AutoQuiz: When Does an Instrument Need to Be Recalibrated?

AutoQuiz is edited by Joel Don, ISA’s community manager.

Today’s 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 information about the CCST program. This question comes from the Level I study guide, Domain 3, Troubleshooting. Level I represents a professional who has a five-year total of education, training, and/or experience.

An instrument calibration error comes to your attention. Your analysis of the calibration plot determines that __________ and the instrument does, in fact, need recalibration.

a) there is a zero error
b) there is a span error
c) there is both a zero and span error
d) there is a linearity error
e) none of the above

   

Click Here to Reveal the Answer

Zero error on an input/output calibration graph has an as-found line that is parallel to the ideal line. The zero error may produce a constant offset throughout the range of the instrument.

Linearity error is only a measure of the straightness of the line and not the slope of the line. A system with gross sensitivity errors can be linear.

Span error is the difference between the actual span and the ideal span. On an input/output calibration graph, span error has an as-found line that is not parallel to the ideal line. The slopes of the two lines are different. That is the case here, and the correct answer is B.

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