Complex impedance, conductance sensor

More reasonable solution is to use analytieal or numerical theoretical models [3-5, 11, 12] to obtain the conductivity value(s), from the measured eomplex impedance of the sensor-coil. Actually, the reverse models are needed here, as we ean measure some kind of the transfer function of the electromagnetic sensors (coUs), e.g. complex impedance at various frequencies. 

Frequency-dependent measurements of the materials dielectric impedance as characterized by its equivalent capacitance, C, and conductance, G, are used to calculate the complex permitivity, e = d — id, where co = 2nf, f is the measurement frequency, and C0 is the equivalent air replacement capacitance of the sensor. 

In view of the above features, oscillometric (conductometric) and potentiometric sensors seem to be best suited for use in industrial and environmental monitors among the electro-analytical techniques. Oscillometric and conductometric sensors fulfil the first and third requirement, the technical problems in connectin with calibration axe not too difficult. There are, however, problems with the fourth requirement, because impedance and the complex dielectric constant both depend on all constituents present, and because these properties do not only depend on one single factor but on several factors. The electrical equivalent circuit for the oscillometric capacities is given in Fig.l. The expression of the h.f. conductance, G, is as follows ... 

A pH sensor, based on a.c. conductivity measurements of a thin polymer film, has been developed. The sensor consists of a planar interdigitated electrode array coated with a polypyrrole multilayer, built-up using the Langmuir-Blodgett technique. Impedance spectroscopy has been used to investigate the complex admittance of the device when exposed to aqueous solutions of different pH. The experimental data have been fitted to the theoretical response of an equivalent electrical network of capacitors and resistors. A response over the pH range 3.5 to 8 has been measured. 

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