>If anybody has knowledge measuring conductivity of water (total dissolved >solids) with a pic, your input would be greatly appreciated. Hi Shane, as I can see, you have already received some input, and although all the advise was quite right, it was maybe too general. If you want to measure conductivity of solution and calculate approximate amount of dissolved salts, it is possible, but be prepared to use much fancier circuit than a capacitor connected to T0CKI. 1. electrodes - mostly used platinum may be replaced with stainless steel ones if you do not dipp them in agresive solutions (strong acids, bases, oxidants). 2. Excitation voltage - 10V pk-pk squarewave will certainly not work. Must use AC with no DC component, sinewave is preferred. Frequency about 1 kHz, amplitude few hundred milivolts. At these conditions the solution behaves like (variable) resistor in parallel with (constant) capacitor. Since the capacitor has water with pretty high dielectric constant (~80) between its electrodes, its capacitance is much higher than one would expect from its dimensions. Therefore a lower frequency is suggested for low conductivity solutions, on the other hand a higher frequency is better for high conductivity solutions (to defeat polarization). 3. Sinewave generator - I use Wien-bridge type, amplitude stabilized with two antiparallel diodes. This works fine for fixed frequency oscilator, several % of THD is not critical. 4. Rectification - I use synchronous detector (analogue multiplexor 4053) and RC filter. 5. The conductivity meter together with the cell is calibrated by KCl standards. 0.1 M : gamma=12.88 mS/cm 0.01 M : gamma=1.413 mS/cm (both at 25 C) 6. Total concentration (salinity) - can be estimated from conductivity as follows: c=75*gamma (c in weight %, gamma in S/cm) This equation is based on quite similar specific molar ion conductivities of most ions and is only approximate. 7. There is some temperature dependance - the specific conductance of most ions increases by ~2% per degree Celsius. 8. PIC will not be much involved in the measurement. It may poll data from the A/D convertor, scale them appropriate, take care of the temperature compensation (if used) and calibration and display/transmit the result. 9. Get an older book of (electro)analytical chemistry to learn more practical details. Regards, Josef