Monday, September 22, 2008

Pneumatic Soda Deployment Device Improvements (Part II)

A quick trip to the party store this weekend provided me with a variety of balloon styles to play with.  The best candidate so far is a "punch-ball" balloon, which is both much thicker than a normal balloon and is also pleated.  The fill tube extends into the neck of the balloon right down to the tip, which makes it easier to insert the whole assembly. Also, before inserting the balloon, I pop in a short length of polyethylene tubing that serves as a bypass and allows soda trapped under the balloon to escape.

The system permits quite precise deployment of the soda, although I am still considering implementing a flow restrictor. In the second image, I've deployed soda most of the way up the tube but halted it at that point. By fully expanding the balloon I can expel almost all the soda, although in practice I'll only need to display a few fluid ounces.

One possible improvement might be to use a smaller bottle and a tube sized so  that when the bottle is completely empty, I have a soda globe of exactly the right size.

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.

Wednesday, September 3, 2008

Mark II Menticulator Cola Deployment Device Tests

Please go down two posts for the original report on our first experiment -- thanks!

As mentioned in the post reporting the results of our initial microgravity tests, the Mark II Menticulator will require a more sophisticated method of deploying a nicely sized sphere of soda. My initial thought was to use a water balloon, but an afternoon of sweaty testing on the back porch lead to the discovery of several serious problems with this approach.

The first is that when the balloon ruptures, friction between the skin of the balloon and the water results in significant spray, some of which is clearly fast enough to overcome the surface tension of the water (and remember, the soda will have a lower surface tension because of the sweetener and other ingredients). If some of this spray coats the camera port, we may not be able to see the reaction. I did multiple tests with different balloon sizes and tensions, and even coated the inside of the balloon with oil, in an attempt to reduce the spray, with no real change in the results.

But an even more serious problem is that unless the balloon is under a significant amount of tension, it will either not puncture at all, or the puncture will not propagate! If this happens in flight, we will experience a total menticulation failure and the entire experiment will be ruined.

So it was back to the drawing board (I have an ACME drawing board, the same brand Wile E. Coyote uses). I had to come up with a method of simply and smoothly deploying the soda. When I'm faced with a problem like this, I like to sit in the workshop and idly play with the various parts and tools that I have available. What I'm looking for is relationships between things that will spark a connection. After a while, this idea came to me.

The initial idea was to use a balloon inside the soda bottle to force the soda out of the bottle and up a tube into the apparatus. I already had the tube -- the Geysertube that Steve Spangler sent to me earlier in the year. I drilled out a hole in it so I could insert some plastic tubing, inserted a bit of copper tubing into the end of the tubing to stiffen it, ziptied a balloon onto the end, and stuck the other end into a hiking bota-bag, which acts as the bellows. A little hot-glue and tape sealed everything up.

Squeeze the bag, the balloon inflates, and liquid is forced up the tube in a reasonably controlled manner (this test had a leak, thus the bubbles).

Okay, so now I've got a basic method of deploying the soda sphere. The next thing to work on is, once I have the sphere deployed, how do I test the convection hypothesis in a manner that leaves no room for doubt about the results, and is as simple and foolproof (always a plus when I'm involved) as possible.

I'm also looking for a more precise method of pumping up the balloon than the bota bag (which was just the cheapest thing I could find that would work reasonably well). Something like a really, really big industrial syringe (500-1000ml!) would be perfect, but I haven't been able to find one yet -- if you know of something that might work, in particular something that's effectively self-locking, so after I get the bubble to the right size, I can just let go and the balloon will stay its current size, let me know!

Subsequent to these experiments, the blog got slashdotted, and I started getting lots of suggestions. One that I'm definitely going to follow up on in the near future is a modification of Ger's suggestion about a box of cola; my current thinking is to build a thin enclosure that will confine the convective flow pretty much to two dimensions.