Practical Guide: Validation of Temperature Sensors (RTD vs Thermocouple) and Drift Analysis

1. Scope and Purpose

This practical guide details the procedures for validating industrial temperature sensors of the Thermocouple (TC) and Resistance Thermal (RTD, Pt100) types through comparative tests and drift analysis. The objective is to ensure the accuracy and reliability of temperature measurements in process systems, identifying deviations and determining the need for calibration or replacement. It is an essential document for maintenance technicians and reliability engineers working on the factory floor, aiming to optimize the performance of control systems and ensure operational safety, in accordance with the guidelines of the ABNT NBR IEC 60751 standard for RTDs and ABNT NBR 13770 for Thermocouples. The application of this methodology is critical in the automotive, aerospace, food, chemical and energy industries, where thermal control is decisive for product quality and energy efficiency.

2. Safety Precautions

WARNING: Failure to follow these precautions could result in serious injury, electric shock, thermal burns, or equipment damage.

• Lockout and Tagout (LOTO): Before starting any work, completely isolate the electrical power to the equipment where the sensor is installed. Apply appropriate locking devices and safety tags, in accordance with NR-10 and internal factory procedures.
• Personal Protective Equipment (PPE): Always wear safety glasses, thermal protective gloves (when handling dry block calibrators or calibration wells with high temperatures), safety shoes and, if necessary, ear protection.
• Hot Surfaces: Dry block calibrators and thermostatic baths operate at high temperatures. Allow complete cooling before touching any part, or use insulated tools and appropriate PPE.
• Electrical Hazards: When handling multimeters and wiring, make sure there are no dangerous voltages. Check the integrity of test leads and use calibrated instruments.
• Fluid Handling: In thermostatic baths, use the fluids recommended by the manufacturer and be aware of spills, which can cause slipping or contact with dangerous substances.
• Work Environment: Keep the work area clean and organized, free from obstacles that could cause tripping. Ensure adequate ventilation, especially when using calibrators that may emit vapors.

3. Tools and Materials Required

The accuracy of the results depends on the quality and calibration of the tools used. Prioritize equipment with a traceable calibration certificate.

4. Pre-Maintenance Inspection Checklist

Before starting validation, perform this visual and functional inspection to identify obvious problems.

5. Step-by-Step Procedure: Validation and Drift Analysis

This procedure describes calibration by comparison, a method widely used for field validation.

5.1. Preparation of the Environment and Equipment

1. Isolation and Security:
   • CRITICAL STEP: Perform the Lockout and Tagout (LOTO) procedure at the sensor measurement point. Confirm the absence of power with a calibrated voltmeter.
   • Wear appropriate PPE (goggles, protective gloves).
2. Sensor Removal:
   • Disconnect the sensor to be tested from the transmitter or control system. Identify wiring for correct reconnection.
   • Carefully remove the sensor from the thermowell. Visually inspect the sensor and well again.
3. Calibrator Setup:
   • Connect the dry block or thermostatic bath calibrator to stable mains power (220 V AC, 60 Hz).
   • Insert the sensor to be tested and the standard sensor into the block or bath, ensuring that their sensitive ends are as close as possible and at the same depth, at a minimum immersion of 150 mm for RTDs and 100 mm for TCs.
   • Apply thermal paste to the sensor rods to optimize thermal contact with the block. Avoid excessive use of paste, which can make removal difficult.
4. Electrical Connection:
   • For RTD (Pt100): Connect the Precision Multimeter to the sensor to be tested using the 4-wire configuration to eliminate cable resistance. The DMM must be in the Ohms (Ω) scale.
   • For Thermocouple (TC): Connect the Precision Multimeter to the sensor to be tested. The DMM must be in the mV DC scale. If it is a Type K Thermocouple, connect the red wire (negative) and the yellow wire (positive). Always check the polarity according to the type of CT and the NBR 13770 standard.
   • Connect the standard sensor (RTD or TC) to the process calibrator or DMM, depending on the sensor type.
   • For Field Thermocouples (Ice Point): If it is not possible to use a calibrator with integrated cold junction compensation, prepare the thermocouple with crushed ice and distilled water. Insert the cold junction of the thermocouple into the ice bath to ensure an accurate 0°C reference.

5.2. Execution of Comparison Tests

5. Definition of Test Points:
   • Select at least 3 temperature points that cover the sensor's operating range, including critical points such as the nominal process temperature. Example: 50°C, 150°C, 300°C.
6. Temperature Stabilization:
   • Adjust the calibrator to the first temperature point (ex: 50 °C).
   • Wait for the temperature in the calibrator to completely stabilize, indicated by a constant reading on the standard display. The stabilization time can vary from 10 to 30 minutes, depending on the mass of the block and sensors.
   • The deviation of the standard sensor in the calibrator must not exceed ±0.05 °C.
7. Recording Readings:
   • With the temperature stabilized, record the reading from the standard sensor in the calibrator.
   • Record the reading of the sensor to be tested (Ohms for RTD, mV for TC) on the Precision Multimeter.
   • Convert the readings of the sensor to be tested to temperature using the appropriate tables (ABNT NBR IEC 60751 for RTDs, ABNT NBR 12771 for TCs) or the process calibrator's conversion function. Make sure to use the correct tables for the sensor type and class (e.g. Pt100 Class B).
   • Calculate the error: Error = Tested Sensor Temperature - Standard Sensor Temperature.
8. Repetition of Points:
   • Repeat steps 6 and 7 for all selected temperature points, both on an ascending and descending ramp to check hysteresis.
   • For critical applications, perform at least 2 measurement cycles at each point.

5.3. Drift Analysis (Metrological Drift)

Drift analysis requires the history of previous calibrations of the same sensor.

9. Historical Data Collection:
   • Obtain the previous calibration certificates for the sensor in question, preferably the last two.
   • Identify the measurement error for each temperature point in previous certificates.
10. Drift Calculation:
    • For each temperature point, calculate the error variation in relation to the previous calibration:
    • $$ \text{Drift (per point)} = \text{Current Error} - \text{Last Calibration Error} $$
    • Drift must be expressed in °C (or the relevant temperature unit).
    • Calculate the annual drift rate if there is enough data: $$ \text{Annual Drift Rate} = \frac{\text{Total Drift}}{\text{Number of Years between Calibrations}} $$
    • A common error is to compare calibration errors performed with different standards or under different conditions. Ensure comparability.
11. Acceptance Criteria:
    • Compare the calculated drift with the tolerance limits defined for the process or sensor accuracy class (e.g. RTD Pt100 Class B has a maximum deviation of ±(0.30 + 0.005 |t|) °C, according to NBR IEC 60751).
    • If the drift exceeds the acceptable limit or Maximum Permissible Error (MPE) of the process, the sensor must be replaced or recalibrated by a specialized laboratory. A drift greater than 0.5°C/year for RTDs or 2°C/year for Thermocouples under stable conditions generally indicates incipient failure.

5.4. Reconnection and Final Check

12. Sensor Reconnection:
    • Remove the sensors from the calibrator after cooling. Clean off any thermal paste residue.
    • Reinstall the sensor in the thermowell, ensuring good mechanical contact. Tighten the cable gland or threaded connection with a torque of 15 to 20 Nm for 1/2'' NPT connections, using a suitable wrench. Do not apply excessive torque, which may damage the sensor housing or well.
    • Reconnect the wiring to the transmitter or control system, observing polarity and the original wiring diagram.
13. Unlocking and Functional Testing:
    • Remove LOTO devices. Re-energize the equipment.
    • Check the temperature reading on the control system. Compare with other nearby instruments, if available, for a quick check.
    • Monitor the process for a few hours to ensure the sensor reading is stable and consistent with normal operation.

6. Post-Maintenance Verification Checklist

After replacing the sensor, confirm functionality and integration with the system.

7. Troubleshooting Guide

Common problems during or after validating temperature sensors.

8. Recommended Maintenance Schedule

The frequencies below are general suggestions and should be adjusted according to the criticality of the process, environment and sensor drift history.

9. UNITEC-D Spare Parts Reference

UNITEC-D GmbH offers a wide range of temperature sensors and accessories, guaranteeing the traceability and quality of components for their maintenance.

To purchase these and other high-quality replacement parts, visit our e-catalog at www.unitecd.com/e-catalog/.

10. References

• ABNT NBR IEC 60751: Industrial platinum thermoresistances.
• ABNT NBR 13770: Calibration of thermocouples by comparison.
• ABNT NBR 13772: Calibration of thermoresistance by comparison.
• ABNT NBR 12771: Reference tables for thermocouples.
• ABNT NBR 13863: Guidelines for using the reference junction in thermocouples.
• NR-10: Safety in electrical installations and services.
• NR-12: Work safety on machines and equipment.