Troubleshooting

PT100 Temperature Reading Is Wrong: RTD Troubleshooting for PLCs

The tank is at 60 °C and the PLC says 64.8 °C. With a PT100, a few ohms of cable are a few degrees of error — here is how to find them.

· 8 min read · By TryPLC Team

  • PT100
  • RTD
  • Temperature
  • Analog Input

The symptom

A PT100 sensor on an RTD input card reads a few degrees high, reads absurd values, or jumps around. A handheld reference thermometer disagrees with the PLC.

Know the numbers

A PT100 has 100 Ω at 0 °C and rises about 0.385 Ω per °C (IEC 60751). So:

  • 1 Ω of error ≈ 2.6 °C of error.
  • At 60 °C a healthy PT100 reads about 123.2 Ω.

    • This makes wiring resistance critical.

    Step 1 — Measure the sensor directly

    Disconnect the sensor at the card and measure its resistance with a multimeter. Compare with a PT100 table at the actual process temperature.

  • Close to the table: the sensor is fine; the error is wiring or configuration.
  • Open circuit: broken element or wire. The card normally shows overrange or a wire-break diagnostic.
  • Near 0 Ω: short circuit, often moisture in the head or a crushed cable.

    • Step 2 — 2-wire, 3-wire or 4-wire?

  • 2-wire: the card measures sensor plus both leads. 20 m of 0.5 mm² cable adds about 1.4 Ω — roughly 3.6 °C high.
  • 3-wire: the card compensates assuming both leads are equal. Good for most process work.
  • 4-wire: separate current and sense leads; lead resistance is eliminated. Best accuracy.

    • Make sure the card configuration matches the physical wiring. A 4-wire card channel set to 2-wire, or a 3-wire sensor with one lead missing, gives a steady offset.

    Step 3 — Sensor type and units

    Check the channel is set to PT100, not PT1000 or Ni100, and to the correct standard (IEC 0.00385 versus older 0.00392 curves). Check the raw value format: many cards return temperature × 10, so 600 means 60.0 °C.

    Step 4 — Connections and moisture

    Loose terminals in the sensor head add resistance that changes with vibration and temperature — an unstable reading is the clue. Corrosion and condensation in the head cause slow drift. Clean, re-terminate and seal the cable gland.

    Step 5 — Noise

    Unstable readings that change when a pump or heater switches indicate noise. Use shielded cable grounded at one end, route it away from power cables, and enable the card's mains frequency suppression filter (50 or 60 Hz).

    Step 6 — Thermowell and placement

    If the electrical side is perfect, the sensor may not be measuring what you think: a probe not touching the bottom of its thermowell, a well in a dead zone of the tank, or no thermal paste. The PLC is then reporting the correct temperature of the wrong place.

    Read about the full instrument chain in the hardware chapter on process instruments.

    Frequently asked questions

    How much error does cable resistance add to a PT100?

    About 2.6 °C per ohm. On a 2-wire connection the resistance of both leads is added to the reading.

    What resistance should a PT100 have?

    100 Ω at 0 °C, rising about 0.385 Ω per °C — about 123.2 Ω at 60 °C.