This automation industry quiz question comes from the ISA Certified Control Systems Technician (CCST) program. Certified Control System Technicians calibrate, document, troubleshoot, and repair/replace instrumentation for systems that measure and control level, temperature, pressure, flow, and other process variables. Click this link for more information about the CCST program.
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The correct answer is b, “ultrasonic”.
The ultrasonic method is the most cost-effective. Floats are very seldom used with any corrosive material, and radioactive measurement is very expensive, due to purchase costs and compliance with federal and state regulations. Capacitance can be adapted to work with a highly corrosive media, but it will not be as cost-effective as ultrasonic measurement.
Reference: Goettsche, L.D. (Editor), Maintenance of Instruments and Systems, 2nd Edition