ISA Certified Control Systems Technician (CCST) program

CCST

Certified Control System Technicians (CCSTs) calibrate, document, troubleshoot, and repair/replace instrumentation for systems that measure and control level, temperature, pressure, flow, and other process variables.

CCST question

As used in instrumentation and defined in the ISA Dictionary, the term, "primary element" usually refers to:

A. outer cascade loop
B. pneumatic control valves
C. transmitters / transducers
D. process detectors or sensors

CCST Answer

The correct answer is D: process detectors or sensors.

According to the ISA Dictionary, a primary element is "The system element that quantitatively converts the measured variable energy into a form suitable for measurement."  Also refer to ANSI/ISA-51.1.

For example, a magnetic flow meter is a primary element which senses/detects the velocity of a flowing conductive fluid through a magnetic field, and converts this measurement into a millivolt signal proportional to the measured velocity (Faraday's Law) . 

The output from a primary element will usually need further processing to be useful in a control system.  In this case, a transmitter would also be required to convert the electrical signal into a standard signal for the control system (4-20ma, HART, ProfiBus, etc.).

Reference: Goettsche, L.D. (Editor), Maintenance of Instruments and Systems, 2nd Edition (2005), ISA Press