Although instrument accuracy and stability ause of drift and tolerance," says Louis Szabo, vice president of marketing and sales, Meriam Process Technologies (www.meriam.com). "All field devices drift, whether via mechanical or electrical means, and drift beyond a specified tolerance range necessitates calibration."
Calibration frequency is subject to a number of variables including manufacturer recommendations, the environment, and the critical nature of the measurement. A manufacturer may recommend annual testing and/or calibration for a given device. However, the environment a device is subjected may accelerate drift.
Devices that are critical to a process will typically have tight tolerances and are often exposed to environmental extremes, leading to an increased calibration frequency. Non-critical devices usually have broader tolerance ranges and need annual or even less frequent calibration cycles.
For calibration of field devices, many companies have adopted a 4:1 accuracy ratio between the measurement standard and the unit under test (UUT). Stated differently, the measurement standard must be four times as accurate as the UUT for the given range. The range of the measurement standard should be as close as possible to the range of the UUT.
The measurement standard must also be tested for accuracy, and this is typically accomplished with laboratory-grade instruments. These instruments operate under controlled environmental conditions and are highly accurate.
The last link in the calibration chain for process plants in the U.S. is the National Institute of Standards and Technology (NIST). "For a calibration to be useful, it must be traceable to the international system of units (SI)," says David Deaver, metrology manager, Fluke (www.fluke.com). "This is usually done through an unbroken chain of calibrations eventually traceable to NIST."
Although it is technically feasible for a process plant to maintain a battery of laboratory-grade test and calibration equipment directly traceable to NIST, this is seldom done in practice. Instead, most process plants rely on self-declared NIST compliance from test equipment and calibration suppliers.
Suppliers typically self-declare the standards to which they comply in certificates provided with their test and measurement equipment. This self-declaration is sufficient for some users, but many others need more assurance of compliance with NIST. These assurances are often a requirement imposed upon end users by regulatory agencies.
Assurance of NIST compliance can be obtained through a detailed audit of supplier testing labs, but audits are very costly and time-consuming for both the user and the supplier. Audit costs can be reduced if a group of firms selects one representative to conduct a supplier audit. This is more cost-efficient than individual audits performed by each company, but there is a new approach that eliminates the need for audits.
Independent laboratory accreditation is growing rapidly in the U.S. to address the issue of laboratory compliance with NIST. "This practice actually began about a decade earlier in Europe," comments Deaver. "An independent accrediting body assesses the laboratory and issues a certificate of accreditation attesting that the laboratory complies with the standards claimed."
But this creates a new dilemma for the end user; namely, the integrity of the independent laboratory accrediting body. The International Laboratory Accreditation Cooperation (ILAC, www.ilac.org) was formed to address this issue. ILAC prescribes the process accrediting bodies must use if they are to be internationally recognized.
There are too many accrediting bodies worldwide for ILAC to effectively manage, so the cooperative now recognize regional bodies that oversee the accreditors. In the U.S., there are about a half dozen of these accreditors. According to Deaver, the National Voluntary Laboratory Accreditation Program (http://ts.nist.gov/ts/htdocs/210/214/214.htm) and the American Assn. for Laboratory Accreditation (www.a2la2.net) are the largest, and both have achieved ILAC recognition.
According to ILAC, laboratory accreditation provides a means of determining the competence of laboratories to perform specific types of testing, measurement, and calibration. It enables people who want a product, material, or instrument to be checked or calibrated to find a reliable testing or calibration service able to meet their needs.
It also allows a laboratory to determine whether it is performing its work correctly and to appropriate standards. Manufacturing organizations also may use laboratory accreditation to ensure that testing of their products by their own in-house laboratories is being done correctly. Laboratory accreditation provides formal recognition to competent laboratories, thus providing a ready means for customers to access reliable testing and calibration services, says ILAC.
ILAC aims to provide users with a method to ensure NIST compliance from instrument and calibration laboratories. This will help end users ensure a robust traceability path from their field devices to established units of measure.
E-mail Dan at [email protected].
