, so they have NPT threads on them, which are different from regular UNC/UNF threads used on common nuts and bolts in the US. Even my usual black-market nut connections couldn't fix me up.  However, I eventually found my way to the illegal nut dealers at Steven's Hardware here in Wilmington, where I learned the magic word. The magic word is "lamp nuts" -- for some reason, they use NPT threads!

The major remaining design issue is the balloon inside the cola bottle.  Sometimes it gets caught on the sides of the bottle and this results in another part of the balloon getting stretched until it breaks. I am currently using double-balloons but I'm thinking that finger-shaped balloons will work better.

I've also been working on a 2-D Cola Convection Display Device, I'll post on that soon.


Mark Mellors said...

Neat trick with using the balloons as pressure gauges.

Sometimes it gets caught on the sides of the bottle and this results in another part of the balloon getting stretched until it breaks

Grease the inside of the bottle?

MadOverlord said...

The problem with greasing the bottle is that it's impossible; the bottle in this instance will be a newly opened bottle of diet coke!