A multifuntion calibrator is most often used when the workload of the technician is unpredictable – the technician knows the work involves parameters that may deal with temperature, pressure, and electrical, but showing up at the jobsite with insufficient equipment means lost time and could be costly.
Multifunction calibrators, as their name implies, combine the 4 to 20 mA function with the following parameters: RTD, T/C, voltage, frequency, pressure, etc. They are reasonably ergonomic, depending on the manufacturer, and their functionality is more than any technician would need to get the job done. There are models that have add-on pressure modules, some that actually generate pressure via an on board compressor, or by a built-in hand pump.
Multifunction calibrators (MFC’s) are sold with different configurations depending on the application. Some units will store calibration ‘as-found and as-left’ data in the field that can be downloaded through a built-in RS232 or mini-USB port. Software to manage the calibration data is usually available for this type of calibration system. The industry refers to these models as “Documenting Process Calibrators”.
Some MFC manufacturers have gone the distance in offering maintenance features that combine not only the multiple features of temperature, electrical and pressure, but have integrated HART communications capabilities that give you the ability to calibrate and maintain HART instrumentation. The majority will support the most popular HART transmitters, and are easily updated as more instruments are added and new HART instruments are released. The HART library updates for MFC’s are usually available to the end user once every 12-18 months, as it is more cost-effective to ‘batch’ the updates provided by the MFC manufactuers. Sometimes the release of these updates is not in sync with the transmitters already operating in the field. For a complete up-to-date library, one may also look at the HART Communicator itself as the ‘defacto standard’ for communication capabilities.
What are some of the other features that you could look for in a multifunction calibrator? Many units include features like limit-switch testing, leak detection, transmitter mode simulation, data logging to name just a few. When selecting an MFC, look for a model that uses a NiMH battery for longer battery life or for an AC adapter if you are doing a lot of bench-top calibration. Many calibrators have energy-saving modes that provide a longer battery life and that only require “AA” batteries.
Even though it looks like we can “have it all” in a multifunction calibrator, there are some drawbacks that should not be overlooked. The initial cost associated with any multi-function calibrator must be considered, as there will be added expense in adding modules, HART libraries, calibration cost, and software to upload and download your stored data. When it comes to performance, MFC’s can accommodate many different functions, but the accuracy and resolution may not be as good as you would see in a single-function calibrator designed for the same job function. Engineering the identical performance of a single-function calibrator into an MFC would drive the cost of the calibrator out of the reach of many maintenance shops. There definitely is a trade-off when choosing one over the other: cost/convenience vs. accuracy/repeatability. One more item that is worth considering; the user-friendliness of the calibrator is not always taken into consideration when we are evaluating the unit for use in the Maintenance Shop. Remember…more features usually require many more key strokes. Note: all calibrators are not created equal!
Selecting the right tool for the right job requires a proper evaluation which may involve a detailed review of the specifications as well as a hands-on trial period of any calibrator being considered for shop use by a team of Instrument Technicians.
And remember this…a well-calibrated plant is a safe plant!
Written by: Stephen J. Kelly, B.Sc., C.E.T., Cameron Instruments Inc.
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