Instrumentation Selection: Getting it Right the First Time

A vital part of any control system is the field instrumentation that connects the system to the process. It is the input from temperature, pressure, flow and other transmitters that provides control algorithms with the necessary process information. Unfortunately, these important devices often do not receive optimal attention needed for a successful installation.




  • Process control & instrumentation

  • Specification software

  • Instrumentment specification

A vital part of any control system is the field instrumentation that connects the system to the process. It is the input from temperature, pressure, flow and other transmitters that provides control algorithms with the necessary process information. Unfortunately, these important devices often do not receive optimal attention needed for a successful installation.

This importance goes beyond the use of standard data sheets as the basis for instrument procurement. Functional requirements of modern instruments can be extensive. Specifiers (usually control engineers) must first determine what is required for the application, then specify that level of performance. To do this successfully, they must specify and document diverse parameters including measurement range, materials of construction, type of control system interface (4-20 mA or network), accuracy, and many others.

Understanding the importance of this type of documentation, ISA, the international society for measurement and control (Research Triangle Park, N.C), has established a standards committee on the topic, ISA SP20—Instrument Specification Forms. The committee aims "to promote uniformity in instrument specifications, both in content and form. Because of the complexity of present day instruments and controls, it is desirable to have some type of specification form to list pertinent details for use by all interested parties." ISA SP20 also defines five benefits in creating good specification sheets. These are:

  • Assistance in preparing complete specifications by listing and providing space for all principal descriptive options;

  • Promotion of uniform terminology;

  • Facilitation of quoting, purchasing, receiving, accounting, and ordering procedures by uniform display of information;

  • Providing a useful permanent record and means for checking the installation; and

  • Improving efficiency from the initial concept to the final installation.

Who needs them?

Developing these specifications also allows communication among involved parties, which helps secure the acceptance critical to instrumentation success. The information these sheets are designed to capture are needed by more than the potential instrument suppliers; good instrument data sheets are also needed by:

  • Process Hazards Analysis committees for hazard analysis, determining mechanical equipment integrity requirements, and management of change.

  • Quality control departments for determining calibration requirements and quality assurance of process variability. They are also needed to determine if instrument resolution and accuracy are adequate for required product specifications.

  • Maintenance departments for determination of component replacement scheduling and ordering and stocking of spare parts.

  • Engineering departments for determination of process and material requirements. They are also required for specifying accuracy and resolution of instruments, connection types, and communication specifications, and engineering standards.

  • Operations departments for determining instrument operational ranges and accuracy.

With procurement patterns changing, so is the importance of detailed specifications. Control Engineering 's 2000 Process Variable Transmitter Study ( CE , Jan.'00, p.51) found that less than half of control engineers responding preferred to buy sensor and transmitter as a package. Good sensor and transmitter data are required to assure compatibility and proper performance of components.

Instrument specification sheets are often neglected for many reasons. These include the tediousness of the task; the relative importance of each individual device—an individual transmitter is perceived as a minor investment when compared to the expense of other, more noticeable, control system elements; and the rush to get systems commissioned.

Errors or lack of clarity in spec sheets can result in instruments that do not function as required and/or fail prematurely; in either case the cost of ownership increases. Another potential result of incomplete or bad information is buying a more expensive instrument than was really necessary for the application.

Spec sheet tools

Standard format specification sheets can easily be created using any software package that supports form creation. Some of the software packages that can be used for this include Microsoft Word, Microsoft Excel, and Adobe Acrobat. Of course, many other packages can also be used including similar commercial offerings. Whichever package is selected, the user needs to determine the desired appearance, then create source forms for each type of instrument required.

As an alternate to creating forms from scratch, the SP20 Committee of ISA has developed a standard series of forms that can be used as a starting point. From these examples custom forms for an individual company can be developed, as can forms for any instrument type not included in the standard set. This standard set is available from ISA in both hardcopy and electronic format.

In addition to tools used to create specific instrument sheets, developers need to track creation of all sheets required for a project. A master list of all instrument numbers from the Piping and Instrumentation Drawings (P&IDs) is a requirement. As a minimum, this list should include the instrument tag name, purpose, and specification sheet ID, and revision number. For simplicity this master list needs to be maintained in a spreadsheet format (Microsoft Excel for example) or a database (Microsoft Access for example).

Lightweight catalogs

Most instrument vendors today offer product catalogs on a CD or web site. These listings generally allow searches for instrument types and provide basic product features, performance specifications, and physical specifications.

Their primary advantages over traditional catalogs are that less "physical" storage space is required—some paper catalogs are enormous; keyword searchabillity; confirmation of current specifications (primarily web site data that are frequently updated); and the ability to cut and paste desired portions into existing Word or Excel spec sheets.

Specing solutions

While it's impossible to look at the scores of examples available, it is helpful to look at several. Ask your suppliers about CD or web site specification options; find vendors a .

Endress+Hauser (Greenwood, Ind.) offers a compact disk titled "Flow Measuring Solutions." This Windows-based CD aids selection and sizing of flowmeters. It walks the user through meter selection. Meters included are orifice plate differential pressure, pitot tube, differential pressure, magnetic, Coriolis mass, vortex shedding, and thermal loss types.

Control engineers can select over 200 media types including pure fluids and some mixtures. A list of synonyms for the currently selected fluids and corresponding standard fluid properties are included. Among fluid properties are chemical formulas, critical constants, abrasiveness values, and conductivity, viscosity, and density values. The program provides warnings regarding meter selection based on these properties.

Once the fluid is selected, users can choose from a range of technologies and corresponding products. Product choice may be a specific technology or a display of all choices. As product choices are highlighted, users can request a product overview and technology summary. Products incompatible with the working fluid are automatically eliminated from the "specing" process.

Should the user insist on an incompatible device, an error box explains the incompatibility. The user is then prompted to set the normal operating conditions and meter size to complete the selection process. Once an instrument is fully specified, a printout summary becomes available allowing the user to enter relevant data into a standard instrument sheet format.

Calling the shots

Rosemount Measurement, a division of Fisher-Rosemount (Grand Prairie, Minn.), also offers software that assists with specification sheet development. Instrument ToolKit software package operates under Microsoft Windows 95, 98, or NT to produce standard format specification sheets for pressure transmitters, flow transmitters (differential pressure, vortex, and magnetic), temperature transmitters (RTD and thermocouple), level transmitters (differential pressure and radar gauge).

This application allows the definition of a project file that includes as many instruments as required for the given system. The instruments are sorted by type and then by user tag ID. This allows locating an instrument by inspection or by use of the tag search function provided.

Development of instrument specification is performed by a series of point-and-click selections. Another important feature is auto error checking, which alerts users to missing requirements and conflicting option selection.

To help resolve questions, Abode Acrobat format files from Rosemount Measurement's product catalog are accessible from the help key on each input page. In addition to product information, the help function provides technical summary pages appropriate for the instrument type under consideration.

The flow portion of this package is provided in a reduced form in the Rosemount Flow Sizing package ( CE , April '99, p. 106), including a wider range of technologies and a selection wizard. Among enhanced features are user-selectable engineering units for input data and a physical properties database for parameter calculations.

ISA-type specification sheets can be printed for all instrument types in this package. Prior to printing, these sheets can be edited directly or saved to Microsoft Excel and edited there. Having the end-product specification sheet in Excel has an advantage beyond accessibility. If another vendor's instrument is purchased, model number fields can be edited to reflect the alternate purchase.

Additional tools available

Many tools are available to individuals responsible for specifying control system instruments. While the capability of the software varies among vendors, it does not replace the need to carefully research and document actual system requirements. In addition, control engineers need to take care to update all instrument specification sheets to "as purchased" form to reflect changes in vendor and model number. Additional tools can be found on other company web sites.

Author Information

Tracy J. Coates, P.E., contributing editor, is a consulting engineer at PCE Engineering, Johnson City, Tenn.

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