bizarre capacitance

David writes: ? Anyone here use an Extech multimeter? This thing seems ? to get confused with certain capacitors. In particular, ? a new 10uF electrolytic register randomly from 1 to 8 ? nF whereas a 1-year-old 1uF electrolytic registers fine. I never trust autoranging multimeters. What's happening is that it is applying a charging current, seeing a dV/dt response that's in a different range, trying to switch to a different range, applying a different charging current, seeing dV/dt in some other range, switching to a different charging current, etc. The textbook definition of capacitance I = C dV/dt works great in a truly linear world but old electrolytics have leakage, soakage, nonlinearities, and other things messing up the textbook definition. Soakage is always fun. Take a big electrolytic, short it out and remove the short, let it sit for a while on the bench, then put a hi-impedance voltmeter on it. It will not read 0V. Tim.