Instrumentation and SCADA · Maintenance and Troubleshooting
What Is an Instrument Calibration Check?
A calibration check tests instrument response against a known reference.
The check may confirm the instrument or show that adjustment is needed. References must be in date, clean, and handled correctly. The plant procedure sets acceptance limits.
A standard solution is expired. The operator replaces it before deciding whether the sensor passed or failed.
A reference is only useful when its value is known and suitable for the instrument range. Reference standards should be traceable to NIST, and a multipoint check commonly tests 0, 25, 50, 75, and 100 percent of range going both up and down, so lag between rising and falling readings (hysteresis) shows up. Protect standards from heat, dirt, evaporation, and cross-contamination. Allow the sensor and reference to reach the required condition before judging the result. If the check fails, repeat only as the procedure allows. Repeating a test until it passes can hide a real problem. Tag or remove an unreliable device when required so others do not use its value unknowingly.
Do not repeat failed checks until one happens to pass.
Key point: References must be in date and clean. Repeating a failed check until it passes can hide a real problem.
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Wastewater Instrumentation, SCADA, and Process Control Fundamentals
This page comes from the course. It covers plant instruments, signals and control, flow, level, pressure and water-quality measurement, SCADA and alarms, and instrument maintenance, one step at a time, with short knowledge checks and a certificate of completion.
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