Device Calibration Trim and EDDL

In my email this morning was the monthly Electronic Device Description Language (EDDL) digest. It’s one of many ISA email discussion lists. Emerson’s Jonas Berge is the driving force behind the EDDL educational information that comes out in this email list and on the website.

One of the items in the email was a mention of an updated section to the EDDL website, Intelligent Device Management. I took a closer look at one of the sections in this category, Calibration Trim. Here are a few points I gleaned from the page and one of the linked whitepapers, Intelligent Device Management: Calibration.

Process manufacturing facilities have hundreds to thousands of field devices including measurement instrumentation, analyzers, and final control elements such as valve positioners. It’s quite a bit of work to setup & configure, calibrate, and perform ongoing troubleshooting and maintenance tasks on all of these devices.

Technology has helped as these field devices have incorporated microprocessors and digital communications protocols such as HART, Foundation fieldbus, and Profibus to connect these devices with asset management and process automation systems. These technologies have made centralized management of all these devices possible.

International standard EDDL (IEC 61804-3) provides a common language for device graphics and procedural wizards to provide a common user experience into these devices from software such as the AMS Device Manager and handheld devices such as the 475 Field Communicator across these various digital communication protocols.

I was casually familiar with device calibration trim, but the whitepaper helped to describe its components: sensor trim, range setting, and current trim. Sensor trim addresses drift which may occur in the transmitter’s sensing device–although modern transmitters are extremely stable over time. An accurate physical input (pressure, level, temperature, etc.) is applied to the sensor. From a handheld device or software screen, the zero, lower, and upper sensor trims can be set.

For HART devices that have a hybrid analog and digital signal, the range setting translates into the analog 4-20mA current range. For instance, a pressure transmitter that is ranged from 0-1000psig has the 4mA output when the sensor measures 0psig and 20mA when the sensor measures 1000psig. The span of the transmitter is 1000psig. For purely digital protocols such as Foundation fieldbus and Profibus, the range is set in the controller. The whitepaper mentions differential pressure (DP) flow and level as an exception.

Current trim is limited to HART devices with their 4-20mA current signals. Current trim adjustments can be performed in the very rare case where the output current circuitry within a transmitter drifts.

EDDL wizards simplify the calibration trim process. Calibration tasks are guided by wizards to perform: sensor zero trim, sensor lower trim, sensor upper trim, lower range value set, upper range value set. For HART devices, 4mA current trim and 20mA current trim wizards assist in the calibration process. The wizards reduce opportunities for technician error and help enforce consistency.

The EDDL standard also supports instructional and procedural documentation, historical device audit trails, and scheduled maintenance tasks. Given the hundreds to thousands of field devices within a typical plant, this common approach from EDDL helps to simplify the process from setup and calibration through lifecycle maintenance.

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