Guide to Diagnosis and Resolution of Discrepancies in Industrial Temperature Measurement

1. Description of the Problem and Scope

This guide addresses discrepancies and errors in temperature measurement in industrial environments, a critical issue that can impact process safety, product quality, and operational efficiency. A temperature reading that is inaccurate, unstable, or has an unacceptable delay can lead to erroneous operating decisions and equipment failure.

Common symptoms include: significantly different temperature readings between instruments, unstable values ​​without apparent cause, false process alarms, or poor temperature control. This guide focuses on issues related to sensor type selection, thermal delay, connection cable resistance, and transmitter configuration.

Affected Equipment: Process control systems, furnaces, heat exchangers, reactors, compressors, refrigeration systems, pipes and storage tanks in sectors such as automotive, aerospace, food, chemical and energy.

Severity Rating:

• Criticism: Directly affects the safety of personnel or the process (e.g. temperature control in exothermic reactions, fire safety systems).
• Major: Impacts product quality, equipment performance or energy efficiency (e.g. temperature control in curing processes, pasteurization).
• Minor: Causes operational inconveniences or inaccurate monitoring without immediate risk (e.g. room temperature readings).

2. Safety Precautions

CRITICAL SAFETY WARNING! Tampering with temperature measurement systems may involve significant risks.

• Lockout/Tagout (LOTO): Always apply lockout/tagout procedures on all power sources (electrical, hydraulic, pneumatic) before beginning any intervention on the equipment.
• Personal Protective Equipment (PPE): Wear heat resistant gloves, safety glasses and appropriate protective clothing. Surfaces can be extremely hot or cold, and fluids under pressure can cause serious injury.
• Stored Energy: Systems under pressure (steam, hot liquids) or with high electrical voltage must be depressurized and completely de-energized before intervention. Confirm the absence of residual energy.
• Hazardous Substances: Some processes may contain corrosive, toxic or flammable substances. Check the safety data sheets (SDS) and use the specific PPE required.
• Work at Height: When it is necessary to work at heights, use safe platforms and a safety harness approved according to UNE-regulations EN 361.
• Explosive Atmospheres (ATEX): In areas classified as ATEX, only use tools and equipment that are intrinsically safe or certified for said classification, in accordance with Directive 2014/34/EU (ATEX).

3. Required Diagnostic Tools

The use of calibrated and appropriate tools is essential for an accurate diagnosis.

4. Initial Evaluation Checklist

Before any direct intervention, collect the following information to narrow down the problem:

5. Systematic Diagnostic Flowchart

Follow this logical sequence to isolate the root cause of temperature measurement discrepancies:

1. Initial Symptom: Temperature reading that is erroneous, unstable or with excessive delay.
   1. Step 1: External Verification and Documentation
      • Is the symptom constant or intermittent? → Register pattern.
      • Have there been any recent changes made to the process or equipment? → Investigate impact of changes.
      • Are there multiple instruments that show the same discrepancy? → Suggests a common source or process problem.
      • Check the reading with a contact reference thermometer (if safe and possible).
      • IF the discrepancy persists and there is no obvious external cause THEN continue to Step 2.
   2. Step 2: Transmitter Output Signal Check
      1. SAFETY WARNING! Electrical hazard. Use appropriate PPE and insulated tools.
      2. Measure the transmitter output signal (e.g. 4-20 mA, digital signal) at the field terminal block.
      3. IF the signal is incorrect or unstable THEN continue to Step 3 (field loop problem).
      4. IF the signal is correct THEN the problem could be in the control system (DCS/PLC) or the input card. Consult with instrumentation and control.
   3. Step 3: Verify Connection Wiring (Sensor to Transmitter)
      1. Disconnect the sensor from the transmitter and process (if safe).
      2. Measure the resistance of the connecting wires with a multimeter.
      3. For RTD (Pt100/Pt1000):
         • Is the resistance of each wire similar and low (usually < 5 Ω per wire)?
         • Is there continuity between all the threads?
         • Is there a short circuit between wires or to ground?
         • IF resistance (high, variable) or continuity/insulation problems THEN wiring is the likely cause.
         • IF wiring is OK THEN continue to Step 4.
      4. For Thermocouple:
         • Is there continuity in the pair of wires?
         • Is there a short circuit between wires or to ground?
         • IF continuity/insulation problems THEN wiring is the likely cause.
         • IF wiring is OK THEN continue to Step 4.
   4. Step 4: Temperature Sensor Verification
      1. Connect the temperature calibrator/simulator directly to the sensor (if possible, disassemble it and expose it to the calibrator).
      2. Compare the sensor reading on the calibrator with the reference value.
      3. For RTD (e.g. Pt100): At 0°C, the resistance should be 100 Ω. At 100 °C, approximately 138.5 Ω (according to UNE-EN 60751).
      4. For Thermocouple: Compare the mV reading of the calibrator with that of the sensor for a given temperature, using reference tables (UNE-EN 60584).
      5. IF the sensor does not correspond to the expected reading or has a significant drift (> ±0.5 °C) THEN the sensor is damaged or incorrect.
      6. IF sensor is OK THEN continue to Step 5.
   5. Step 5: Verification of the Temperature Transmitter
      1. Connect the temperature calibrator/simulator directly to the transmitter input.
      2. Simulate a known temperature signal (e.g. 25%, 50%, 75% of range).
      3. Measure the transmitter output (ex. 4-20 mA) and compare with the expected value (ex. 8 mA, 12 mA, 16 mA).
      4. Check transmitter configuration (sensor type, range, units, linearization).
      5. IF the transmitter output is incorrect for a known input or the configuration is incorrect THEN the transmitter is failing or misconfigured.
      6. IF the transmitter is OK THEN continue to Step 6.
   6. Step 6: Thermal Delay Analysis
      • If the problem is slow response or unstable control: Is the thermometer sheath thick-walled or is the diameter large? Is the sensor in direct contact with the bottom of the pod?
      • Use a datalogger to record the sensor's response to an abrupt change in temperature. Compare to expected response time (OEM specification).
      • IF response time is excessively slow THEN thermal lag is the likely cause.

6. Matrix of Failures and Probable Causes

This table details the common symptoms, their most probable causes, the diagnostic tests to be applied and the expected results.

7. Root Cause Analysis for Each Failure

7.1. Incorrect Sensor Type Selection

Explanation: Each type of temperature sensor (RTD, Thermocouple, Thermistor) has specific characteristics of range, precision, linearity and response. Using a Pt100 in a temperature range >600°C or a Type K thermocouple in high-precision, low-temperature applications can lead to systematic errors. Thermocouples generate a voltage (Seebeck Effect) that depends on the temperature difference between the measurement junction and the reference junction, while RTDs (Resistance Temperature Detectors) measure changes in the resistance of a pure metal (e.g. Platinum) with temperature. The response curve (resistance vs. temperature or mV vs. temperature) is unique for each type.

How to Confirm: Review P&IDs and process data sheets to determine the required temperature range and required accuracy. Compare this information to the physically installed sensor type and sensor specification (e.g. label on sensor head, certification). A temperature calibrator is essential to verify the behavior of the sensor at different points in its range.

Harm if not Resolved: Incorrect process data leading to poor control, out-of-specification product, inefficient energy consumption, or even hazardous operating conditions if the safety system depends on this measurement.

7.2. Connection Wire Resistance (RTD)

Explanation: 2-wire RTDs are highly susceptible to errors due to the resistance of the connection cable, since this resistance is added to that of the sensing element, making the reading appear higher than real. 3- and 4-wire RTDs compensate for wiring resistance. A 3-wire RTD measures the resistance of the sensor plus that of a connecting wire, and then subtracts the resistance of the third wire (assuming they are equal). A 4-wire RTD measures current through the sensor with one pair of wires and voltage through another pair, completely eliminating the influence of resistance from the measurement leads.

How to Confirm: Measure the resistance of each wire individually from the sensor head to the transmitter with a precision multimeter. In a 2-wire system, any additional resistance will be added to the sensor value. In 3 or 4 wire systems, the resistances of the compensation wires should be very similar. A difference >0.5 Ω between wires can introduce errors.

Harm if not Resolved: Reading errors that are magnified with wire length and gauge, resulting in inaccurate process control and potential energy waste.

7.3. Excessive Thermal Delay

Explanation: Thermal delay refers to the time it takes for a sensor to respond to an actual change in process temperature. It is influenced by the mass of the sensor, the material and thickness of the thermometer sheath, the sheath filling fluid (if any), and the degree of contact of the sensor with the bottom of the sheath. In dynamic processes, excessive delay can cause overshoots and oscillations, making control ineffective.

How to Confirm: Use a data logger to observe the sensor response to a known, sudden change in process temperature. Compare the observed response time (e.g. time to reach 63.2% of total change, T63) with the manufacturer's specifications or a faster responding sensor (e.g. exposed tip thermocouple, if safe). A thermal imaging camera can help visualize temperature gradients in the sheath.

Damage if not Resolved: Unstable process control, out-of-specification product, increased wear on control valves due to constant and aggressive correction action, and energy inefficiency.

7.4. Misconfiguration of the Transmitter

Explanation: Temperature transmitters are intelligent devices that convert the signal from a sensor (resistance, mV) into a standardized signal (e.g. 4-20 mA, HART, Profibus). Incorrect configuration of the sensor type (e.g. transmitter configured for Pt100 when a Type K Thermocouple is present), measurement range, units or linearization can generate significant and constant reading errors.

How to Confirm: Connect the temperature calibrator/simulator directly to the transmitter. Simulate various temperature points within the operating range and compare the transmitter output to expected values. Access the transmitter using its configuration software (e.g. HART communicator) and verify all parameters against the P&IDs and equipment data sheet. Ensure that the transmitter input range matches the process temperature range and that the sensor type selected is correct.

Damage if not Resolved: Failures in the automatic control of the process, false or ignored alarms, erroneous readings in the control system and difficulty in diagnosing other problems.

7.5. Sensor Failure

Explanation: Sensors can fail due to short circuits (wires in contact), open circuits (broken wires), drift (permanent change in their response curve due to thermal or mechanical stress), or contamination. A short circuit in an RTD will result in a lower resistance reading and therefore a lower temperature. An open circuit will result in infinite resistance and the lowest possible transmitter reading (e.g. 3.6 mA) or an error. In thermocouples, an open circuit will also lead to the minimum reading.

How to Confirm: Use a multimeter to check continuity and resistance (for RTD) or a calibrator to measure the mV output (for Thermocouple) at the sensor head. A drift is confirmed by calibrating the sensor at various points and comparing with standard tables (UNE-EN 60751 for RTD, UNE-EN 60584 for Thermocouples).
WARNING! If the sensor is in an active process, the temperature is likely to be high. Take precautions.

Harm if not Resolved: Ineffective process control, equipment failure due to overheating or excessive cooling, and safety risks.

7.6. Electromagnetic Interference (EMI) or Electrical Noise

Explanation: Temperature signals, especially low-level signals such as those from thermocouples, are susceptible to interference from electromagnetic fields generated by motors, frequency converters, transformers or high-voltage lines. Inadequate cable shielding or poor grounding can allow electrical noise to induce spurious voltages in the sensor wiring, resulting in erratic or unstable readings.

How to Confirm: Inspect the wiring for braided or foil shielding and proper single-point grounding. Note the proximity of the sensor cable to sources of electrical noise. Use an oscilloscope to view the mV signal from the sensor or transmitter output for noise spikes correlated with activation of nearby equipment. A thermal imaging camera can help identify abnormal heating in faulty splices or terminal blocks that may generate noise.

Damage if not Resolved: Process instability, false alarms, premature wear of control elements and difficulty in accurately diagnosing other failures.

8. Step-by-Step Resolution Procedures

Once the root cause is identified, follow these procedures to correct the problem.

8.1. Temperature Sensor Replacement

1. SAFETY WARNING! Apply LOTO and ensure process depressurization/cooling before removing sensor.
2. Loosen and remove the sensor connection head. Disconnect the cables, noting their position.
3. Remove the sensor from the thermometer sheath. If the sensor and sheath are integrated, remove the entire sheath from the process.
4. Verify the compatibility of the new sensor (type, range, immersion length, diameter, sheath material if applicable) with the equipment P&ID and data sheet. Use a CE certified sensor.
5. Apply high conductivity thermal paste between the sensor and the thermometer sheath (if a sheathed installation) to ensure optimal thermal coupling.
6. Insert the new sensor into the pod and gently tighten the connection.
7. Reconnect the wiring to the sensor head, making sure the wires are tight and to the correct terminals (especially important for 3- and 4-wire RTDs).
8. Perform a continuity and resistance test of the wiring if it is an RTD.
9. Reconfigure the transmitter if the type or range of the new sensor has changed.
10. Remove LOTO and put into service, checking the temperature reading.

8.2. Correction of Wiring and Connections

1. SAFETY WARNING! Apply LOTO to the affected control loop.
2. Visually inspect the entire cable run from the sensor to the transmitter and/or control system. Look for physical damage, crushing, abrasions, or signs of overheating.
3. On all terminals and connections (sensor head, transmitter, junction boxes, control panels):
   • Clean any signs of corrosion or dirt.
   • Firmly tighten each terminal. Typical tightening torque: 0.5 to 0.8 Nm (UNE-EN 60947).
   • Check that the wire insulation is not damaged at the connection points.
4. If a damaged cable run is identified, replace it with one of the same specifications (insulation type, gauge, shielding). For thermocouples, always use compensated extension wire of the same type as the thermocouple. For RTD, use shielded multi-pair copper cable.
5. Ensure that the cable shield is grounded at a single point (usually on the control system or transmitter side).
6. After corrections, check the continuity and resistance of each wire with a multimeter (end to end). Resistance values ​​should be low and stable.
7. Remove LOTO and put into service, monitoring the stability of the reading.

8.3. Thermal Delay Optimization

1. SECURITY WARNING! Requires access to the process. Apply LOTO and depressurize/cool if necessary.
2. Thermometric Sheath Evaluation: If the current sheath is excessively thick or long for the dynamics of the process, consider replacing it with one of smaller diameter, material with better thermal conductivity (e.g. certified 316L stainless steel) and/or with an optimized immersion length to reach the point of greatest turbulence in the fluid flow.
3. Improving Sensor-Sheath Coupling: Verify that the sensor is making firm contact with the bottom of the sheath. Apply heat-conducting thermal paste (e.g. silicone base with metal oxide particles) to the inside of the sheath before inserting the sensor to eliminate air spaces that act as an insulator.
4. Sensor Relocation: In cases where the sensor is in a 'dead zone' or low flow, evaluate the possibility of relocating the measurement point to an area where the fluid has greater velocity and mixing, ensuring a more accurate representation of process temperature.
5. Consider Fast Response Sensors: For highly dynamic processes, evaluate the feasibility of using exposed tip thermocouples or low thermal mass thin film RTDs, as long as the process environment allows it without compromising sensor life.
6. After modifications, use a data logger to verify improvement in response time.

8.4. Transmitter Configuration and Calibration

1. SAFETY WARNING! Apply LOTO to the control loop.
2. Isolate the transmitter from the process (if a head-mounted transmitter) or the control system (if a panel-mounted transmitter).
3. Connect the temperature calibrator/simulator to the transmitter input.
4. Connect a precision multimeter in series with the 4-20 mA output of the transmitter.
5. Access transmitter settings using a HART communicator or the manufacturer's proprietary software.
6. Check and adjust the following parameters:
   • Sensor Type: Make sure it exactly matches the physical sensor installed (e.g. Pt100, Type K Thermocouple).
   • Measurement Range: Set the range (Lower Range Value - LRV and Upper Range Value - URV) according to the process operating range.
   • Units: Verify that the temperature units are correct (°C).
   • Linearization: Confirm that the linearization curve corresponds to the sensor type.
   • Zero and Span Calibration: Perform a two-point calibration (zero and span) using the temperature calibrator to apply known inputs and adjust the transmitter output.
7. Disconnect test equipment, reconnect transmitter to control loop, remove LOTO and verify operation.

9. Preventive Measures

Implement these strategies to minimize the recurrence of temperature measurement discrepancies.

10. Spare parts and components

Having the right spare parts is essential for quick troubleshooting. Consult our e-catalog to verify availability and specifications.

To view our complete catalog of sensors, transmitters, thermometric pods and accessories, and place efficient orders, visit our e-catalog: https://www.unitecd.com/e-catalog/

11. References

• UNE-EN 60751:2009. Industrial platinum resistance temperature detectors.
• UNE-EN 60584-1:2014. Thermocouples. Part 1: Thermo-electromotive force tables and tolerances.
• Directive 2014/34/EU (ATEX): Harmonization of the laws of the Member States relating to devices and protection systems for use in potentially explosive atmospheres.
• UNE-EN 361:2002. Personal protective equipment against falls from height. Anti-fall harnesses.
• UNE-EN 60900:2019. Work in tension. Hand tools for nominal voltages up to 1000 V in alternating current and 1500 V in direct current.
• Original equipment manufacturers (OEM) operation and maintenance manuals.
• ISA (International Society of Automation) Recommended Practice Guides.