A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: FW: Plexiglass horizon
From: Jared Sherman
Date: 2003 Jul 15, 05:40 -0400
From: Jared Sherman
Date: 2003 Jul 15, 05:40 -0400
Gentlemen... I'm afraid Bruce Stark's attraction to the black plexi and RIchard Pisko's mention of levels that are accurate within 20 seconds of tilt crawled under my brain and forced me to do some long thinking. Bear with me, this is either madness or fiendish elegance.
I realized first that I had no idea of how a spirit level (a simple tube with a bubble in it) works. Only one internet source mentioned accuracy of their level, in that case a circular design used for leveling silicon wafer fabrication equipment to 30 seconds of accuracy with no price mentioned. I assume that means "terribly dear". I suspect the readable accuracy of a spirit level tube is enmeshed in the flatness of the tube walls, the surface tension of the liquid (less being better), the density of the gas bubble (less being better?) compared to the fluid, and the length of the tube and bubble, with a long tube and long bubble being more accurate for the same reasons that a "longer" base would make a ssiphon level more accurate. But without any certainty about this, I decided to pass up thinking about using 12" long glass chemistry tubing for spirit levels and did some rough calculations about a siphon level. That's the kind where you take a "U" of clear tubing, fil it with liquid, hold up the two ends and sight across them. As the water seeks its own level, the water line in each end will be at equal heights and sighting across them gives a level line between the two columns of water. So...for an accuracy of 20 seconds, 13 degree, I wanted a measurement that could be read easily with the eye and picked a convenient millimeter as being sufficiently precise, or crude, as you would have it. If you took a mark one millimeter wide to be 1/3 of a degree, you would need a circle with a circumference of 360x3, or 1180 millimeters, to simply mark the 1/3 degree intervals so you could sight across them and line two of them up equally. That would be a circle with approximately a 7.5" radius, i.e. 15" diameter. Now, if I haven't made too many wrong assumptions yet, that would indicate that taking a siphon level "around" the bottom half of a 15" diameter circle (held vertically) would allow one to sight across the middle of the circle, and assuming your eye could even up the two ends of the water column within that one millimeter of being even...You would have established a level line accurate to 20 seconds. I wrestled with how to apply this to a black plexi plate and think it can be translated somewhat simply and accurately. Suppose that we take a bar of 1" thick plexi, selected arbitrarily because we're already working with plexi and the flatness and eveness of the sides of the bar should be fairly good from the manufacturing process. (By all means, use Jacobsen blocks if you have them. ) Cut two 1" long pieces from the bar, keeping "this side up" the same for both of them in order to keep them even in case they are not perfectly made. You now have something like two 1" cubes of plexi. Glue the two cubes along the edge on top of a black plexiglass sheet, 15" apart from each other's centers. Use plexi cement so they weld properly. You know have a black plexi sheet with two "dice" glued on top of it, next to one edge. Now, drill a convenient 1/4" wide hole down the niddle of each cube, all the way through the plexi sheet. use a drill press to make it nicely vertical, although exactness will not cause great problems. Insert a 2" long piece of 1/4" wide glass tubing in each hole, so that it extends below the plexi, and above the cube, mainly above the cube. Connect the two bits of tubing with some hose (poly, rubber, whatever) beneath the plexi. Slowly fill the tubing with alcohol, perhaps with a bit of bright food coloring in it, until it is just level with the tops of the two cubes, or perhaps several millimeters above them. Stop. Connect the tops of the two tubes with another bit of tubing--they must be connected so the air can move and teh fluid can find its own level in the two columns. You should now have a siphon level with a 15" base, capable of resolving 20 seconds or better of "level". Adding a scale to the columns, or choosing the fluid/plexi colors so as to increase contrast, all are icing on the cake. Construct a second such level along another side of the plexi, and you've now got a lack plexi sheet 15" square with two highly accurate levels fixed into it. Add screw legs below to adjust it, and I suspect you've got a fiendishly elegant artificial horizon. Problems, caveats, finessing: At 15", the plexi will have to be a bit thicker in order to avoid sagging. You may chose to brace it from below (i.e. by adding Sintra or other lighter weight rigid plastic) or use thicker plexi, and add plexi "beams" below it to stiffen it. And you could, of course, literally carve channels into the bottom of the plexi so no tubing was needed beneath it and the liquid was captive between two plexi sheets, in a channel. My numbers are rough and the accuracy and size you chose ill be your own compromise, but I think it should scale marvelously. If "spirit levels" can be accurately made by simply taking a 12" long glass tube, filling it mainly with spirits, and marking the distance from the ends (and a longer bubble being better, so you'd want to get an air bubble that nearly came to the ends?) then of course it could be even simpler. I confess, my mind is churned to butter from trying to figure out how spirit levels REALLY work, somehow we never learned the physics of them in school. Perhaps one of you could be so kind as to explain this to me. Bruce and company...if you're feeling really ambitious and you DO build one of these, and it DOES work...I'll expect you to send one to me as well! And if you go into business making them, I'll expect a partnership, I could use a new job. If my thinking on all this is hopelessly wrong, or highly accurate commercial levels really can be found inexpensively...Well, it was good exercise anyway.