Hi Peter, Normaly you would use oil as the dialectric, chose a thicknes of oil that = suits the responce speed you require. The closer the plates the higher the capacitance, too close and you may = have problems with capillary action. You Can't use Mercury as a dialectric in a capacitor, the plates would be = shorted all the time (good tilt switch). Regards Peter W. >>> "Peter L. Peres" 10/25 6:11 AM >>> Hi, I am interested in integrating a $SUBJ very tightly with a very small board. That means build it, as all parts are too big for this. I am thinking of the halfcircle-shaped cavity differential type, built between a sandwich of 3 FR4 boards, top and bottom 0.5mm and middle (spacer with halfmoon/halfcircle channel machined into it) 1.5mm with no traces. The electrodes will be etched directly into the top and bottom thin boards. This and running the hot electrode off the PIC's clock through a buffer and using two rectifying detectors to feed two AD channels is the easy part. Now what juice do I use ? Mercury is very tempting but I'd rather not. Some sort of electrolyte is needed that will not foam and not stick to the walls too much. High specific gravity is nice. Stopping short of 'milking' a large electrolytic capacitor for the prototypes, what could I use ? Salt water should work, some sort of light oil seeded with something conductive ? Come to think of it, some blood plasma will do fine according to specs . Peter -- http://www.piclist.com hint: To leave the PICList mailto:piclist-unsubscribe-request@mitvma.mit.edu=20 -- http://www.piclist.com hint: To leave the PICList mailto:piclist-unsubscribe-request@mitvma.mit.edu