It should come as no surprise that there are quite a few international standards that define how to determine instrument performance. The most significant of these standards are developed by a core team of volunteers within IEC SC65B’s Working Group 6 (WG6), “Testing and evaluation performance.”
The scope of WG6 is “to prepare methods of evaluating the performance of system elements and functions used in industrial process measurement, and control with special regard for harmonization.” It's presently responsible for 21 standards on this topic.
Standards developed by WG6 fall into three categories:
- Process measurement devices – IEC 61298 and IEC 62828 series;
- Final control elements – IEC 61514 series; and
- Control systems as the remainder of the series.
These documents can also be classified into those for “testing and evaluation” or for “inspection and routine testing,” with testing and evaluation documents being in the majority.
The committee is presently working on the “new” 62828 series to address requirements for digital measurement transmitters, while the 61298 series was created when analog devices were in the majority. So, though some content won't change, there's a risk of contradiction between the documents. For this reason, as soon as the 62828 series is complete, the 61298 series will be revised to ensure alignment.
Topics covered in the process measurement device series include:
- Specific terms and definitions;
- Typical configurations and architectures for the various types of measurement transmitters;
- Hardware and software aspects;
- Interfaces (to the process, to the operator, to the other measurement and control devices);
- Physical, mechanical, and electrical requirements and relevant tests;
- Performance specifications of required tests, including calibration traceability requirements;
- Environmental protection, hazardous areas application, functional safety, etc.; and,
- Structure of the technical documentation.
These standards address the issues associated with the devices proper. However, correct installation and process connections—including how the device connects to the associated I/O card—also play a critical role in overall system performance (but that's a topic for multiple other books.)
Where performance becomes especially important is in custody transfer, when ownership of a commodity changes, and coincidentally where royalties, taxes, etc., are collected.
To reinforce the importance of accuracy, let’s take the example of the Transmountain Pipeline running from Edmonton, Alberta, to the Port of Vancouver, British Columbia, with a capacity of 300,000 barrels/day. Assuming an accuracy of 0.1% of reading, the potential error is ±300 barrels, which at $75/barrel works out to $22,500/day or $8.2 million/year. Of course, because the error in theory is plus/minus it “should” average out over that time, but unless you're able to know for sure…
This is one reason why there are a range of industry standards from other organizations specific to the industry with domain expertise, such as the American Petroleum Institute for hydrocarbon liquids and American Gas Association for gases. The National Institute of Standards and Technology also provides guidance, and then there are government regulations, such as Alberta’s Directive 017, “Measurement requirements for Oil and Gas Operations,” and associated similar requirements for operations and safety from the U.S. Dept. of Transportation.
Meter performance, or at least accuracy, may not be critical within plant boundaries, where repeatability counts for closed-loop control. But once we cross the property line, money is involved, and accuracy, as defined by the standard—and traceable to an auditable source—becomes critical.
About the author: Ian Verhappen
