If you read my article, “Why and When is RTD Calibration Necessary?” you learned how Resistance Temperature Detectors (RTDs) such as our S81 and S50 are a better choice than thermocouples for temperature process measurement because of their repeatable, high-accuracy measurement capabilities. More specifically, RTDs are ideal for demanding applications like chemical and refining processes that depend on precise temperature measurements. But to really understand RTD calibration, you also need to know how an RTD works.
When I joined Ashcroft — a leader of temperature and pressure instrument solutions across the globe— I brought more than 20 years of temperature application experience with me, and I am happy to share my knowledge with you.
In the following article, you learn more about how RTDs work, the different levels of accuracy in RTD calibration and the national and international standards that they are measured against. You will also be directed to additional resources and contact information in case you are ready to take the next step in your decision to select the best temperature measurement solution for your specific application.
How does an RTD work?
The resistance of an RTD increases proportionally and predictably with an increase in temperature. This relationship is ideal as it is repeatable through the RTDs temperature coefficient, providing a linear relationship between both resistance and temperature.
Customers can set up process control loops that manage control valves, heaters, pumps and other equipment that depend on a temperature measurement.
4 steps to configure an RTD element inside the probe.
The most common RTD element is the 100 Ohm Platinum with a 385 curve or temperature resistance coefficient, Class B wire wound. Here's how it's done:
- Start with platinum as the resistance material.
- Use a transmitter, PLC or DCS to measure the resistance signal. (At 0° C the RTD will produce a predictable 100.0 Ω of resistance.)
- Select tolerance class.
- Select temperature range and construction type based on the tolerance class and element style.
See the chart below for an example that shows a temperature range based on Class B tolerance class and thin film element style.
Figure 1: RTD, Resistance to Temperature Relationship.
Figure 2: RTD Element Type, Tolerance and Temperature Range
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.
How RTD calibrations are performed.
Customers must be assured that their temperature measurements meet the accuracy specifications for new EPC installations or critical applications. Calibration, complete with a certified Calibration Report is the best way to validate sensor accuracy and provide correction factors that can help improve process measurement.
Comparison calibration process
The most common calibration method is comparison calibration, which is performed by lab engineers and technicians. In this process, RTDs are calibrated using a high-accuracy reference sensor, typically a Platinum Resistance Thermometer (PRT). The sole job of the PRT is to calibrate multiple sensors that will be installed at plants. PRTs are certified through accredited laboratories to ISO 17025, and traceable through national laboratories such as NIST to the International Standard, ITS-90.
Figure 3: Traceable Link to National Institutes and International Standards.
In turn, the PRT is calibrated by either a “Standard” PRT or a “Primary” PRT”. In the figure below, a reading is taken by high-accuracy meters from the PRT and the RTD to be installed into the plant. Within the Calibration Report, the RTD to be installed at the plant is referred to as the UUT or Unit Under Test. Other common terms are SUT (Sensor Under Test) and DUT (Device Under Test).
Figure 4: Comparison Calibration Example.
RTDs are calibrated at either temperature points specified by the customer or preset points offered by the sensor manufacturer.
Calibration certificates are provided with the results and contain pertinent information on all test equipment and reference sensors used in the calibration process.
Figure 5: Ashcroft Sample Certified RTD Calibration Chart.
The Calibration report will indicate the error for the test sensor (UUT, Unit Under Test) at each temperature point as compared to the measurement of the reference PRT sensor.
Figure 6: Sample Error Readings in Calibration Report.
Recommended RTD calibration intervals.
RTDs do not experience significant drift as seen in thermocouples due to many factors, including the materials of construction and the temperature ranges that each type of sensor experiences.
However, RTDs are used in applications where an out-of-tolerance situation may have a more profound adverse effect, especially since they are selected for applications that require high accuracy. These applications include high vibration, severe temperature cycling, Safety Shut Down Systems with SIL requirements and others. In these cases, RTDs are preferable as they typically fail open as opposed to Thermocouples which may continue to produce a signal with an undetected failure.
RTDs are more common in regulated industries that mandate measurement validation using calibration programs. As shown below, RTD recalibration schedules will depend on the process application. For example, high tolerances such as in the food and pharma industries, require more frequent calibration to ensure processes are running efficiently.
Figure 7: Application Process Tolerances
Next steps regarding RTD calibration.
Now that you know how RTD calibrations are performed, you can research the solution that best suits your application. Keeping these factors in mind can help ensure you avoid problems and keep your process running safely and reliably.
We understand that every situation is different. That’s why you can trust us to guide you through every step of your instrument selection and application needs, including MRO and turnarounds, 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. Thermocouple
- 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.
