In message , Pic Dude writes >This sounds neat, but I worry that if we're measuring capacitance, then >external influences may throw off the calibration. For example, when I >first saw this post, I related it to a problem I had building a corner >scale for a vehicle. It is very necessary to keep the wheels level. I >wonder how the large chunk of metal in the area (the car) will affect >the sensor output. It will but not as much as you'd think. The sensor capacitance to the water is defined by its area, the thickness of the tube( say 1mm) and its dielectric constant say 5. The sensor capacitance to the car has a much larger distance (~100mm) and relative dielectric is only 1. Ignoring (large) edge effects, the effective area of the sensor-car capacitance is still defined by the sensor dimensions so we have 500:1 ratio in sensitivities at that range. I can report that after extensive research, a beer filled 85Kg test object sitting on the edge of the bath cutting it's toenails does not upset the sensor by more than a few percent. The main problem is that most of the capacitance is due to the film of water on the inside of the tube. It should be coated to break the film quickly. A better technique, which I don't use for fear of electrocuting customers (generally not a good USP), is a self heated NTC thermistor, immersed in the water and measuring the voltage across it as the water dissipates the self heating. Regards Martin -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads