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Field calibration of instruments and analyzers used to be a simple, albeit time-consuming affair. No more.
Now new methods of field calibration feature paperless systems that start with calibration management software (CMS). CMS packages contain extensive data on each instrument, including calibration procedures. The CMS interfaces to a digital calibrator and downloads instrument data. The instrument technician then takes the digital calibrator to the field and calibrates each instrument. Calibration results are automatically uploaded to the digital calibrator from each instrument, and uploaded to the CMS.
Field calibration is faster and more accurate with digital calibrators and a CMS, but there’s a catch. CMS packages are expensive to install and maintain. Digital calibrators are also much more expensive than their older analog counterparts. Finally, instrument technicians need to be trained to work with these new systems.
“We use several handheld digital devices for configuring and calibrating instruments. We primarily use HART handheld communicators for configuration and calibration of field instruments. The HART handhelds do not source any signals for calibration; they are only used to aid a mechanic’s calibration of a transmitter,” says Ryan Hodges, the instrument supervisor at the Harmac Pulp Division of wood-and-pulp maker Pope & Talbot.
Hodges touches on the first layer of complexity. It usually takes two devices to calibrate an instrument, one to digitally transmit and record data and a second to source signals. Combined devices that perform both of these functions are available, but they are expensive and necessarily complex.
“We have a couple of Fluke HART-capable calibration/configuration devices that are used to calibrate ISO 9000 and ISO 14000 transmitters. The Fluke devices are capable of sourcing and measuring RTD/thermocouples, pressures and electrical current, voltage and frequency,” adds Hodges.
Fieldbus and digital network control systems present a new challenge to the technician. “Fieldbus instruments, either Profibus or Foundation fieldbus, have eliminated the analog output from the instruments that technicians formerly used to determine if the system had an accurate correlation between input and output,” says Jim Shields, Fluke’s product marketing manager for process calibration tools.
“Without an analog output, one option is to install all field instruments with a local indicator, apply the process variable and directly compare the value from the calibrator to the indicator. This option adds additional cost to each instrument purchased. Another method is to locally apply a process variable to the transmitter and get the output value back from the control room from an operator via radio. This approach is good in that the entire loop from process to control room is tested as a complete system, but it requires two people to perform the task. Lastly, a communication tool can be used to get the digital process variable from the instrument, but this requires intervention of the digital segment and often requires that the segment not be operational during the test,” concludes Shields.
The instruments are also more complex. “We use a laptop computer loaded with various vendor-specific software packages for advanced configuration of radar level transmitters, ultrasonic level transmitters, brightness transmitters and valve positioners,” reports Hodges.
The final layer of complexity is training. “All 15 of our instrument mechanics need to be trained on the software and calibration devices and receive significant hands-on time to retain training benefits. The calibration management software needs to be more flexible and allow manual entry of analytical measurements like pH,” according to Hodges.
Digital calibrators and CMS packages are worth their added complexity and expense. “Advantages to going digital are more accurate calibration records and quicker calibrations. For instance, the Fluke sources a simulated temperature, and then it measures the temperature transmitter’s 4-20mA output. It automatically ramps up and down through the temperature range of the transmitter while it logs the as-found measurement. After the calibration procedure, it logs the as-left values. The values are downloaded and stored in our CMS. The software can plot various statistics for analysis, and the records are also permanently stored for future reference,” concludes Hodges.
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