January/February 2010

ISA Certified Control Systems Technician (CCST) program

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

An electronic d/p cell and pressure transmitter device has been suspected of reading incorrectly during some high-pressure plant operations. The instrument supervisor sends her best technician during the next shutdown to diagnose the problem. The as-found condition of the pressure transmitter as recorded by the instrument technician has given the following values:

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What should the instrument technician report to the supervisor?

A. There is no apparent problem with the pressure sensor/transmitter.

B. The pressure sensor/transmitter is giving a non-linear response; replacement of the sensor and/or the transmitter will be required.

C. The pressure transmitter span has drifted; the pressure transmitter should be recalibrated.

D. The pressure transmitter zero has shifted; the pressure transmitter should be recalibrated.

CCST answer

The correct answer is B, the pressure sensor/transmitter is giving a non-linear response; replacement of the sensor and/or the transmitter will be required.

Response A (no apparent problem) would be appropriate only if the % Deviations calculated for the as-found conditions were all very close to zero (e.g., <0.01%).

Response C (span drift) would be indicated by % Deviations all increasing in the positive direction with a very small deviation (e.g., <0.01%) at the 0% value.  In addition, the % Deviation for increasing and decreasing input values would need to be very close in value, indicating hysteresis is not a problem.

Response D (zero shift) would be indicated by % Deviations of roughly all the same magnitude, either (+) or (-), including the 0% values. In addition, the % Deviation for increasing and decreasing input values would need to be very close in value, indicating that hysteresis is not a problem.

From the table of as-found values, the % Deviations cross from positive to negative several times, indicating a non-linear response. In addition, the as-found % Deviation values as the input signal is decreased are quite different from the values found with increasing input signal, indicating hysteresis in the output signal.  This may indicate the d/p cell is damaged or the pressure transmitter electronics are not functioning properly.

Reference: L.D. Goettsche; Maintenance of Instruments & Systems, 2nd Edition, ISA Press, 2003.