Modern temperature instrumentation simplifies technician access, calibration and SIL procedures. This feature originally appeared in the May 2022 issue of InTech Focus Temperature & Pressure.
As temperature instrument technology has matured, there is less focus on the actual measured value and more on added value. Accuracy and stability are considered basic thresholds, and what process owners really want to know is how a temperature instrument can further improve the overall process. For most process owners, added value falls into one of three basic categories: 1. Simplicity. For plants faced with continued attrition of onsite maintenance and service support, can the instrument assist with simplifying tasks such as installation, commissioning, calibration and troubleshooting? 2. Safety and risk reduction. Will the instrument reduce the risk to personnel, property, or the process? 3. Productivity and profitability. How can the instrument improve process efficiency, reduce labor cost and increase profitability? Temperature, arguably the technology with the most measurement points in many process plants and facilities, presents a huge benefit potential in all three categories.
The most time-consuming activities associated with any temperature instrument are commissioning, calibration, and service. Technicians on the plant floor need to access data in the instrument, such as status and diagnostic messages, to perform these services. Most temperature instruments support a standard 4-20 mA HART communication protocol. While robust and well accepted, this two-way form of communication also requires specialized tools, such as a HART modem, software, and the correct device type manager (DTM) and device driver (DD), depending on the interface device employed. The technician goes to the instrument, plugs in the interface device, and performs the required maintenance tasks. The advent of wireless access to temperature transmitters changes all this. Many instrument vendors now offer WiFi, WirelessHART, and Bluetooth connections to temperature transmitters. For example, all a technician needs to interface with a Bluetooth-capable instrument is a smart device, such as a handheld computer, smartphone, or tablet, and the corresponding app. The technician can communicate securely with the device from distances of up to 40 feet away (Figure 1). A technician can see the output of the device, as well as perform all the same commissioning and diagnostic functions normally associated with a HART modem and DTM. The ability to access a temperature instrument from a distance means that the technician doesn’t need to climb scaffolding or ladders to reach instruments installed on tops of tanks or in otherwise inaccessible locations. This increases technician safety and reduces the amount of time needed to perform maintenance tasks. Figure 1: Technicians can communicate and perform commissioning and diagnostic functions using a handheld computer, smartphone, or tablet and the appropriate app from up to 40 feet away.
In certain critical temperature measurements, temperature instrument calibration can be required every six to 12 months. Calibration is necessary to detect measurement drift and to permit adjustment or replacement of the instrument. The exact timing of calibrations is determined via a risk/cost analysis. A more frequent calibration cycle reduces risk to the facility but, conversely, increases cost. Figure 2: Some resistance temperature detectors (RTDs) automatically perform an in-situ calibration once a predetermined set of conditions are met. Some resistance temperature detectors (RTDs) (Figure 2) automatically perform an in-situ calibration once a predetermined set of conditions is met. This occurs with no interruption in the process or measurement signal. The two conditions required are a process temperature that has met or exceeded 253 degrees F (123 degrees C) followed by a controlled cooling cycle to below 244 degrees F (118 degrees C). This most commonly occurs during a steam-in-place sterilization or some other type of aseptic process. Once these conditions are met, the RTD’s integrated transmitter automatically compares the measured value against an embedded traceable reference. Any deviation is identified, and if it exceeds the customer-specified tolerance, a notification occurs. The safety or metrology department can then determine if adjustment or replacement of the existing RTD is needed. Figure 3: Some temperature transmitters have a two-channel function that can detect drift between sensor 1 and sensor 2, a broken sensor, and corrosion on the sensors, as well as provide automatic sensor backup if one sensor fails. This in-situ calibration eliminates the need to periodically remove the instrument’s sensor from the process, take it to a calibration lab, and reinstall it, thus saving time and money. Due to the popularity of this feature, instrument vendors are looking to develop additional temperature thresholds. Temperature sensors and transmitters also are becoming available with advanced diagnostics to simplify the job of maintenance technicians. Some temperature transmitters (figure 3), for example, have a two-channel function that detects drift between sensor 1 and sensor 2, detects a broken sensor, detects corrosion on the sensors, and provides automatic sensor backup if one sensor fails. When the temperature transmitter detects a problem, it sends a standard NAMUR NE017 diagnostic message to the control system via its 4-20 mA HART, Profibus, or Foundation Fieldbus interface. When the maintenance technician accesses the temperature transmitter from a tablet or computer, all diagnostic messages and data are available.
Temperature is a critical measurement in a safety instrumented system (SIS). As parity among instrument vendors regarding safety integrity level (SIL) certification has been achieved, the more significant risk is human error associated with commissioning and proof testing. The typical commissioning of a SIL-rated device in a SIS involves setting the SIL relevant parameters (SRPs) in conjunction with an instrument-specific SIL checklist developed by the safety engineer. Since a technician may need to enter six to eight values, there is ample opportunity for errors due to inexperience or interruptions. Also, during proof testing, the correct instrumentspecific SIL checklist must be located, and then each of the SRPs must be confirmed. Figure 4: Transmitters designated as SIL capable are factory programmed with SRPs. New SIL-capable temperature transmitters have eliminated much of the risk inherent with this process. Transmitters designated as SIL capable (figure 4) are factory programmed with SRPs. While field customization is still possible, the amount of human interaction and setting adjustments is minimized. These same transmitters can produce and store the SIL checklist. Upgraded DTMs permit the safety engineer to access this information directly from the transmitter and print a PDF on demand, reducing lost or duplicated paperwork. Checksum is a unique numeric code generated by the transmitter based on which SRPs are used. During a proof test cycle, a manual review of the six to eight SRPs is required. But a simple comparison of the current checksum value currently stored in the instrument against the last stored SIL checklist will confirm if any SRPs have been altered. If the checksum numbers match, no changes have occurred, and the technician can safely and efficiently proceed with the balance of the proof test. A 96 percent proof test coverage can typically be performed in approximately 10 minutes with a SIL-capable instrument, compared to 20 to 30 minutes for instruments without this capability. It will vary depending on the experience and efficiency of the technician.
Temperature instruments need to be calibrated, which can be a time-consuming procedure that might require the process to be shut down, thus adversely affecting productivity. In most cases, calibration requires the following sequence:
Typically, this process takes a minimum of 30 minutes per instrument and adds additional risk to the process, including damage to wiring from connecting and disconnecting the leads, possible damage to the RTD or thermocouple (T/C) sensor when it’s transported to and from a calibration lab, and the possibility of miswiring the leads during reconnection. A quick disconnect feature (Figure 5) can reduce calibration and downtime by two-thirds while reducing risk by eliminating the need to remove leads from the transmitter. This also makes it much more practical to perform calibration at the measurement point using a mobile reference. The time saved with easier instrument access (i.e., no scaffolding or ladders involved in reaching instrumentation) combined with time savings from self-calibrating sensors, SIL-capable transmitters, and quick-disconnect sensors can add up to considerable labor savings. In a plant with 1,000 or more temperature instruments, the labor savings can amount to several hundred thousand dollars per year, with additional savings achieved by reducing risk and increasing uptime. Figure 5: A quick disconnect feature can reduce calibration and downtime while reducing risk by eliminating the need to remove leads from the transmitter.
While it seems a monumental task to upgrade a plant with 1,000 or more temperature instruments, such a project may not be as big as it appears, due to the following factors:
The best way to determine the next steps in an upgrade project is to perform a plant-wide installed base analysis of temperature instruments to identify what’s installed, check to see if the installed instruments have the appropriate technology, and determine what needs to be done. Any major instrument vendor can assist with this type of project. All images courtesy of Endress+Hauser. This feature originally appeared in the May 2022 issue of InTech Focus Temperature & Pressure.
Keith Riley is the national product manager for level and pressure at Endress+Hauser USA. He has been with the company since April 2008. Prior to this, he was a product manager and regional sales manager with L.J. Star Inc., as well as a product manager for Penberthy (Tyco Valves). He has more than 20 years of sales, marketing, and instrumentation experience in the process industries.
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