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

In a level control system using a capacitance probe as the sensor, if the process liquid medium changes:

A. the transmitter will need to be recalibrated.
B. the probe’s grounding system will need to be relocated.
C. adaptive control action will need to be added to the controller.
D. there should be no effect on the control loop performance.

CCST Answer

The correct answer is A: The transmitter will need to be recalibrated.

For a capacitance level sensor, capacitance (C) is proportional to the dielectric constant of the material in the tank.  The capacitance level sensor measures the change in capacitance as the liquid level rises or falls.

The metal rod of the probe's electrode acts as one plate of the capacitor and the tank wall (or reference electrode in a non-metallic vessel) acts as the other plate. As the liquid level rises, the air or gas normally surrounding the electrode is displaced by material having a different dielectric constant. A change in the value of the capacitor takes place because the dielectric between the plates has changed.

At 0% or LRV, the capacitance will be Cempty. When full, or URV, the capacitance will be Cfull.  If the material in the tank is changed, the capacitance measured when full will be different.  Materials have different dielectric constants, and Cfull will be greater if the new material has a higher dielectric constant than the original material, and Cfull will be less if the new material has a lower dielectric constant.

A change in the values of LRV or URV for a sensor will require a recalibration so that the sensor will correctly sense and detect the proper level in the tank.

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