Theory and design of an electrodeless portable conductivity meter

The method of electrodeless conductivity measurement using two coils wound on toroidal ferrite cores in close proximity, coupled by the induced current in the electrolyte and operating in the audio range of frequencies has been used in process control since its introduction in the 1950s. It presents several advantages over the traditional method of measuring conductivity most important of which are stability, ruggedness, and maintenance free operation. Its major disadvantage is the large sample size required for correct operation. Despite its long established use in industry with various configurations of probe design, there is no published theory of its operation. A model of the probe operation is presented which takes into account the pattern of current flow induced in the electrolyte. This allows the electrolyte to be modelled as a resistance and the effect of geometrical factors upon this resistance is described. The operation of the probe can be represented by an equivalent circuit and theoretical expressions for the cell constant are derived. The correctness of this theory is confirmed by practical measurements. Factors affecting the performance of the probe such as operating frequency, cable length and coupling have been examined. The design and development of a conductivity meter which operates with an existing electrodeless sensor is described. It comprises analog circuitry which interfaces the sensor to a microcontroller. The microcontroller provides synchronisation, analog circuit control, calculations, display update, and interfacing with a small keyboard. A requirement that the meter be portable necessitated the minimisation of component count, cost and power consumption. Addition of reference curves for various chemicals stored in the microcontroller's EEPROM allows the instrument to be used as a concentration meter.