Conductivity Measurement circuit for embedded processor

We are looking for design of a circuit which will return a value to our microprocessor which corresponds to the conductivity of water with upto 5000ppm of nacl. The circuit has source voltage of 5vDC or upto 24VAC and can be designed to output a signal to an atmel processor The probes are stainless steel spaced 1cm apart and will be used in apparatus to measure the conductivity of a water solution with a reading of 0-20000 ms/cm. For this reason several electrical characteristics must be taken into account. We would like the components to be simple and affordable as it will effect the base cost of the unit we are developing. Schematics will need to be provided as well as a description of how the circuit will work. In summary: a. Measure conductivity of water b. Source voltage on circuit will upto 24VAC c. Design needs to be simple and affordable d. Probes on metering device is 1cm apart and stainless steel e. Voltage used to measure cannot be DC as this will corrode the probes in liquid f. Schematics and description of operation must be submitted g. We are already using a microcontroller which will read the output of the circuit provided.