Tutorial: Digital fieldbus device calibration

-- Control Engineering, 6/4/2008

Houston, TX – Sure, analog devices drift and need recalibration, but digital? Yes, while the frequency of required calibration is less, even digital temperature transmitters, pressure transmitters, and other digital network devices connected to FOUNDATION fieldbus and Profibus PA networks need attention.

Bruce Campbell, with Beamex, performs a calibration adjustment on PT-101 on a Mobile FF Cart.

“Why is calibration needed?” asks Hakon Dybwad, field service engineer, New England Controls. Electronics drift with temperature extremes, he says. Keeping transmitters calibrated is especially important with custody transfer requirements, the need to decrease outages and augment efficiencies, and compliance with regulations and ISO 9000, ISO 14,000 initiatives. While conventional analog transmitters do not provide much information, Dybwad says, fieldbus transmitters using digital two-way signals provide a lot of information about the process and about the sensors and logic interacting with process. 

Dybwad gave an end-user presentation on fieldbus device calibration at a Foundation Fieldbus Users Forum in Houston, TX, April 10, and provided more information and photos to Control Engineering after the meeting. Read more advice and learn about the 2008 series of free Fieldbus Foundation presentations. 

When trimming a device using the Beamex MC5 calibrator, the user takes device to out of service mode, selects “User Trim Mode” vs “Factory Trim” and proceeds with the trim.

At the meeting Dybwad showed how a Beamex MC5 Documenting Process Calibrator and FI-5 (Fieldbus Interface Module) can be used. He says the Beamex calibrator is the only one that can read the output of a Foundation Fieldbus device digitally and calibrate it, in one step, without tying up two or more individuals. (Using another interface module, the calibrator also can calibrate Profibus devices, though that wasn’t discussed at the Foundation Fieldbus meeting.) 

In time Fieldbus transmitters must be calibrated to ensure they’re generating accurate process output readings. Devices cannot be tested remotely. At the FOUNDATION fieldbus device, the calibrator reads the device’s digital output values, documents the readings, and allows the operator to adjust the devices output if needed. Until recently, two people were required, one at the device and the other in the control room; a method prone to inaccuracies and erroneous results, Dybwad says. The calibrator simultaneously simulates inputs and reads the digital output of a FOUNDATION fieldbus instrument. 

With a device trim using Emerson’s AMS Device Manager or Emerson’s DeltaV, fields pop up in sequence for the user to enter relevant information (see screen shot, next).

When the name of person performing the test or trim is stored in the device, it can be retrieved with, for instance, the Emerson AMS Device Manager or Emerson DeltaV. The fields Snsr Cal Date, Snsr Cal Loc, and Sensor Calibration can be written (see photos), but these tasks are not part of the trim routine and require diligence to take the extra step(s) by accessing Transducer Block Information and scrolling up or down to update the respective parameter fields, he said.
In the view and edit functions, the user connects, tests, documents automatically, and trims if required. Upon connecting the calibrator to a segment, it scans for devices. After they appear, select the device, the calibrator takes a reading. If the device descriptor (DD) for a particular device does not reside in the calibrator, the DD for the device can be downloaded from the calibrator vendor (Beamex) or device manufacturer; and loaded into the calibrator , so that the calibrator can communicate with the Foundation Fieldbus device in question. 

Device tests can be copied from a similar device test residing in the calibrator and modified as appropriate to facilitate quick and efficient field testing of the specific desired devices in question. The device must be taken out of service mode using calibrator before desired adjustments to the device output in question can be made. Upon test completion, the device must be placed back in service mode.

User configurable information includes date, time, calibrator location, and person name; that information corresponds with the following fields: Snsr Cal Date, Snsr Cal Loc, and Sensor Calibration in image.

Some suggestions on calibration (device must be out of service):
-Common sense should help decide what to do when. For instance, if an instrument is known to be accurate plus or minus 2% or 3%, and the process only requires plus or minus 5% accuracy, why check it? Go somewhere else.
-Press the device information key. Review the test scheme. To determine hysteresis errors 9 point tests are necessary. Unless testing for hysteresis errors Dybwad recommends 5 point tests. In the case of 0-100 degree C test, ensure that output range and units match the input range and units. Pressing the edit key allows full review.
-Start the test away from 0 if you’re doing a square root test. 
-Set point delay depends on the filtering delay in the device; it is typically 10 times the value defined in device;
-Automatic temperature device test: To simulate input for the first point and capture device output digitally, press the start key. Force accept (indicates that calibrator will automatically accept each point after the predetermined delay has lapsed). Save as found. Make adjustments using pull down options. Unless you want to restore factory settings, select the user trim method. The calibrator asks if you want to send a value. Check any value. If the answer is acceptable, end the adjustment. Test results may be saved, and calibration certificate made, with notes added, if needed. If calibration was OK as it was, hit the save as both key.
-In viewing calibration results, the graph shows maximum errors. The displayed trend indicates whether or not the device under test needs adjustment.
-Parameters where the manufacturer will not allow changes are grayed out.
-Return the device to service. 

Orderly calibration or repairs are preferred rather than have a device cause an unexpected outage, Dybwad says. Copies of an instrument’s information may be made, edited, and copied to the next device, if needed, saving time and reducing opportunities for typing errors. 

The test described above typically takes a half hour for each temperature device when the calibrator simulates temperature element input values. Calibration takes perhaps 2 hours per device when the temperature element is placed in a temperature bath during testing, he says. 

Beamex calibrators are in use in many locations with digital fieldbus installations, Dybwad says. Foundation Fieldbus instruments are deployed widely, he adds, including Drax Power Station, U.K.; Port Westward combined cycle plant, U.S.A.; Scarab Saffron Project, Alexandria, Egypt; a Chile pulp and paper mill; and in various deep sea oil rigs.

– Mark T. Hoske, editor in chief
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