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Fieldbus transmitters must also be calibrated

31 March 2008

The main difference between a fieldbus transmitter for pressure or temperature and conventional or HART transmitter is that the output signal is a fully digital fieldbus signal. The other parts of a fieldbus transmitter aren’t any different from conventional transmitters. Changing the output signal does not change the need for periodic calibration. Although modern fieldbus transmitters have been improved compared to older transmitter models, it does not eliminate the need for calibration.

Beamex fieldbus calibrator
Beamex fieldbus calibrator

There are also many other reasons for calibration, such as quality systems and regulations, that make the periodic calibrations compulsory.

CALIBRATING FIELDBUS TRANSMITTERS

The word ‘calibration’ is often misused in fieldbus terminology. In the world of fieldbus, ‘calibration’ is often used to mean the configuration of a transmitter. In metrology, ‘calibration’ means that you compare the transmitter to a traceable measurement standard and document the results.

In many cases, calibrating a fieldbus transmitter can be cumbersome, time-consuming and may require an abundance of resources.

For example, it is not possible to calibrate a fieldbus transmitter using only a configurator or configuration software, and it is not possible to calibrate a fieldbus transmitter remotely.

Fieldbus transmitters are calibrated in very much the same way as conventional transmitters: you need to place a physical input into the transmitter and simultaneously read the transmitter output to see that it is measuring correctly. The input is measured with a traceable calibrator, but you also need to have a way to read the output of the fieldbus transmitter. Reading the digital output is not always an easy thing to do.

When fieldbus is up and running, you can have one person in the field to provide the transmitter input while another person is in the control room reads the output. Naturally these two people need to communicate with each other in order to perform and document the calibration.

While your fieldbus and process automation systems are idle, you need to find other ways to read the transmitter’s output. In some cases you can use a portable fieldbus communicator or a laptop computer with dedicated software and hardware.

A NEW FIELDBUS CALIBRATOR

Beamex’s MC5 Fieldbus Calibrator is the solution to the problem: it can be used for calibrating Foundation Fieldbus H1 or Profibus PA transmitters. The instrument is a combination of a multifunction process calibrator and a fieldbus configurator.

The most important advantage of the MC5 Fieldbus Calibrator is the possibility to calibrate, configure and trim the Foundation Fieldbus H1 or the Profibus PA transmitters using a single unit. Fieldbus configurators and configuration software are not able to do this; they can only be used to read/change configurations. Calibration can therefore be performed by one person instead of two.

The MC5 simultaneously generates and measures the transmitter input and also reads the digital fieldbus output of the transmitter. It can also be used to change the configurations of a fieldbus transmitter. If you find that the fieldbus transmitter fails in calibration, you can also use the instrument to trim/adjust the fieldbus transmitter to measure correctly. Being a documenting calibrator, it automatically documents the calibration results in its memory, from where the results can be uploaded to calibration management software. This eliminates the time-consuming and error-prone need for manual documenting using traditional methods.

The MC5 includes an integral power supply for powering up a stand-alone transmitter during calibration. Therefore, the MC5 can also be used during commissioning when the fieldbus and control systems are still idle.


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