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1 April 2002

Get with the program

By Richard Pirret

Paradigm shifts, users must stick to disciplined calibration program

With global competition getting more intense today, companies must change their approach to maintaining and calibrating instrumentation. Engineers and technicians in instrument maintenance departments face new challenges in both technology and procedure.

A desire for increased quality and control has fueled a drive toward improved instrument accuracy through regular calibration. Calibrators must be significantly more accurate than the instruments they calibrate. The accepted standard is that process calibrators be four times more accurate than the calibrated instrument (4:1 test uncertainty ratio). With better instrument accuracy, greater accuracy and resolution is needed in process calibrators.

As process instrumentation shifts from analog to digital technology, instruments are delivering multiple parameter capability, digital communication (HART, Foundation fieldbus, Profibus), better time and temperature stability, and lower failure rates. For the instrument shop, this adds up to longer intervals between calibrations, which reduces maintenance expense.

While the industry has always been sensitive to operational safety, the philosophy that has developed is that a well-calibrated plant is a safer plant. In addition to ensuring that instruments perform to specification, a sound maintenance program often detects and corrects noninstrument problems such as orientation effects, obstructed pressure lines, incorrect or defective thermocouple, and installation errors.

Field test

To limit downtime, the cost of instrument removal and installation, and the expense of spare instruments, the emphasis has shifted from in-shop to in-field maintenance. Errors due to temperature and humidity, lead configurations, ground loops, local power supplies, pressure sources, improper isolation, and electromagnetic interference/radio frequency interference sources are best detected and controlled by calibrating in the field.

The change to in-field calibration has been made possible by the emergence of highly accurate, field-tolerant process calibrators that deliver near-laboratory accuracy in field environments. These calibrations are not only more cost effective but also typically superior because they take into account the local environment.

The days of calibration only after an instrument has failed are gone. Users can avoid unplanned shutdowns and better control costs if maintenance becomes more proactive. By evaluating the performance of critical instruments over time, users can even predict out-of-tolerance performance and failures. This is not possible without regular collection of calibration performance data for an instrument.

Users should most frequently calibrate the instruments that have the greatest impact on product quality or plant safety. The time between calibrations, or "cal interval," should be as long as possible while still meeting performance needs, keeping costs as low as possible without impacting operation.

In preventive maintenance programs, users calibrate on a fixed time schedule, and the time interval lengthens or shortens as experience with a family of instruments dictates. However, using predictive maintenance, users precisely observe and record the performance of a particular instrument over time and adjust or replace that instrument only as its particular performance dictates.

Today, the most proactive instrument shops boast a ratio of planned maintenance calls to emergency calls of greater than 10:1. A proactive program, however, requires a disciplined calibration program.

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

• At regular intervals
• Using documented procedures
• With standards of suitable accuracy that are traceable to recognized standards
• While capturing and documenting the results

While the program's benefits are obvious, it can become burdensome to the company. To ease the burden of such a program, a company can adopt today's sophisticated documenting calibrators that automatically capture results, calculate errors, compare the errors to tolerance, and flag out-of-tolerance results. Relevant traceability data (i.e., tag ID, instrument serial number, day, date, time, and operator identification), make, model, and serial number of the calibrator are all captured.

More for less

In many plants today, maintenance is one of the last remaining opportunities for significant expense savings. Instrument shops feel this is a drive toward greater productivity.

Because of the need to work smarter, instrument shops are increasingly implementing instrumentation management software systems. These systems can address the thousands of instruments in a large plant by allowing the shop supervisor or lead technician to do the following:

1. Search and sort out your instrument population
2. Select items you want for maintenance or calibration
3. Preprogram the calibration procedures
4. Load procedures to a field calibrator
5. Unload results from a calibrator
6. Preserve the results data
7. Generate certificates and reports

To meet market demand, a much broader range of maintenance tools is available today. The anticipated application should dictate the choice. Along with a greater variety in maintenance tools comes a greater complexity in selecting the best tools for your applications. In evaluating new tools, look for these characteristics:

• Analog source/measure functions required by the application
• Capability to perform daily troubleshooting and maintenance tasks as well as calibration duties
• Conservative specifications adequate to cover the instrument under calibration
• Size, weight, durability, and reliability appropriate to in-field use
• Readily available calibration, service, parts, and support

Know your specs

The initial selection of a process calibrator is often based on a specification sheet - a written description of the equipment's performance in quantifiable terms that applies to all calibrators with the same model number. Any single calibrator would meet all of the specifications and usually significantly exceed most of the various specifications.

Specifications do not equal performance; they are performance parameters. Whether they are conservative or aggressive, manufacturers do not have to abide by any convention on how they present specifications. Reputable manufacturers will attempt to describe performance of their products as accurately and clearly as possible without hiding areas of poor performance by omitting relevant specifications. Some manufacturers specify their products conservatively, and their calibrators usually outperform their specifications. Other manufacturers manipulate specifications to make an instrument appear more capable than it really is.

Good specifications should be complete, be easy to interpret and use, and include the effects found in normal usage, such as environment and loading.

Completeness requires that sufficient information be provided so users can determine the bounds of performance for all anticipated outputs (or inputs), all possible and permissible environmental conditions within the listed bounds, and all permissible loads.

Ease of use is also important. Many specifications are confusing and difficult to interpret, thus causing mistakes in interpretation that can lead to application errors or faulty calibrations.

The challenge of specification design is to mutually satisfy both the requirement for completeness and the ease of use, which you can sometimes accomplish by bundling the effects of many error contributions within a useful and common window of operation. For example, "the listed performance may be valid for a period of six months when used in a temperature range of 23 ±5°C and in humidity up to 80% and for all loads up to a specified maximum rating." This is a great simplification for the user because the error contributions of time, temperature, humidity, and loads are included in the basic specification, and the user can ignore them as long as the device operates under these conditions.

Total accuracy package

A common specification for process tools is reference uncertainty. Reference uncertainty is the measurement of the performance of a device when the company manufactures it. Its purpose it to provide a reference specification that can be universally applied so buyers can compare tools. Unfortunately, these specifications typically apply over the shortest time interval and smallest temperature span and are sometimes a relative specification that comes from using a nonconservative confidence level.

Accuracy is always more than a single number. A better reference in comparing tools is to look at total performance or total uncertainty. Total uncertainty takes the following into account:

• Reference uncertainty. Short-term stability.
• Time. Periods of 30, 90, and 180 days are common; one or two years is a more reasonable usage period between calibrations. 
• Temperature. Make sure the temperature intervals specified match your working environment.
• Standards. Evaluation of uncertainty specifications must be relative or total. Relative uncertainty does not include the additional uncertainty of the reference standards used to maintain the calibrator. Total uncertainty includes all uncertainties in the traceability chain - the relative uncertainty of the calibrator plus the uncertainty of the equipment used to calibrate it.

A calibrator's performance must be better than what it tests. By examining all the specifications to understand a calibrator's total performance, technicians can choose the tool most appropriate for the application. The challenges instrument maintenance departments face may seem daunting at times, but application of contemporary process calibrators and supporting software permits these shops to step up to the demand for greater productivity while fulfilling the requirements of a disciplined calibration program. IT

Behind the byline

Richard Pirret, P.E., is business unit manager for process tools at Everett, Wash.-based Fluke Corp. His e-mail is rick.pirret@fluke.com. Specifications do not equal performance; they are performance parameters. Whether they are conservative or aggressive, manufacturers do not have to abide by any convention on how they present specifications.

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