Troubleshooting discrepancies in temperature measurement: analyzing sensor selection, thermal inertia, line resistance and transmitter settings

1. Description of the problem and scope of application

These instructions serve to systematically identify and eliminate measurement deviations in industrial temperature measuring points. Discrepancies between the process value (ACTUAL temperature) and the measured value (display/control) can lead to inefficient system states, unacceptable product qualities or, in critical cases, to the triggering of safety shutdowns.

Affected equipment: Resistance thermometers (Pt100, Pt1000), thermocouples (type K, J, N, S, R), measuring transducers (transmitters) 4-20 mA/HART, process control systems (PLS) and PLC input cards.

Severity classification:

• Critical: Temperature monitoring for safety functions (e.g. according to SIL, explosion protection/ATEX). Deviations > 1% of the final value require immediate correction.
• Major: Temperature control in quality-critical processes. Deviations lead to rejects.
• Minor: Monitoring functions without a direct influence on product quality.

2. Safety instructions

ATTENTION: DANGER CAUSED BY ELECTRICAL VOLTAGE. Before starting work on electrical connections or measuring transducers, the system must be activated in accordance with DIN VDE 0105-100 (five safety rules). For temperature measuring points on pressurized systems, the risk from thermal energy and potentially dangerous media (e.g. steam, chemicals) must be taken into account. Wearing personal protective equipment (PPE), consisting of safety glasses, heat-protective gloves and flame-retardant work clothing, is mandatory.

3. Required diagnostic tools

4. Initial assessment protocol

5. Systematic diagnostic flow

1. Step 1: Plausibility check of the display
   
   • Does the discrepancy agree with the process physics? (e.g. heating due to friction).
   • IF deviation > 5 °C (stationary) → Continue to step 2.
2. Step 2: Check the measuring transducer (transmitter)
   
   • Measure the input signal of the sensor directly at the transmitter (resistance/voltage).
   • Check the output signal of the transmitter (4-20 mA).
   • IF mA signal does not correspond to the sensor input → Configuration error/defect.
   • IF mA signal correct, but display incorrect → Check PLC/PLS scaling.
3. Step 3: Check sensor and cabling
   
   • Measure the resistance of the sensor supply line (especially with a 2-wire connection).
   • Check insulation between sensor and protective tube.
   • IF resistance values outside the DIN EN 60751 tolerance → Sensor or cabling defective.

6. Error-cause matrix

7. Root Cause Analysis

7.1 Cable resistance (resistance thermometer)

For Pt100 sensors with 2-wire connection technology, the resistance of the supply lines is added to the sensor resistance. 1 ohm line resistance corresponds to approx. 2.5 °C offset. Since this resistance is temperature dependent, this leads to a non-linear measurement error.

7.2 Thermal inertia

A protective tube that is too massive (thermowell) or poor heat transfer between the sensor and the protective tube leads to a time delay (T90 time). This is critical for fast control processes.

7.3 Calibration errors/configuration errors

Incorrect input type (e.g. Pt100 vs. Pt1000) on the transmitter or incorrect zero point/span adjustment leads to systematic deviations across the entire measuring range.

8. Step-by-step resolution

1. Correction of line resistance: Conversion to 3 or 4-wire technology. If technically not possible, measure the line resistance and adjust it in the transmitter as an offset (if possible).
2. Correction thermal inertia: Check thermowell geometry. If necessary, use thermal paste (temperature-resistant up to the target temperature) or switch to sensors with a lower mass (sheathed thermocouples).
3. Correct transmitter configuration: Read configuration via HART protocol or software. Check input type and scaling. Simulation of the 4 mA and 20 mA values ​​and comparison with the PLC display.

9. Preventive measures

10. Spare parts and components

Detailed specifications and ordering options can be found at: https://www.unitecd.com/e-catalog/

11. References

• DIN EN 60751: Industrial platinum resistance thermometers.
• DIN EN 60584: thermocouples.
• VDE 0105-100: Operation of electrical systems.
• UNITEC-D maintenance guide: Installation of temperature measuring points (internal link).