Three steps to pinpoint thermocouple/resistance temperature detector (RTD) faults on-site

Jan 06, 2026

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Multimeter + Observation Method: Troubleshooting in 10 Minutes, Saving You Thousands in Repair Costs!

🔥 Case Warning

The temperature of a chemical plant's reactor suddenly dropped to room temperature, triggering an alarm in the DCS system. Using only a multimeter, engineers pinpointed the fault:

→ Thermocouple compensation wire was broken (measured resistance ∞Ω)

→ Replacing the wire prevented a total production stoppage and a loss of 300,000 RMB!

I. Four Major Causes of Thermocouple Failure

1. Open Circuit → Most Common!

Symptoms: The meter displays an over-range reading (e.g., -999℃) or room temperature value.

Diagnosis:

✨ Multimeter Resistance Measurement: Disconnect the terminals and measure the resistance at both ends → Infinite resistance (∞Ω)

✨ Voltage Detection Method: Set the multimeter to the mV range. A normal output should be 0.1~50mV (varying with temperature). No output = open circuit.

2. Short Circuit (Ground/Line-to-Line Short Circuit) → Most Dangerous!

Phenomenon: Sudden drop in displayed value (e.g., 300℃ → 50℃)

Diagnosis:

✨ Measure terminal-to-ground resistance: Resistance < 100Ω → Insulation failure leading to grounding

✨ Measure electrode-to-electrode resistance: Approaching 0Ω → Electrodes touching

3. Poor contact → Most insidious!

Phenomenon: Large temperature jumps/fluctuations

Diagnosis:

✨ Measure circuit resistance with a multimeter: Gently shaking the terminals causes resistance to fluctuate greatly.

✨ Observe the terminals: Oxidation and blackening, loose screws

4. Electrode deterioration → Most fatal!

Symptom: Abnormally high output value (e.g., originally 600℃, now displaying 800℃)

Diagnosis:

✨ Visual Inspection: Blackening and green oxidation on the electrode surface (NiChromium-NiSilicon thermocouple)

✨ Slope Test: Output should ≈0mV in an ice-water mixture; deviation >0.1mV → failure

⚠️ Golden Rule: 80% of thermocouple failures are in the compensating wires! First, check the wiring from the junction box to the instrument.

II. Two Major Cracks in RTD Faults

1. Open Circuit → 100% Production Shutdown

Symptoms: Displays maximum value (e.g., 999℃) or open circuit alarm.

Diagnosis:

✨ Measure the resistance at terminals AB/AC/BC (three-wire system):

→ ∞Ω between any two points = open circuit

2. Short Circuit → Hidden Error Killer

Symptoms: Displayed value is significantly lower and remains constant.

Diagnosis:

✨ Measure the resistance between the wires: If the resistance between A-B/C ≈ 0Ω → internal short circuit

✨ Check for wiring errors: A two-wire system incorrectly connected to a three-wire system can cause an error of up to 10℃!

III. Three Essential Troubleshooting Techniques

Tools: Multimeter, screwdriver, insulating tape

Fault Type, Operation Steps, Diagnostic Standards

Thermocouple: 1. Disconnect terminals and measure resistance → 2. Measure mV output → 3. Check visual resistance: ∞ = open circuit, 0 = short circuit

RTD: 1. Measure resistance between three wires → 2. Check lead resistance: any value ∞ = open circuit, resistance approaching 0 = short circuit

Ultimate Technique:

➡️ Ice Water Bath Verification Method: Immerse the sensor end in 0℃ ice water. The thermocouple should output 0±0.2mV, and the Pt100 should display 100±0.5Ω.

IV. Three Preventative Measures

Monthly Inspection:

→ Thermocouple: Tighten junction box screws (to prevent vibration from loosening)

→ RTD: Check lead resistance (>1Ω requires replacement)

Interference Eliminator:

→ Single-end grounding of the thermocouple shielding layer

→ The RTD signal line should be separated from the power line by 30cm.

Hequan Instruments' Cutting-Edge Technology:

→ HqT300 Series Transmitter Automatic Diagnosis:

Thermocouple: Real-time cold junction compensation + open circuit alarm

RTD: Three-wire adaptive detection, lead resistance self-calibration

Conclusion: Faults are not terrible, misdiagnosis is fatal!