How to Troubleshoot Industrial Temperature Transmitters ?

A temperature transmitter is an instrument that converts temperature variables into standardized output signals (e.g. 4-20 mA or 0-10 V). It is widely used in industrial processes for temperature measurement and control. During operation, transmitters may encounter faults due to environmental, operational, or installation factors. The following guide provides common troubleshooting methods and maintenance tips for resolving these issues effectively.

Procedure for Troubleshooting Temperature Transmitters in PLC Analog Input Loop

Safety Precautions

• Identify potential hazards such as electrical shocks, moving machinery, or hot surfaces.
• Wear appropriate personal protective equipment (PPE).
• Follow the site’s risk assessment procedures.

• Isolate the loop from the process or bypass the signal to avoid triggering interlocks or alarms.
• Obtain approval through the site’s work permit system before proceeding.

• Use caution when bypassing signals to ensure interlocks are not unintentionally disabled.
• Notify relevant personnel about the bypass.

Required Tools and Documentation

• Multimeter.
• Loop calibrator.
• HART communicator.
• Screwdrivers and other basic hand tools.

• Loop drawing and circuit diagram.
• Manufacturer’s datasheets and manuals.
• Work permits and safety checklists.

Visual Inspection

Cable and Connection Checks

• Ensure cables are securely connected and there are no loose or corroded terminals at the transmitter, junction boxes, or PLC analog input (AI) module.
• Confirm the transmitter’s proper grounding.

Verify the Power Supply

• Check that the transmitter is receiving the required power (e.g., 24 VDC for 2-wire transmitters).
• Use a multimeter to confirm voltage at the transmitter’s power terminals.

Use a HART Communicator (If Transmitter Supports HART Protocol)

• Connect the communicator across the transmitter terminals or in a loop junction.
• Verify the HART communicator can detect the transmitter. If not, check wiring and loop power.

• Read diagnostic messages and alarms directly from the transmitter.
• Verify the sensor range, configuration, and failure conditions.

• Ensure the correct temperature sensor type (e.g., RTD/PT100, thermocouple type) and scaling range (e.g., 0-100°C = 4-20 mA).
• Check damping settings if the output signal is fluctuating excessively.
• Use the HART communicator to force the transmitter to output specific current values (e.g., 4 mA, 12 mA, or 20 mA) and verify the PLC input matches these values.

 Inspect the Sensor side

• Use an ohmmeter to check an RTD’s resistance (e.g., PT100 = ~100 ohms at 0°C).
• For thermocouples, measure millivolt output and verify it corresponds to the expected temperature (using a thermocouple chart).

• Ensure that the transmitter or PLC has proper cold junction compensation enabled to account for the reference junction temperature.
• Verify the ambient temperature at the cold junction is within the specified operating range of the transmitter.
• Inspect the cold junction compensation circuitry for faults, if applicable.

• Look for open circuits (infinite resistance), shorts (low resistance), or sensor wiring errors (e.g., 3-wire RTD connected as 2-wire).
• For thermocouples, inspect the polarity and wiring to ensure proper installation.

• Ensure the sensor is not exposed to temperatures or environments outside its rated range.
• Verify that thermocouples are not subjected to contamination or physical damage, which could impact their accuracy.

Measure and Verify Loop Current / Voltage

Test Loop Signal

• Use a multimeter or loop calibrator to measure the loop current (4-20 mA) or voltage (e.g., 0-10 V) at various points:
  1. Transmitter output terminals: Ensure the transmitter outputs a current or voltage corresponding to the actual process temperature.
  2. PLC analog input module terminals: Verify that the signal at the PLC matches the transmitter output.
• Compare the measured values to the expected process condition. For example, at 50% of the transmitter range, the current should be 12 mA.
• If discrepancies occur, identify whether the issue lies in the transmitter, wiring, or PLC AI module.

Loop Power Check

• Confirm that the loop power supply is providing sufficient voltage and current to drive all devices in the loop, including the transmitter and connected instruments.
• Measure the voltage across the transmitter’s power terminals to ensure it meets the manufacturer’s specifications (e.g., 24 VDC for most 2-wire transmitters).
• Check for excessive loop resistance caused by long cable runs or poor connections. High resistance can drop the voltage below the required level, leading to transmitter malfunctions.
• Verify the integrity of the power supply and ensure it is stable and free of fluctuations.

Signal Barrier and Marshalling Panel Components

• Inspect the signal barrier for proper operation in intrinsically safe circuits. Ensure it matches the hazardous area requirements and does not introduce excessive loop resistance.
• Bypass the signal barrier if safe to do so, to confirm it is not introducing errors.
• Verify the integrity of fuses and other protective components in the marshalling panel. Replace blown fuses and ensure proper wiring.
• Confirm proper grounding and shielding to avoid signal interference.

How to check temperature transmitter?

Calibration and Verification

Calibration Verification

• Use a loop calibrator or simulation tool to inject known signals (e.g., 4 mA, 12 mA, 20 mA) into the system.
• Verify that the PLC analog input module reads and scales these signals accurately within the expected range.

Click here for How to Calibrate RTD transmitter?

Sensor Simulation

• For RTDs, use a decade resistance box to simulate specific temperatures by inputting resistance values corresponding to the RTD’s curve (e.g., 138.5 ohms for 100°C for a PT100).
• For thermocouples, use a millivolt simulator to inject equivalent voltage signals based on the thermocouple type and temperature chart.
• Verify that the transmitter and PLC display the correct simulated values.

Refer the below link for How to simulate RTDs and Thermocouples using Multifunction calibrator?

Check the PLC Analog Input Module

PLC Analog Input Card Configuration

• Check the configuration settings of the PLC AI card, ensuring:
  • The correct signal type is selected (e.g., 4-20 mA or 0-10 V).
  • Cold junction compensation (CJC) is enabled and properly calibrated for thermocouples.
  • Scaling parameters match the transmitter’s range.
• Calibrate the analog input card using manufacturer-recommended procedures if necessary.
• Check whether the channel is enabled in the PLC program or hardware configuration.

Channel Integrity

• If possible, swap the input channel to another unused one to rule out a faulty channel.
• Test the PLC AI module and program logic independently with simulated signals.

Signal Verification

• Inject a known signal (e.g., 4 mA or 20 mA) directly to the AI module terminals and confirm the PLC registers or ladder logic reflect the expected value.

Click here for How to calibrate Thermocouple Transmitter?

Check PLC Ladder Logic or Function Block Program

Scaling Block

• Verify the scaling parameters in the PLC program match the transmitter range (e.g., 4 mA = 0°C, 20 mA = 100°C).

Fault Alarms and Overrides

• Review the logic for high/low alarms, fault conditions, or forced values that may be overriding the actual signal.
• Ensure alarm setpoints and overrides are properly programmed for operational safety.

Debugging Tools

• Use online monitoring tools in the PLC software to trace the signal from the AI module to the program logic.
• Verify that the raw data from the AI module matches the expected values.
• Ensure intermediate calculations or processing logic does not introduce errors.

How to troubleshoot a temperature transmitter?

Common Problems in Temperature Transmitter and How to Rectify

Click here for What is Burnout Function of a Temperature Transmitter with an example

De-Isolation & Completion and Final Verification

• Once repairs or adjustments are made, test the entire loop end-to-end.
• Compare readings on the transmitter, HART communicator, and PLC input to ensure consistency.
• Conduct a functional test by varying the temperature and confirming the expected response across the system.

• Close the work permit once the troubleshooting is complete.
• Remove all bypasses, reconnect safety systems, and restore normal operation.
• Perform a final functional test to ensure the system operates as expected under process conditions.
• Record all calibration and troubleshooting results for future reference.
• Obtain final approval from operations before handing the system back to production.

 Checklist for Troubleshooting Temperature Transmitters

Download this comprehensive temperature transmitter troubleshooting checklist in Excel format to ensure systematic diagnostics, covering safety, tools, inspections, power checks, and final verification. Refer the below link