If you are an industrial instrument designer, you have several temperature measurement options available to you. These include Infrared, Bi-Metal, Gas-actuated thermometers, Thermistors, Thermocouples and RTDs.
Industrial automation and process applications predominantly use thermocouples and Resistance Temperature Detectors (RTDs) instrumentation such as our S81 and S50 because of their proven ability to provide accurate and reliable readings. Each of these sensors has unique advantages and can be configured to match the application needs for optimum service life and performance.
Based on my experience working with instrumentation and reliability teams, I’ve seen and compared the benefits of both options. Thermocouples, for example, are often chosen for a robust signal, cost-effective price and ability to withstand most process plant specifications, including harsh vibration and high-temperature range up to 2300° C. RTDs, on the other hand, are the top choice for applications demanding higher accuracy levels and minimal drift.
In this article, you will learn more about the benefits of RTDs and why RTD calibration is important to maintain highly accurate and repeatable readings for a longer life span.
RTDs are more accurate than thermocouples.
RTDs provide highly accurate and consistent measurements and have a temperature range that spans
-196° C to 600° C. Unlike thermocouples, RTDs do not require cold junction compensation or compensated alloy lead wire to measure temperature, which results in a more reliable signal. That’s why RTDs are the preferred temperature measurement instrument for demanding applications.
Common applications for RTDs.
The following applications rely on RTDs for the instrument’s high accuracy in repeatable process variable measurements.
• Clean In Place (CIP)
• Pharmaceutical
• Chemical and Refining Processes
• Cryogenic
• Custody Transfer
• Critical Control Loops
• Safety Shut Down Systems
• Asset Protection
• Compressor Efficiency Monitoring
Expected tolerances for RTD elements.
The chart below illustrates the expected tolerances for the most common RTD elements used in temperature measurement, specifically the 100 Ohm Platinum sensing element with a 0.00385 Temperature Coefficient.
Thin Film and Wire Wound elements are the two types of configurations classified through ASTM and IEC 60751 in Tolerance Class A, Class B, and Class AA. IEC Tolerance Designations include the element construction as demonstrated in the chart. For example, Class AA in Wire Wound is designated as W0.1 and in Thin Film as F0.1.
Figure 1: Expected Tolerances for RTD Elements.
Note: Operating the RTD Element above the specified maximum temperature range may stress the sensor and lead to aging effects. Stressing the sensor can lead to deviation within the specified range and affect performance.
Each element type and accuracy class are limited by their respective temperature ranges shown in the chart. While Class AA has the highest accuracy in this group, Class B in a Wire Wound format has the widest temperature Range. RTD elements can measure above these ranges including up to 850° C, however, stressing the element above their specified range can lead to aging effects and affect sensor performance.
Why RTD calibration may be necessary.
With the level of accuracy RTDs provide (shown above), in most cases, there is no need to calibrate these instruments. Like thermocouples, RTDs can be installed out of the box and used to measure temperature with a high confidence level for a long period of time.
However, there are certain times when plant personnel should consider making the additional investment to calibrate these instruments to ensure the RTD is performing properly and to the specified tolerances. Here are four examples of when calibration may be necessary and can provide substantial benefits:
1. Chemical, refining, power, process and automation improvements
Process Engineers and Safety Teams must closely monitor chemical reactions to prevent “Run away Temperature Excursions”. Temperature differentials monitor changes in temperature and must be able to analyze/assess in real time if the change presents an abnormal condition that requires preventive measures such as placing a process into a safe mode via an emergency shutdown.
Calibration will provide the error for each RTD element which process engineers can use to eliminate uncertainty in these critical measurements. Operation teams can take the information from calibration reports for each sensor and utilize as correction factors to support both safety and process efficiency improvement programs.
Applications that require the detection of small increases in temperature may also benefit from a specialized calibration process and calculation called Callendar Van Dusen, often referred to as CvD. Through this process, the actual resistance to temperature relationship for each individual element is calculated using coefficients generated by calibration.
2. Industry and plant requirements
Industries that depend on tight temperature tolerances such as pharmaceuticals, life sciences, and Clean in Place (CIP) must certify that all instrumentation tied to production is calibrated. CIP pasteurization units must maintain small temperature differences between the inlet and outlet temperatures for a successful operation. Calibrated RTDs provide the necessary accuracy for these units and validate the temperature measurements as needed.
Aside from company, corporate and local plant standards, regulatory agencies such as the FDA will mandate or put forward best practices including recalibration schedules. These timetables can be predetermined, or instrument data-driven schedules.
3. Engineering procurement construction (EPC) projects
EPC construction projects require calibration for most new instruments. Here’s why:
- Calibration provides traceable confirmation that instruments meet specifications.
- Inspection teams rely on documentation that assures RTDs meet the specified accuracy level per the instrument data sheet.
- New instrument installations provide an opportunity to benchmark expected results.
- End users receive all pertinent documentation including calibration reports prior to start-up and commissioning.
- Project Documentation Requirements (PDR) include calibration reports in a controlled and auditable process.
- This supports safe operations and maintenance of process and automation plants.
Reliability and Health Safety Environment (HSE) engineers who take over plant unit operations may use data from calibration reports to help establish expected unit performance for various equipment including compressors, turbines and other critical assets.
Runtime engineers and maintenance departments can use the data to help investigate process discrepancies and implement as needed corrective and preventive action plans.
4. Quality manufacturing program assurance
The accuracy stated by ASTM and IEC in the Chart above are for the element only (Thin Film or Wire Wound). Calibration of a probe will provide the tolerance error for the entire sensor probe including the lead wire.
Any measurement errors introduced during the manufacturing process will be captured in the calibration process.
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Selected Wire Wound and Thin Film elements for industrial use are embedded into metal sheaths with mineral insulation for added protection.
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The small platinum wires from these elements (0.006” to .008” in diameter) are welded to much larger diameter conductors and subsequently to flexible lead wires. The probe is sealed with hermetic potting to protect the integrity of the electrical signal.
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This process has many steps and, when not performed correctly, can introduce errors into the sensor assembly.
For example, an inadvertent additional resistance of 1 ohm in the manufacturing process for a platinum RTD can lead to an error as high as 0.4° C. 
RTD wiring configurations
Industrial plant RTD Wire Configurations include “3 wire” and “4 wire”. Three Wire is the most common, however, this configuration requires lead wire lengths to match, unlike in a four-wire RTD.
In a three-wire configuration, the electronics within a transmitter, DCS, and PLC will not be able to account for any resistance difference in the third leadwire where the lengths do not match. This can occur since an assumption is made that the measured resistance of the second leg is the same as in the third leg.
The calibration process will capture these types of discrepancies and help certify not only the element but also the complete sensor.
Figure 2. Wire Configuration Examples.
RTD –calibration traceability
To avoid costly events like unplanned shutdowns or product quality issues, Procurement and Inspection departments must rely on both Manufacturer’s Quality Programs and Traceable Calibration Reports. Calibrations should be traceable to the international standard ITS-90 Scale via ISO/IEC 1702S certified labs and equipment.
The Chart below shows the link between the installed sensors and the standards used to complete the calibration process.
Figure 3. Unbreakable Traceable Chain.
When should I recalibrate my RTD?
Recalibration schedules will depend on the application and industry. Unlike thermocouples, RTDs will not experience significant drift and therefore will not typically require a recalibration. Below are conditions that may stress an RTD and prompt a calibration:
• Operating sensor element above its specified temperature range
• High vibration applications
• Severe temperature cycling
RTDs will offer stable and reliable signals when configured correctly. A recalibration can be utilized as a confirmation of the signal integrity and increase the confidence level in critical temperature measurements.
Final thoughts on RTD calibration benefits.
Now that you know more about why and when RTD Calibration may be necessary, you can research the solution that’s best suited for your application. Keeping these factors in mind can help ensure that you avoid problems and keep your process running with reliable and accurate temperature measurements.
You deserve to feel confident in your measurement equipment. At Ashcroft, we understand that not all applications are the same and requirements can vary. We provide support for your everyday operational needs, maintenance, repair and operations (MRO), working with engineering procurement construction (EPC) and corporate engineering firms on large capital projects with a dedicated support team.
If you want to learn more about RTDs and other temperature sensors, check out some of our other blog posts:
- When to Use an RTD vs. a Thermocouple Temperature Sensor
- Product Review: New RTDs and Thermocouples
- Fitting a Thermowell to Bimetal Thermometers or RTDs/Thermocouples
- How Much Do Temperature Sensors Cost?
Or reach out to one of our product experts with questions.