Abnormal PT100 reading? Use this 5-step troubleshooting method to quickly identify the issue.

"The PT100 temperature has jumped again!"  "The display reads 5°C higher than the actual temperature!"  "Why does it still show incorrect readings even with a three-wire connection?" — These are the most common complaints heard in industrial settings.

As the cornerstone of industrial temperature measurement, PT100 sensors can experience abnormal readings that may cause data distortion affecting process control or trigger false interlock protections. At Weilian Fengran, a seasoned expert in Temperature Sensor solutions, we've developed the most practical "5-step troubleshooting method" for field engineers—requiring only a multimeter to quickly identify PT100 faults within 10 minutes.

1. Five-step troubleshooting method: Rapid localization procedure for abnormal PT100 readings

[01] Step 1: Visual inspection (initial assessment after power interruption)

First, disconnect the instrument's power supply, then open the PT100 junction box and carefully check three key details:

• Inspect the junction box: Check for moisture, copper green deposits, or dust; verify that screws are securely tightened. This is the primary inspection point for abnormal PT100 readings in high-humidity environments.
• Inspect the protective tube: Check for cracks, severe corrosion, or dust accumulation, especially during liquid temperature measurement or under negative pressure conditions.
• Inspect the leads for damage, compression, or signs of aging-related breakage.

💡 Experience: Over 50% of PT100 failures can be detected at the visual inspection stage alone.

[02] Step 2: Measure resistance with a multimeter (core step)

Disconnect the connecting wires from the sensor terminals and measure the resistance between the terminals using a multimeter set to the 200 Ω ohm range. This is the most critical step in troubleshooting PT100 faults.

At room temperature, the standard resistance value of PT100 is approximately 109.73 Ω (at 25°C). The measurement results are as follows:

• Resistance value ≈ 109–111 Ω → The Pt100 Sensor itself is functioning normally; the issue lies in the instrument or wiring circuit.
• Resistance value → ∞ (infinite) → open circuit fault; check the leads or replace the sensor
• Resistance value ≈ 0 Ω or significantly lower → Short-circuit fault, typically caused by moisture exposure or insulation damage

💡 Advanced: Use a megohmmeter to measure the insulation resistance between the lead and the shielding layer; it should be ≥10 MΩ. A value below this indicates moisture presence.

[03] Step 3: Wire Connection Verification (Three-Wire System Confirmation)

The three-wire system is crucial for minimizing lead resistance errors, yet 90% of PT100 reading anomalies stem from incorrect wiring connections.

• Conversion from three-wire system to two-wire system → Lead resistance not compensated, resulting in overall high temperature readings
• Reversed connection of compensation wire → Measurement deviation may exceed ±5°C
• Multi-point grounding of the shielding layer → induces circulating interference, causing reading fluctuations

Correct procedure: Connect wires strictly according to the three-line system specification (red–red–white, or follow manufacturer markings); ground the shielding layer at a single point only on the control cabinet side, while leaving the local terminal ungrounded.

💡 Quick diagnosis: Short-circuit the PT100 wiring at the instrument end; the instrument should display the corresponding resistance value based on room temperature. Significant deviation indicates a circuit fault.

[04] Step 4: Interference Diagnosis (Signal Stability)

If the resistance value is normal but the PT100 displays jump readings or instability, there is a 90% probability of an interference issue. Key troubleshooting points:

• Are the signal lines installed parallel to the power cables? They should be routed separately with a minimum spacing of 30 cm.
• Are there any high-power devices such as frequency converters or solenoid valves nearby? Install surge protectors if necessary.
• Is the shielding layer grounded at a single point? Multi-point grounding may instead introduce interference.
• Are the DCS/PLC channel filtering parameters appropriate? The damping time may be appropriately increased.

💡 Typical characteristics: The temperature suddenly spikes to an abnormal value, then recovers within seconds, exhibiting "pulse-like" fluctuations—almost always due to interference.

[05] Step 5: Calibration Verification (Accuracy Confirmation)

The first four steps can resolve approximately 90% of PT100 faults; the final step is to verify accuracy.

• Ice-water mixture test: Insert the PT100 probe into a pure ice-water mixture (using 0°C as reference) and measure a resistance of 100.00 Ω ± 0.1 Ω (Class A accuracy).
• Comparison with a standard thermometer: Under a stable heat source (e.g., a constant-temperature water bath), compare the readings at three temperatures of 0°C, 50°C, and 100°C with those obtained using a calibrated standard thermometer.
• Long-term stability test: According to IEC 60751 standards, the accuracy of Class A PT100 sensors is ±(0.15 + 0.002 |t|) °C; deviation beyond this range requires replacement.

💡 Recommendation: Calibrate PT100 sensors at key measurement points every six months and maintain a log to track drift trends.

1. Quick Reference Table for Common PT100 Malfunctions (Recommended for Storage)

1. The "Three Habits" to Minimize PT100 Malfunctions

[Model Selection Phase] Choose the appropriate model based on temperature measurement range, accuracy class, and installation environment to avoid using a model that is too small for the task or operating it under prolonged high-temperature conditions.

[Installation Phase] Connect wires strictly according to three-wire or four-wire standards; ensure single-ended grounding of the shielding layer, and keep signal lines separate from power cables.

[Maintenance Phase] Maintain a PT100 usage logbook and perform calibration every six months, with particular attention to equipment operating in high-humidity, vibrating, or corrosive environments.

Don't panic if your PT100 reads abnormally – follow these 5 steps to diagnose the issue in just 10 minutes.

Appendix: Schematic diagram of the PT100 fault diagnosis procedure