Trends in process calibration

Key concepts Process calibrator accuracy must be greater than the instrument it measures.

By Richard Pirret, PE, Process Tools, Fluke Corp., Everett, WA
12/01/2000


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Process calibrator accuracy must be greater than the instrument it measures.

Instruments that have the greatest impact on product quality or plant safety should be calibrated most frequently.

The drive for greater productivity brings instrumentation management software systems into play.

The internet is changing how information is gathered and purchases are made. Wireless phone technology is altering communication.

As the economy becomes global, competition increases. Cost management is critical and reducing downtime is a key factor in managing these costs. To maximize instrument maintenance cost effectiveness, plant engineers should be aware of several trends.

Accuracy and performance

The desire for increased quality and better process control fuels the drive toward improved process instrument accuracy. Process calibrators must be significantly more accurate than the instruments they calibrate.

Today, a test uncertainty ratio of 4:1 is commonly accepted. As a result, greater instrument accuracy creates a need for higher accuracy and resolution in process calibrators.

As process instrumentation shifts from analog to digital technology, instruments are delivering multiple-parameter capability, digital communication, better time and temperature stability, and lower failure rates. For the instrument shop, these factors mean longer intervals between calibrations and a resultant reduction in maintenance expense.

Safety and maintenance

While the process industries have always been sensitive to operational safety, an appreciation is developing that a well-calibrated plant is a safer one. In addition to assuring that instruments are performing to specifications, a sound maintenance program often detects and corrects noninstrument problems, such as orientation effects, obstructed pressure lines, incorrect thermocouple types, and installation errors.

The desire to limit process downtime, cost of instrument removal and installation, and expense of spare equipment drives the shift from "in-shop" to "in-field" maintenance. This shift has been made possible by the emergence of highly accurate, field-tolerant process calibrators that can deliver near-laboratory accuracy in field environments.

Not only are these calibrations more cost-effective, they are often superior to shop calibrations because they take into account the local environment. Errors due to temperature and humidity, lead configurations, ground loops, improper isolation, and EMI/RFI sources are effectively detected and controlled by calibrating in the field.

Proactive maintenance stance

The days of calibration only after an instrument fails are gone. Unplanned shutdowns can be avoided and costs controlled better if maintenance becomes more proactive. The instruments that have the greatest impact on product quality or plant safety should be calibrated most frequently.

Time between calibrations, or "cal interval," should be as long as possible and yet still meet performance needs. Under preventive maintenance, calibration is done on a fixed time schedule and the time interval is lengthened or shortened as experience with a family of instruments dictates.

Under predictive maintenance, the performance of a particular instrument is precisely observed and recorded over time. Then adjustments or replacements of that instrument are done only as its particular performance dictates.

Today, the most proactive instrument shops proudly point to a ratio of planned maintenance calls to emergency calls of more than 10:1. However, a proactive program requires a disciplined calibration program.

Rigorous calibration and documentation

Quality programs, such as ISO 9000, and environmental, occupational safety, or consumer protection regulations, are now requiring complete records of instrument maintenance. A rigorous calibration program ensures all instruments that impact product quality are calibrated:

  • At regular intervals

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      • Using documented procedures

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          • With standards of suitable accuracy traceable to recognized standards

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              • While capturing and documenting results.

                • The burden of such a program is eased by the adoption of today's documenting calibrators that automatically capture results, calculate errors, compare errors to tolerance, and flag out-of-tolerance results. Relevant traceability data is captured-tag ID, instrument serial number; operator identification, and day, date, and time-as well as the make, model, and serial number of the calibrator used.

                  Emphasis on productivity

                  In many plants today, maintenance is viewed as the last remaining opportunity for significant expense savings. The instrument shop believes this view is a drive for greater productivity. Instrument shops are working smarter, as well as harder. They are often bringing instrumentation management software systems into play. These systems can address the thousands of instruments in a large plant by allowing the shop to: