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.
Which of the following describes a controller in which an increase in the process variable results in an increase in the output to the manipulated variable?
a) reverse acting
b) direct acting
d) fail safe
e) none of the above
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The correct answer is B, direct acting. For a direct-acting controller, the resulting output movement is in the same direction as the movement of the process variable.
Here is an example of a direct-acting controller. If we have an air-to-open tank outlet valve, as the level rises (PV) in the tank, we would want the outlet valve to open (controller output increases).
Reference: Goettsche, L.D. (Editor), Maintenance of Instruments and Systems, 2nd Edition