Stan: What are the relative merits of field and bench-top calibration?
Glenn: Field calibration tends to be more efficient, causes less process exposure and minimizes loop downtime. Bench-top calibration enables visual inspection of the primary element and allows the technician to use more advanced sourcing equipment such as temperature baths and pressure controllers. In addition, the bench-top model is useful during plant outages as it allows the most skilled calibration technician to focus on calibrating an uninterrupted flow of devices at the bench top.
Greg: Since pH electrodes are especially fragile, references may take hours to days to equilibrate, and aging and coatings are best detected by an increase in response time, the best practice is to do pH checks and tests at the measurement location. For a response time test, the electrode can be removed and inserted into the lab sample after the pH of the sample was verified. Response times greater than 20 seconds indicate significant aging or coating of the glass. The aging is not visually perceptible, and seemingly insignificant coatings of a few millimeters may not be obvious.
Stan: How is the data managed?
Glenn: The best practice is to store calibration records in a dedicated calibration management system (CMS). A CMS makes it easy to trend as-found and as-left data, track calibration assets, identify past-due calibrations, and audit previous work. About a third of existing process plants use this method. Alternatively, results can be stored in Excel, in the Enterprise Resource Planning (ERP) system, or on paper. I'm not a big advocate of these latter methods. Excel files are very prone to human error in data entry, and ERP or paper-based systems do not easily enable proactive tracking and trending.
Greg: What about instrument management systems?
Glenn: Instrument management systems (IMS) are particularly useful for tracking configuration data on field devices. For supported intelligent devices, automated diagnostics can also be sent. Configuration files can be created in advance and downloaded upon device installation, which uncouples the installation and configuration tasks. Both instrument and calibration management systems are often available as modules in a broader software system category known as an asset management system (AMS).
Stan: Progress has been made in making the AMS accessible on operator screens so the information is not held captive in an instrument shop, but a lot more needs to be done to use the ever expanding diagnostic and analysis capabilities.
Glenn: Several vendors are working on fleshing out the various modules of an AMS, including areas such as vibration, thermography, thermodynamic efficiency, RCM/FMEA analysis, machinery reliability metrics, etc. It's definitely an exciting area.
Greg: The AMS SNAP-ON application for smart instrumentation has already proved valuable, as noted in the May/June 2012 Plant Engineering and Maintenance (PEM) article "Refined Monitoring." What are some of the other trends developing from smart transmitters?
Glenn: Smart transmitters will lead vendors to increasingly include digital communication protocols in their field calibrators, making them useful for both commissioning and calibration workflow. Some plants will favor purchasing large quantities of a few standard smart transmitters which they can configure on-site to save costs, while others will lean towards having the OEM custom pre-configure each transmitter.
Stan: What can be done to minimize spares?
Glenn: The greater configuration flexibility of smart transmitters means that a spare can be used for many different applications. This can translate to fewer on-site spares and faster delivery of those spares by the supplier.
Greg: I see an opportunity to use wireless instruments to do spot checks that would automatically get the data into AMS. If an additional pressure, temperature or electrode process connection is provided, a wireless transmitter verified in the shop can be used as the precision measurement for the spot check. I particularly see the advantage of wireless for pH. Lab meters have the standard pH electrode temperature compensation (mv/degC), but generally do not have the unique pH solution temperature compensation (pH/degC) needed for each application stream. Wireless pH transmitters can also function as spares and lab meters for extended testing with process samples to determine life expectancy to find the best electrode technology and calibration schedule. For situations where sensors have limited life expectancy, or where a bad batch or false trip can cause a loss of several hundred thousand to millions of dollars, triple redundancy and middle signal selection should be used that can inherently ignore a single failure of any type. Finally, if your want to retire, use the online top ten list to make management think everything is wonderful.
Top Ten Ways to Impress Your Management with the Trends of a Control System
(10) Make large setpoint changes that will zip past valve dead band and local nonlinearities.
(9) Change the setpoint to operate on the flat part of the titration curve.
(8) Select the tray with minimum process sensitivity for column temperature control.
(7) Pick periods when the unit was down.
(6) Decrease the time span so that just a couple data points are trended.
(5) Increase the reporting interval so that just a couple data points are trended.
(4) Use really thick line sizes.
(3) Add huge signal filters.
(2) Increase the process variable scale span so it is at least ten times the control region of interest.
(1) Increase the historian's data compression so changes are screened out as insignificant.