ISA Certified Automation Professional (CAP) program
Certified Automation Professionals (CAPs) are responsible for the direction, design, and deployment of systems and equipment for manufacturing and control systems.
The following question comes from the CAP study guide, Performance Domain III, System Design, Design, specify, and procure the hardware/software used in the system
To measure the level of a liquid with a dielectric constant greater than 2 inches a horizontal vessel with changing composition and temperature, the device with BEST reproducibility is a:
A. Radar level detector
B. Ultrasonic level detector
C. Capacitance level detector
D. Differential pressure transmitter
The correct answer is A, radar level detector.
The fact that the tank is horizontal implies the angle of the beam will be narrow at the surface and the vessel walls will not interfere. Radar can detect the smallest change in surface level. It does not depend upon the temperature or composition of the fluid as long as the beam is narrow and the dielectric constant of the fluid is greater than two.
Ultrasonic level detectors are affected by changes in the speed of sound with temperature and are not quite as sensitive as
Capacitance level detectors are affected by changes in the dielectric constant with composition and are not quite as sensitive as radar.
Differential pressure transmitters are affected by changes in liquid density with composition and temperature and are not as sensitive as radar.
Reference: Terrence L. Blevins, Gregory K. McMillan, Willy K. Wojsznis, and Michael W. Brown; Advanced Control Unleashed: Plant Performance Management for Optimum Benefit; ISA Press, 2003.