Calibration Trends

Greg: An instrument is only as good as its calibration. Instruments have significantly improved, but are not perfect. Resources have dramatically declined. Mistakes can be made by the design engineer, vendor, manufacturer and the maintenance technician. Good calibration procedures and the leading tip "Always Ask Why" from 101 Tips for a Successful Automation Career can catch these mistakes.

Stan: Instruments are the only window into the process. Errors and incorrect ranges can result in inefficient and unsafe operation. We asked Glenn Gardner, Fluke product planner, to help complete our perspective on calibration by taking a look at future trends.

Greg: Stan and I enlisted the help of engineers who were proficient in the design, installation, checkout and start-up of electrical devices and got specialists to do the high-voltage stuff. We could focus on instrumentation. In the 1990s we saw the start of the decline of instrument engineers and technicians in the plants and the rise of groups for distributed control systems (DCS) and information technology (IT). Who is making sure the best instrument technology is used and maintained? Are the plant instrument engineer and technician becoming extinct?

Glenn:  On the engineering side, the workload is often split between an instrument engineer responsible for the instrument management system, and a controls engineer who focuses on the DCS configuration and performance. On the technician side, workforce reductions have caused a trend towards hybrid "E&I" techs that maintain both automation and electrical systems, including drives and switchgear. Overall, the trend is towards generalism: Anyone working in automation is expected to understand the basics of asset management systems, data historians, DCS hardware and software, fieldbuses, power distribution, MCC's, networks, wireless, etc.

Stan: How are the technicians adapting?

Glenn: The instrument techs seem to be able to pick up how to support the electrical and power systems. Electrical techs are having a tougher time learning how to calibrate, configure and troubleshoot instrumentation systems.

Greg: I saw a disturbing trend where the emphasis was becoming more on moving data around than getting better data from better instrumentation and process control or better presentation and analysis of the data by better operator interfaces and data analytics. We ended up with deluge of data, and not knowing how good the data was or what the data meant. Some IT departments thought they should be responsible for the generation and use of data.

Glenn:  The confusion seems to be lessening, with IT groups staying on the business technology (BT) side. It's noteworthy that the IT workforce tends to be fairly young, whereas the plant technician workforce in the USA and Europe is nearing retirement. I'm concerned about who will inherit the tribal knowledge required to maintain the plant systems. The inverse of this problem is present in emerging regions: There are plenty of young engineers and techs at process plants, but very few experienced mentors.

Stan: What are the trends in calibration practices from the decrease in resources and expertise?

Glenn: Spot checks (in-situ where possible) are used to make a one-point calibration. A zero adjustment is used to compensate for an offset between the transmitter output and a precision measurement at the transmitter (e.g., precision pressure gage near pressure sensor). A span adjustment requires the removal of the transmitter and a source to provide at least two operating values of the process variable measured.

Greg: The measurement errors seen in a control loop are more from offsets or zero shifts than from changes in span. Also, a zero adjustment can compensate for a span error for control at a setpoint. If there is poor control, the span error shows up as a change in process gain. Since there are much larger sources of nonlinearity, the span error is mostly a consideration when operating at vastly different setpoints. We take advantage of this understanding in pH measurements by taking a sample and using a lab meter at the pH measurement point to make an offset adjustment. To make span adjustments to account for changes in electrode efficiency would require removal of the electrodes and testing in buffer solutions, upsetting the equilibrium of the reference electrode. 

Stan: How does this method fit in with a documenting calibrator?

Glenn: Spot check results and the criticality of the measurement are used to help determine when a full calibration should be done with a documenting calibrator.

Greg: What are the choices for a calibrating a temperature transmitter?

Glenn: We have three ways of doing temperature transmitter calibrations. First, we can use a thermocouple millivolt or RTD resistance source to calibrate the temperature transmitter output. Obviously, this does not test the accuracy or condition of the sensor. Second, we can pull out the sensor and insert it into a temperature bath and perform a full calibration. This method is much more common when the calibration is performed in the shop rather than the field. Third, some thermowells have a slot for the direct insertion of a reference probe that can be used to perform a single-point calibration in-situ.

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.