Finding center of a spherical flask
Jonathan
Posts: 1,023
Hi All,
This is way OT,, but there are so many smart folks here I just thought I'd ask.
I am working on a sonoluminescence set up. Basically, you blast a spherical flask with ultrasound, a small bubble forms in the center and glows. I am about to glue the transducers to a new flask, and I want to get them lined up perfectly.
I have played with a laser, and yes I can see where the incident beam overlaps the reflected beam, but is very hard to mark, and the optical quality of the flask is fairly poor. I'm looking for an accurate way to place two dots directly opposite each other in the center of the flask, to be used as targets when gluing the transducers. Any ideas?
Thanks!
Jonathan
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www.madlabs.info - Home of the Hydrogen Fuel Cell Robot
This is way OT,, but there are so many smart folks here I just thought I'd ask.
I am working on a sonoluminescence set up. Basically, you blast a spherical flask with ultrasound, a small bubble forms in the center and glows. I am about to glue the transducers to a new flask, and I want to get them lined up perfectly.
I have played with a laser, and yes I can see where the incident beam overlaps the reflected beam, but is very hard to mark, and the optical quality of the flask is fairly poor. I'm looking for an accurate way to place two dots directly opposite each other in the center of the flask, to be used as targets when gluing the transducers. Any ideas?
Thanks!
Jonathan
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www.madlabs.info - Home of the Hydrogen Fuel Cell Robot
Comments
Here's a real wild idea, but it might just work if you're VERY careful. How about a lathe! You may be able to "chuck" the neck of the flask in a three-point chuck. A three point chuck guarantees centering. Then use any suitable marker (a china marker comes to mind) against the slowing rotating flask. Obviously the lathe would have to be variable speed.
Regards,
Bruce Bates
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When all else fails, try inserting a new battery.
Post Edited (Bruce Bates) : 11/28/2008 4:04:58 PM GMT
I don't know what kind of accuracy you need, but here's a random untested idea:
Consult the crude sketch I've attached to this post.
Consider filling the spherical part of the flask half way with fluid (a task which, in itself, might be tricky and take some trial and error). Place the flask on three pointy things that make like a tripod that cradles the spherical part of the flask. The neck of the flask is pressed against a cylindrical rod of small diameter but good stiffness (a polished metal rod, precision ground). If you rotate the flask with the fluid inside maybe about 15 degrees and mark the fluid level after each rotation, you could theoretically get a series of lines that intersect at two points directly opposite the sphere. On the sketch, the axis of the aforementioned rotation would be coming out of the paper. The cylindrical guide rod (against which the flask neck is pressed) and the axis of flask rotation will have to be perpendicular to each other and to gravity.
One problem with this set up might be the meniscus of the fluid - you would ideally want a fluid that doesn't "ride up" on the glass too much, so you might need to find a fluid with a Goldilocks surface tension, but this could contaminate your fluid sample later on. Mercury will have a reverse meniscus, so that wouldn't help much. Maybe some kind of alcohol or other organic that can evaporate after you're done with it???
Anyway, it's just a thought. I've never seen anybody try this before.
Good luck,
Mark
PS. My sketch shows the pointy tripod and guide rail and gravity not perpendicular to each other, but I hope you get the idea.
Post Edited (ElectricAye) : 11/28/2008 5:36:51 PM GMT
**
Duffer
Thanks again for the help folks! And if anyone ever wants to attempt sonoluminescence, lemme know, it isn't all that hard and it sure is neat!
Jonathan
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www.madlabs.info - Home of the Hydrogen Fuel Cell Robot
And is it spherical, but for the neck, or does it have a flat spot on the bottom too? Seems to me, you can just build a simple fixture, and use a plumb bob to determine horizontal. Use the fixture to locate a point, then rotate and view the point from the other side, to locate the other one.
Cross section shown. Build the fixture with two sides, or three, such that the flask just sits in there. 3.5" flask shown.
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if you want to stick to using a laser, you can improve the optics of the flask by filling the flask with a clear fluid whose index of refraction matches that of the glass from which the flask was made. For example, borosilicate glass has an index of refraction of about 1.5, which is about the same for benzyl alcohol. By filling the flask with benzyl alcohol, you will effectively eliminate the internal surfaces of the flask insofar as optics goes, which leaves only the outer surface (air to glass interface) to distort the laser significantly. Since a laser is only a single frequency of light, you don't have to worry about matching the Abbe numbers of the fluid and glass.
In other words, with only air in the flask, your laser is playing off the outer air-glass interface, then strikes the glass-air interface, then travels across the diameter of the flask, next hits the air-glass interface on the other side, then encounters the glass-air interface also on the other side of the flask. By filling the flask with an index-matching fluid, you rid your laser path of the inner reflections and distortions. Thus, the only thing the laser light is going to suffer are the very outsides of the flask as the laser enters and exits the flask.
Fundamentally, you are trying to get accuracy out of a flask that is not manufactured for such accuracies. It's not an easy task, but maybe your apparatus can live with some slop.
Keep us posted on your progress. This is a cool topic and if you need help using Propeller chips, etc. for instrumentation, I have found that the people here practically walk on water.
good luck,
Mark
I had the idea put up by Duffer but the trouble is you only get one point - the lowest.
The solution (pun intended) is as follows
1 - fill flask FULL with water and drop a small lead shot in it (not ROHS)
2 - plug and seal the flask
3 - rotate on its side so its level, filling neck running at parallel to the ground
4 - the lead shot sinks to the bottom - an air bubble rises to the top
(air bubble too big/small? you know what to do... repeat until happy)
5 - mark as required
good luck!
Post Edited (sully) : 12/1/2008 7:32:02 AM GMT