by 2seater » Fri Apr 29, 2005 9:47 pm
I tried a few different materials for the tubing in the manometers and I finally settled on Teflon tubing, 1/4" od x .031" wall thickness. It is not clear but is translucent and colored fluid can be seen through it very easily. I am using Marvel Mystery Oil @ .830 sp.gravity, which works well, doesn't freeze, is cheap and seems to dampen oscillations in the readings a bit. For support I dado cut a groove in a hardwood board , slightly undersize for the tube diameter so it can be pressed into place and it is screwed to the wall. The Marvel oil has pretty good film strength and other materials would cause the reading to recover relatively slowly and slipperiness of teflon has helped the response time. I did try smaller diameter tubing, and that worked okay for high inclinations, but in my high resolution, low rise manometer, the response was very slow. My suspicion is the surface tension of the fluid causes a need for a minimum tube size of some sort to respond correctly. In the quest for best accuracy, I modified the Excell spreadsheet formula for rise in the manometer, to add the ratio of the well size to the tubing i.d. In my case the ratio is 502:1, so the spreadsheet adds 1/502 x the rise to the inches of rise in the manometer. Picking nits I guess.