1. Description of the problem and scope of application
Inconsistencies in temperature measurement in industrial processes lead to a decrease in product quality, the transition of equipment into emergency mode, or irrational consumption of energy resources. This manual covers the diagnosis of measurement deviations caused by sensor selection, thermal inertia, resistance of connecting wires and incorrect setting of transducers. Criticality classification: critical for safety processes, high for heat treatment processes.
2. Safety measures
WARNING: Before starting work with temperature sensors, it is necessary to perform the procedure of blocking and marking (LOTO) of energy carriers. The equipment may be under pressure or have a high surface temperature. Use appropriate personal protective equipment (PPE), including heat-resistant gloves and safety glasses.
3. Necessary diagnostic tools
| Tool | Specification/Model | Measurement range | Purpose |
|---|
| Digital multimeter | TRUE RMS, class 0.05% | 0-100 MΩ, 0-1000 mA | Checking the resistance of the sensor, the integrity of the lines |
| Temperature calibrator | Dry block or liquid bath | -50...+600°C | Checking the accuracy of the sensor and transducer |
| Thermal imager | Resolution 320x240 | -20...+1000°C | Detection of local overheating or heat leaks |
| Resistance shop | Accuracy class 0.01 | 0-2000 ohms | Pt100 signal simulation for converter diagnostics |
4. Initial assessment checklist
| action | The goal |
|---|
| Checking the history of alarm signals (Alarm Log) | Detection of patterns of signal drift |
| Fixation of environmental conditions | The effect of air temperature on the connection line |
| Overview of terminal connections | Detection of corrosion or loosening of contacts |
| Checking the sensor type in the PLC/transducer settings | Troubleshooting configuration errors (eg Pt100 vs Ni100) |
5. Systematic diagnosis diagram
- Symptom: Permanent reading offset (Offset)
- IF resistance sensor (RTD) → check line resistance → IF > 2 Ohm/wire → probable cause: long cable length or section.
- IF thermocouple (TC) → check the integrity of the thermocouple cable → IF existing connections with other metals → probable cause: parasitic thermo-EMF.
- Symptom: High inertia (slow response)
- Check sensor immersion depth → IF insufficient → probable cause: contact with protective sleeve, not medium.
- Check the presence of thermal paste in the sleeve → IF missing → probable cause: air gap (thermal resistance).
- Symptom: Jumps in readings (Noise)
- Check for sources of electromagnetic interference (frequency converters, motors) near the signal cables → IF present → probable cause: lack of shielding.
6. Matrix of malfunctions and causes
| Symptom | Probable reasons | Diagnostic test | Expected result |
|---|
| Constant deviation | Calibration error | Comparison with the standard | Deviation > 0.5°C |
| Temperature drift | Moisture in the sensor head | Measurement of insulation resistance | Resistance < 10 MΩ |
| Slow reaction | Air in the sleeve | Thermal imaging control of the sleeve | Temperature gradient |
7. Root cause analysis
Resistance of connection lines: When using a 2-wire Pt100 connection scheme, the resistance of the wires is added to the resistance of the sensor. The resistance of a copper wire (0.5 mm²) is approximately 0.035 Ohm/m. At 50 meters, this gives 3.5 ohms, which corresponds to an error of ~9°C.
Thermal inertia: Stainless steel protective sleeves have a high heat capacity. If the sensor does not have a tight thermal contact with the walls of the sleeve, there is a significant delay in heat transfer, which makes it impossible to control dynamic processes.
8. Step-by-step solution procedures
- Fixing the effect of wire resistance: Switch the RTD sensor to a 3- or 4-wire connection scheme. If it is not possible, calibrate the offset (Offset) in the converter to the value of the line resistance.
- Correction of thermal inertia: Clean the protective sleeve from the remnants of the old paste, apply fresh heat-conducting paste (up to 250°C) or insert a metal sleeve-insert.
- Transducer Settings: Make sure the transducer range (4-20mA) matches the process range to minimize error.
9. Preventive measures
| The root cause | Strategy | Monitoring method | Interval |
|---|
| Oxidation of contacts | Use of anti-corrosion sprays | Overview of terminals | 6 months |
| Characteristic drift | Scheduled calibration | Comparison with the standard | 12 months |
10. Spare parts and components
| Description | Specification | When to replace | Category UNITEC |
|---|
| Sensor Pt100 | Class A, 3-wire | If the deviation is > 0.5°C | Temperature sensors |
| 4-20 mA converter | Programmable, HART | When the exit is refused | Electronics |
The full range of components is available in our catalog: https://www.unitecd.com/e-catalog/
11. Links
- DSTU EN 60751:2015 (Platinum resistance thermometers)
- ISO 9001:2015 (Quality management systems)
- Technical regulations of the sensor manufacturer (OEM Manuals)
