Dedicated impedance sensors with reduced influence of undesired physical effects

This paper discusses the problems of impedance sensors in their application. Four case studies of industrial sensors are presented and show how a systematic approach can reduce the effects of undesired physical influences. It is shown that in a first case of ideal environmental conditions, with capacitive sensors even with simple interface electronics a very high precision can be obtained. A second case study shows that when the sensing elements have resistive leakage, the related problems can easily be solved by modifying the interface circuits. The third case study concerns capacitive detection of water content in soil. It is shown that the presence of salt in water creates measurement problems related to physical conditions and phenomena. With a special probe construction and dedicated electronics, the water content in soil with high salinity can be detected. However, because of physical effects such as skin and proximity effects, the water content is only measured for the local environment of the probe. Finally, the physical problems of a blood-impedance sensor are discussed. Also in this case, application of dedicated measurement techniques and a special probe construction enables to solve the problems. A number of experimental results are given.