Factors for Electrical Resistivity Units

A factor which previously limited installation of automatic corrosion monitoring systems was the cost of cabling between sensors and control room instrumentation-this was particularly relevant to the electrical resistance (ER) systems. Developments to overcome this have included transmitter units at the probe location providing the standard 4-20 mA output (allowing use of standard cable) for onward transmission to data systems or the use of radio linkage which has been successfully used for other process-plant instrumentation. 

The resistance variation is measured by Keithley unit as shown. The chemiresistor-type sensor is mounted on hot plate (filament heater) which is coupled with ceramic base stand. The two leads of thin copper wires are attached to interdigited electrodes by silver paste for electrical connections. The electrical connections for gas sensing measurements, thermocouple and temperature variation are made using instrumentation feed through. The response of sensor is monitored in terms of the normalized resistance calculated by response = Ro/Rg and the sensitivity factor is monitored in terms of % sensitivity = dRJRo. Where dR is the variation in resistance of sensing materials after exposure to gas, Rg is the resistance of the sensor in presence of gas and Rq is the initial resistance with gas. 

Hall effect is the most widely used technique to measure the transport properties and assess the quality of epitaxial layers. For semiconductor materials, it yields the carrier concentration, its type, and carrier mobility. More specifically, experimental data on Hall measurements over a wide temperature range (4.2-300 K) provide quantitative information on impurities, imperfections, uniformity, scattering mechanisms, and so on. The Hall coefficient and resistivity (p) are experimentally determined and then related to the electrical parameters through (for n-type conduction) ffn = fulne and M-h = f n/P. where n is the free electron concentration, e is the unit electronic charge, Ph is the Hall mobility, and Th is the Hall scattering factor that depends on the particular scattering mechanism. The drift mobility is the average velocity per unit electric field in the limit of zero electric field and is related to the Hall mobility... 

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