Calibration


Incorporating metrology fundamentals in calibration


Chuck Boyd, Beamex, discusses some critical items to address for a calibration programme based on sound metrology fundamentals without a complete overhaul of that programme


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mplementing metrology-based principles does not have to be a dramatic change. Metrology as a science has an immense


number of elements to consider, but initially focusing on the following areas will provide huge strides in building and maintaining a calibration programme that provides confidence in measurement accuracy for process control instrumentation: measurement tolerance and pass/fail determination; test strategy including hysteresis; maintaining acceptable test uncertainty ratios; and securing information assets.


MeasureMent toleranCe and pass/fail deterMination The calibration tolerance assigned to each instrument is the defining value used to determine how much measurement error is acceptable. This subject is one that should rely heavily on the requirements of the process and not by looking at what the instrument is capable of performing. Ideally, the tolerance is a parameter that is set in process development where the effect of variation is measured. Unfortunately, there is no hard-and- fast formula for developing a tolerance value, it should be based on some combination of the process requirement, manufacturers’ stated accuracy of the instrument, criticality of the instrument and intended use. Care should be taken not to set a range too tight as it will put pressure on the measurement to be unnecessarily accurate. Tolerance can be stated as a unit of measure,


percentage of span, or percentage of reading. It is critical during calibration to mathematically calculate the error value in order to determine pass/fail status. This calculation is an additional step in the process and particularly with tolerances defined as a percentage of span or reading, mathematical calculations invite opportunities for errors. As calibration programmes evolve this aspect of the calibration process will get relegated to the calibration technician’s discretion. There have been occasions where the decision on pass/fail is relegated to technician experience, or gut feeling. This is a practice that provides questionable results and although the resulting calibration certificate may show the measurement is within tolerance, the instrument is recorded as within tolerance in perpetuity when in fact this result was not mathematically confirmed. More importantly, plant operators could be making


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decisions based on wrong data. This method of determining pass/fail should not be allowed and enforced either procedurally, which should require recording of the error limits as well as the calculated error, or enforced programmatically, having the inputs entered into a computer-based system where the pass/fail is indicated automatically.


test strategy inCluding Hysteresis Hysteresis errors occur when the instrument responds differently to an increasing input compared to a decreasing input and is almost always caused by mechanical friction on some moving element. These types of errors rarely can be rectified by simply making calibration


adjustments and typically require replacement of the instrument or correction of the mechanical element that is causing friction against a moving element. This is a critical error due to the probability that the instrument is failing. Most calibration test strategies will include a test


point at zero and a test point at span, and typically is at least another test point at mid-range. This three-point test can be considered a good balance between efficiency and practicality during an outage. The only way to detect hysteresis is to use a testing strategy that includes test points up the span and test points back down the span. It is critical that the technician does not overshoot the test point and reverse the source signal, approaching the test point from the wrong direction. Technicians should be instructed to return to previous test point and approach the target point from the proper direction.


Maintaining aCCeptaBle test unCertainty ratios Measurement uncertainty is an estimate of the error associated with a measurement. In general, the smaller the uncertainty, the higher the accuracy of the measurement. The uncertainty of the measurement standard (i.e. – calibrator) is the primary factor considered along with potential errors introduced in the calibration process to get


September 2018 Instrumentation Monthly


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