Field or Workshop Calibration?

Is it better to calibrate process instruments in the field or in a workshop, i.e, on a bench?

That’s a question frequently posed to experts with Beamex, Marietta, GA (beamex.com). According to a blog post by Heikki Laurila, the correct answer depends on various factors. He offered the following arguments for these different types of calibration approaches.

FIELD CALIBRATION

• The field approach (the most common method) ensures that instruments are calibrated under actual operating conditions. It also can be more effective than workshop calibration, assuming that the field instruments have been designed and installed for easy access.

• If you want to calibrate a complete measurement loop in one go, from the measurement point in the field to the control room display, you’ll need to go out in the field to start from the process sensor/transmitter. If the total loop has too big of an error, then the individual parts of the loop need to be calibrated separately.

• If you don’t want to remove—or can’t remove—an instrument from its installed location, field calibration is, essentially, your only option. 

WORKSHOP CALIBRATION

Sometimes, actual field conditions can be very challenging/harsh. In those cases, workshop calibration is more appropriate. Plus, in a dedicated workshop, all required equipment is typically at hand and ready for use. While usually less efficient, in the long run, workshop calibration can be more ergonomically friendly, not to mention more convenient, than field calibration.

If you calibrate instruments during the commissioning phase, i.e., before they are installed in the field, it’s more convenient to do so in a dedicated calibration workshop.

If your requirement is “best possible uncertainty,” it’s often easier to get better total uncertainty in a workshop than out in the field. This is achieved using dedicated stationary, high-accuracy calibration equipment and controlled environmental conditions and processes.

If you use rotating spares, or want to calibrate loose spare devices before installing them in the field, doing so in a workshop is the most practical approach and can offer the same efficiency gains. EP

Heikki Laurila is a frequent blogger for Beamex Oy Ab (Pietarsaari, Finland). Read more of his posts at blog.beamex.com.