Accurate temperature measurement is critical in industrial processes, yet technicians often encounter a common problem:
the multimeter shows a “normal” resistance or mV value, but the temperature reading in the control system is still wrong.
In most cases, the issue is not with the multimeter, but with how the measurement should be interpreted.
By understanding the core behavior of Pt100 RTDs and K-type thermocouples, you can quickly estimate temperature, identify abnormal signals, and determine whether the sensor or wiring is faulty — all with a standard multimeter.
1. Key Principles Every Technician Must Know
1.1 Pt100 RTD — Resistance Increases Linearly with Temperature
At 0°C, the resistance is exactly 100 Ω.
The resistance increases by approximately 0.385 Ω per °C.
A multimeter measures resistance (Ω) directly; higher resistance = higher temperature.
This simple linearity is the foundation for quick field evaluation.
1.2 K-Type Thermocouple — Voltage Generated by Temperature Difference
A thermocouple produces a small voltage (mV level) based on the temperature difference between:
The measuring (hot) junction
The reference (cold) junction — usually the termination point or the multimeter connection
A common mistake:
👉 The multimeter does NOT read actual temperature — it reads temperature difference.
Therefore, the cold-junction temperature must always be known or compensated.
2. Quick Temperature Estimation Methods
2.1 Pt100 RTD — Two Practical Methods
Method 1: Fast Estimation Formula
For 0–100°C (excellent linearity):
Temperature (°C) ≈ (Measured Resistance − 100) / 0.385
Tip: For quick mental math, use 0.4 instead of 0.385 (only ~3% error).
Example:
Measured resistance = 115 Ω
(115 − 100) / 0.4 ≈ 37.5°C
(Actual value ≈ 39°C)
This accuracy is more than enough for troubleshooting.
Method 2: Table Lookup (100% Accurate)
Use the official Pt100 IEC751 table.
Key reference points:
0°C → 100.00 Ω
50°C → 119.25 Ω
100°C → 138.50 Ω
200°C → 175.86 Ω
If the measured resistance lies between two values, simple linear interpolation provides the actual temperature.
Practical Field Tips (RTD)
✔ Check the wiring method
2-wire: includes lead resistance, must subtract cable resistance.
3-wire: two measured values must be equal → wiring OK.
4-wire: highest accuracy, no compensation needed.
✔ Quick Good/Bad Judgment
At room temperature (≈25°C), normal Pt100 ≈ 109–110 Ω.
Abnormal signs:
Near 0 Ω → short circuit
Very high resistance or OL → open circuit
2.2 K-Type Thermocouple — Two Methods to Estimate Actual Temperature
Method 1: Approximate Calculation (Fast Troubleshooting)
In 0–100°C range:
K-type ≈ 0.04 mV per °C
So:
Temperature ≈ (Measured mV ÷ 0.04) + Cold-Junction Temperature
Example:
Environment = 25°C
Measured = 2.0 mV
(2.0 ÷ 0.04) + 25 = 75°C
This method is for quick screening, not calibration.
Method 2: Accurate Table-Based Calculation
Essential for high temperatures or commissioning.
Step 1 — Measure
E_mv = thermocouple output (mV)
T_ref = cold-junction temperature (°C)
Step 2 — Lookup cold-junction voltage
Example:
25°C ≈ 1.000 mV
Step 3 — Calculate total thermoelectric voltage
E_total = E_mv + E_ref
Step 4 — Reverse lookup
Find temperature corresponding to E_total using the standard K-type table.
Example:
Measured: E_mv = 20.0 mV
Cold junction: 25°C → 1.000 mV
E_total = 21.0 mV → corresponds to ≈ 506°C
Practical Field Tips (Thermocouple)
✔ Cold-junction compensation is mandatory
If cold junction is 25°C but ignored, the error will be 25°C immediately.
✔ Quick Good/Bad Check
Short-circuit test:
Short the TC wires → multimeter should read mV corresponding to ambient temperature
(≈1.0 mV at 25°C)Heat test:
Apply a flame/heat → mV must rise quicklyReverse polarity:
Positive wire of K-type is magnetic (usually yellow or red depending on region coding)
Wrong polarity → negative mV → wrong temperature
✔ Typical expected values
At room temperature: 0.1–1.0 mV
If tens of mV are seen at room temperature → likely wrong thermocouple type or wiring error.
3. Practical Troubleshooting Guide for Technicians
1. Always record ambient temperature first (for thermocouples).
2. Compare multimeter readings with DCS/PLC values.
A difference within ±5°C (allowing for cable resistance and cold-junction error) is generally acceptable.
3. Maintain a printed or digital copy of the Pt100 and K-type tables.
Having the tables accessible significantly speeds up diagnostics.
4. Ensure personal safety
High temperature and live circuits can be dangerous.
Use gloves, insulation tools, and maintain safe working distance.
4. Summary
Using a multimeter to diagnose temperature sensor problems does not require complex theory.
You only need to remember two fundamentals:
✔ Pt100: approximately 0.385 Ω per °C
✔ K-type: always apply cold-junction compensation
By combining quick estimation formulas, table lookup methods, wiring checks, and simple functional tests, technicians can efficiently determine whether the sensor, wiring, or transmitter is at fault — often within minutes.