Correct instrument readings are crucial for safety, product quality, regulatory compliance and optimum operation of process plants. Ensuring the hundreds, or even thousands, of sophisticated instruments on a site perform accurately and reliably requires regular inspection, testing, calibration and repair.
In addition, extensive industrial experience has established the value of carefully recording the details of those inspections, tests, calibrations and repairs. More than a best practice, such highly specific and accurate recordkeeping often is necessary to prove that full value is delivered to customers and the health and safety of citizens are protected.
However, traditional testing, calibration and documentation practices are labor-intensive. Fortunately, new methods and technology coupled with best practices can reduce the overall cost of performing calibrations and producing documentation while providing additional productivity and operational reliability benefits.
Calibration may take place where the device is located or in an instrument shop. It involves checking and adjusting, as needed, components of the device.
Most process instruments consist of two parts: a primary element and a transmitter. Primary elements include flow tubes, orifice plates, pressure sensors, wet chemistry sensors such as pH, ORP and conductivity probes, level gauges of all types, temperature probes and others. A primary element typically produces a signal — usually voltage, current or resistance — that is proportional to the variable (e.g., level, flow or temperature) it is designed to measure. The generated signal serves as input to a field transmitter that processes the signal — first characterizing it in linear format and applying engineering unit coefficients to it, before transmitting it in analog (usually 4–20-mA dc) or digital format (via some variety of fieldbus).
Analog and HART devices. Analog devices — often called “4–20-mA loop” devices — transmit a signal that is an electrical-current analog representation of a measured physical quantity (temperature, for example). The transmitted current is proportional to the magnitude of a measured physical quantity, with 4 mA representing the minimum scaled value and 20 mA the maximum scaled value. HART (Highway Addressable Remote Transducer) smart devices operating in the analog output mode function similarly.
Digital devices. These devices convert a measured physical value into a digital signal. The process industry uses many different digital-encoding methods, including Foundation Fieldbus, Profibus and HART.
Analog devices remain common at process plants. However, deployment of digital devices is increasing strongly. Here, we’ll focus on field calibration of digital devices.
There’s a widespread — but erroneous — belief that digital field devices don’t require calibration. Although a fieldbus signal (whether Foundation Fieldbus, Profibus or connected HART) gives diagnostic information, it doesn’t provide information about the accuracy of the device nor does it verify that the device is reporting the process accurately and precisely. Most, if not all, smart devices have an input analog/digital converter that requires occasional adjustment; for HART smart devices in the analog mode, the output mA signal needs checking and periodic adjustment.
Control valves. These valves have actuators that require calibration to adjust for wear, repacking of the valve for leakage remediation and the effects of stiction. Often, to check for dependable operation, these valves must undergo a full or partial stroke test if actuation doesn’t occur regularly. Adjustment ensures the position of the valve matches that mandated by the control system.
Administrative tasks, from getting permits to documenting and filing results, can add to the cost and time required to perform even a calibration. Ian Verhappen, a former chair of the Fieldbus Foundation User Group, notes: “In many cases getting all the necessary paperwork (permits, isolation, etc.) in order often takes longer than the work itself.”
Documenting a calibration traditionally has meant using a logbook to hand-write the date and time, the pre-calibration and post-calibration readings, and any other observations by the technician. Surprisingly, many plants continue to document calibration work by hand. But pencil-and-paper documentation has many shortcomings.
First, it both produces and perpetuates errors. The data in hand-written records often are simply illegible or insufficient. Facilities that use a computerized maintenance management system (CMMS) then must account for the extra time required to manually enter hand-written data and contend with additional possibilities for error.
Beyond documentation, newer digital devices require a seemingly infinite number of new configurations and settings. So many in fact that it’s important to keep your test instruments up to date with the latest information. With that in mind, some calibrator makers such as Fluke Corp. offer special calibration assistant tools that have updated libraries of digital device descriptions (DDs) on a tablet computer that connects to the internet for updates.
Achieving Greater Efficiency
When a field instrument is manufactured, both the primary element and the transmitter (or the actuator, if a control valve) are calibrated at the factory; the calibration information is supplied with the unit. These calibration data often are lost, though. To prevent this, establish a standard practice of entering the information into centralized calibration records when the device is put into service. Centralizing calibration information not only increases efficiency but also ensures the facility retains knowledge even as teams change.
An important way to boost worker productivity is to use “smarter” field calibration tools. These combine multiple tools into one and perform functions beyond basic test and measurement — such as assisting with analysis and documentation.
Multifunction “documenting process calibrators” are handheld, electronic test tools that consolidate multiple calibration steps and functions into a single device, sourcing simulating and measuring pressure, temperature and a wide variety of electrical and electronic signals. Their benefits include:
• fewer tools that technicians must train on and carry into the field;
• similar calibration processes and data output across multiple devices instead of different processes and data outputs for each tool;
• automated procedures that replace many manual calibration steps;
• faster calibration time per device; and
• simpler error calculation — for a single tool rather than adding the errors of several tools.
Know your device descriptions. Many multifunction calibrators include support for a basic set of digital fieldbus protocols such as HART. Nevertheless, given the number of new devices coming on the market each year, it’s not generally feasible to have every conceivable DD needed for calibration.
A device such as the Fluke 154 HART Assistant, for example, works with the 754 documenting process calibrator to calibrate a much greater range of HART devices at process plants. The assistant device connects a modem directly to a HART transmitter. An app on a tablet computer then wirelessly communicates with the device under calibration through the modem and also connects to the documenting and calibration functions of the multifunction calibrator. Because the tablet can connect to the internet, HART DDs can be added at regular intervals or on-demand. Moreover, after configuring a device, you can send the configuration to the tablet memory to use the next time you must calibrate that or an identical device in the same application.
Use calibration routes. A documenting calibrator includes a built-in route management tool. This enables using a single set of permits and paperwork for an entire set of calibrations — reducing costs considerably.
Implement a management system. Unlike paper documentation, calibrator data never are illegible, cryptic or partial. You can directly download documenting calibrator data into asset-management or calibration-management systems or a CMMS with no transcription or filing.
Because documenting process calibrators automatically record the as-found and as-left state of each field device in situ and can be operated by a single technician, route-based practices utilizing such tools can save as much as 50% of the time and cost of traditional manual, single-device calibration methods. Stated differently, the same small team can accomplish twice as many calibrations in a given period of time.
Good calibration methods, documentation and practices, including updating with the latest digital configurations and DDs, help improve productivity, increase quality and reduce the risk of dangerous incidents. In the event the process goes awry, leading to bad batches or other issues, or if a disaster strikes, good calibration records can prove valuable for getting back on track or even for mounting an effective defense in a legal action. Moreover, compliance with industry and government standards and regulations often requires good calibration records.
FREDERIC BAUDART is a product application specialist for Fluke Corporation, Everett, Wash. Email him at [email protected].