Troubleshooting temperature measurement errors: diagnostics of sensors, thermal inertia and transmitter configuration

Description of the problem and scope of application

This technique is intended for diagnosis and elimination of temperature measurement errors in industrial control and automation systems. Discrepancies in displays can cause critical deviations in technological processes, lead to product shortages and emergency situations.

Severity Classification:

• Critical: Error >±5°C in processes with a narrow temperature range
• Significant: ±2-5°C error in standard manufacturing processes
• Minor: Error <±2°C in wide tolerance processes

Types of equipment: Thermocouples, RTD thermocouples, 4-20 mA temperature transmitters, temperature controllers, SCADA systems.

Safety measures

ATTENTION: Before starting diagnostics, be sure to take the following safety measures:

• Use PPE: dielectric gloves, safety glasses, flame-resistant clothing
• Lockout/Tagout: Disconnect power before working with wiring
• High Temperature: Allow the equipment to cool down to <60°C
• Chemical Hazards: Check for aggressive media around sensors
• Electrical risks: Check insulation when working with 24V DC transmitters

Diagnostic tools are required

Initial assessment checklist

A systematic scheme of diagnostics

1. Checking the indication and emergency signals
   • If there are no indications at all → go to point 2
   • If the readings are unstable/noisy → go to item 3
   • If the readings are stable, but inaccurate → go to item 4
2. Power and signal diagnostics
   • Measure the power supply voltage at the transmitter terminals
   • If <21.6V або >26.4V → power supply problem
   • Measure the circuit current of 4-20 mA
   • If <3.8 mA або >20.5 mA → circuit damage
3. Analysis of unstable readings
   • Measure noise with an oscilloscope (amplitude >50 mV - unacceptable)
   • Check cable shielding
   • Check ground (resistance <1 Ω)
4. Measurement accuracy diagnostics
   • Compare with reference thermometer
   • If the error is >±2°C → go to the cause matrix
   • Check sensor calibration

Cause and symptom matrix

Detailed analysis of the reasons

Incorrect choice of sensor type

The main reason is the inconsistency of the sensor characteristics with the process conditions. Type K thermocouples cannot be used in reducing environments at temperatures >800°C.

Diagnosis: Check sensor specifications against process conditions.

Consequences: Degradation of accuracy, reduction of service life, emergency shutdowns.

Thermal inertia (time constant)

Sensors with a large thermal mass do not have time to track rapid temperature changes. The standard time constant τ63% should be <10 sec for most processes.

Diagnosis: Measure the reaction time to a sudden change of 50°C.

Criteria: τ63% >30 sec indicates a thermal inertia problem.

Resistance of communication lines

For RTD Pt100, the resistance of each wire should not exceed 5 Ω. With a 3-wire connection scheme, the resistance of the wires causes an error of +0.4°C for every 1 Ω.

Diagnosis: Measure the resistance of each wire from the transmitter to the sensor.

Critical values: >10 Ω - critical, 5-10 Ω - needs correction.

Incorrect transmitter configuration

Errors in setting ranges, sensor type, cold junction compensation (for thermocouples) lead to systematic errors.

Diagnosis: Compare the settings with the technical requirements of the process.

Step-by-step elimination procedures

Procedure 1: Sensor selection correction

1. Determine the real conditions of the process (temperature, pressure, environment)
2. Select the appropriate sensor type according to DSTU EN 60584-1:2014
3. For high temperature applications (>800°C) use R or S type thermocouples
4. For accurate measurements (<200°C) use RTD Pt100 class A
5. Install a new sensor with an immersion depth of ≥10x the diameter of the sensor
6. Check the readings by comparison with a reference thermometer

Procedure 2: Reduction of thermal inertia

1. Measure the current time constant τ63%
2. If τ63% >20 sec, replace with a smaller diameter sensor
3. Improve heat transfer: use heat-conducting paste
4. Check the contact of the sensor with the surface/environment
5. Measure τ63% again - should be <15 sec
6. Document the new time constant for the controller setup

Procedure 3: Correction of wiring resistance

1. Measure the resistance of each wire with the sensor disconnected
2. If the resistance is >5 Ω, replace the cable with a larger section (min. 1.5 mm²)
3. For long lines (>100m) use a 4-wire circuit for the RTD
4. Install an intermediate transmitter near the sensor if the length is >200m
5. Check the quality of the connections: tightening torque 0.5-0.8 Nm
6. Measure the error after correction: it should be <±1°C

Procedure 4: Transmitter configuration

1. Connect a HART communicator or a computer with the appropriate software
2. Check sensor type settings (RTD: Pt100, TC: K, J type, etc.)
3. Set the correct measurement range according to technical requirements
4. For thermocouples, adjust the cold junction compensation
5. Perform 2-point calibration (0% and 100% scale)
6. Set damping for 2-5 seconds to reduce noise
7. Save the configuration and check the operation

Preventive measures

Spare parts and components

To order spare parts, visit the UNITEC-D electronic catalog: UNITEC-D E-Catalog

Reference materials

• DSTU EN 60584-1:2014 - Thermocouples. Part 1. EMF specifications and tolerances
• DSTU IEC 60751:2009 - Industrial platinum resistance thermometers
• ISO 5168:2005 - Measurement of fluid flow - Procedures for estimating uncertainties
• EN 50446:2008 - General requirements for methods of measuring electromagnetic fields
• Transmitter manufacturers' manuals (Rosemount, Endress+Hauser, WIKA)
• UNITEC Maintenance Guides - Calibration of measuring devices
• UNITEC Maintenance Guides - Protection against electromagnetic interference