A common misconception is that the accuracy and stability of HART instruments eliminate the need for calibration. Another misconception is that calibration can be accomplished by re-ranging field instruments using only a HART communicator. Still another misconception is that the control system can remotely calibrate smart instruments. These are not true. All instruments drift. Re-ranging with just a communicator is not calibration.
A precision calibrator or standard is required. Regular performance verification with a calibrator traceable to national standards is necessary due to:
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Shifts in performance of electronic instruments over time, due to exposure of the electronics and the primary sensing element to temperature, humidity, vibration, and other field environmental factors.
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Regulations governing occupational safety, consumer safety, and environmental protection.
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Quality programs such as ISO 9000 standards for all instruments that impact product quality.
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Commercial requirements such as weights, measures and custody transfer.
A calibration procedure consists of a verification (As Found) test, adjustment to within acceptable tolerance if necessary, and a final verification (As Left) test if an adjustment has been made. Data from the calibration is collected and used to complete a report of calibration, documenting instrument performance over time.