digital computer and networks for management, data logging, monitoring, automatic and manual control of industrial process variables; namely, flow of materials, temperature, pressure and liquid level, comprised in whole or in part of central processors, microprocessors, memory units, multiplexers, teletypewriters, CRT terminals, paper tape readers and punchers, analog/digital converters, and operator interface consoles; industrial process control operators; namely, automatically operated fluid flow control valves, liquid and gas analyzing apparatus and instruments; namely, liquid and gas chromatographs, octane analyzers, gas analyzers using infrared techniques and data recording and indicating apparatus and instruments for the afore-mentioned goods; apparatus and analytical instruments for measuring, indicating and recording chemical properties, pH and conductive properties of solutions; namely, pH electrodes, reference electrodes, electrode holders and mechanical electrode cleaners; particle size analyzing apparatus and instruments; namely, instruments for detecting very small particles in liquids and large particles in mixtures; apparatus and instruments for sensing and indicating the interface between different fluids; fluid density measuring and indicating apparatus and instruments; instruments and apparatus for measuring pressure using pressure sensitive and semiconductor sensors and parts for the afore-mentioned goods