### **PR235: How to Diagnose Thermocouple and RTD Faults?**

In the intricate world of process control and calibration, temperature sensors are the unsung heroes, providing critical data for everything from safety interlocks to quality control. The Emerson Rosemount PR235 Multifunction Process Calibrator is an indispensable tool for technicians tasked with maintaining these sensors. However, when a temperature reading seems inaccurate or a loop fails, quickly and accurately diagnosing whether the fault lies with the thermocouple (TC), the Resistance Temperature Detector (RTD), the wiring, or the receiving instrument is a core skill.

Misdiagnosis can lead to extended downtime, unnecessary parts replacement, and persistent process issues. The PR235, with its versatile measurement capabilities, is perfectly suited to perform a systematic and conclusive diagnosis. This article provides a comprehensive guide on how to use the PR235 to diagnose common failures in both thermocouples and RTDs.

#### **1. Initial Setup and Safety First**

Before beginning any diagnostic procedure:

*   **Understand the Process:** Know the expected temperature and ensure it is safe to disconnect the sensor.

*   **Isolate the Sensor:** If possible, disconnect the sensor from the control system to avoid live loops or back-powered signals that could skew readings or damage the calibrator.

*   **Use Personal Protective Equipment (PPE):** Wear appropriate PPE for the process environment, especially when dealing with high temperatures.

*   **Prepare the PR235:** Ensure the PR235 is calibrated and functioning correctly. Use high-quality, insulated test leads.

#### **2. The Foundation: Understanding the Sensors**

To diagnose faults, you must know what a healthy sensor looks like.

*   **Thermocouples:** Generate a small DC voltage (mV) proportional to the temperature difference between the measuring junction (hot end) and the reference junction (cold end, compensated inside the PR235). They are prone to issues related to their millivolt-level signals.

*   **RTDs:** Change their electrical resistance predictably with temperature. The most common is the Pt100, which has a resistance of 100 ohms at 0°C. They are prone to issues that affect resistance.

#### **3. Systematic Diagnosis of Thermocouples with the PR235**

**Step 1: Visual Inspection**

*   Check the thermocouple sheath for physical damage—kinks, cracks, or corrosion.

*   Inspect the connection head for moisture, corrosion, or loose terminals.

**Step 2: Measure the Output Signal (mV)**

*   Set the PR235 to the correct thermocouple type (e.g., Type J, K, T) and to the **mA/mV DC measurement** function.

*   Connect the PR235 test leads to the thermocouple wires at the connection head.

*   **Observe the reading:**

    *   **Plausible Value Fluctuating:** The sensor is likely functional and reading the process temperature.

    *   **Zero or Very Low mV:** Indicates an open circuit (broken wire) or a short circuit.

    *   **Erratic/Jumpy Reading:** Often caused by intermittent connections, EMI noise on the poor signal, or moisture.

    *   **Stable but Incorrect Value:** Could indicate degradation/contamination of the thermocouple wire.

**Step 3: Check for Open or Short Circuit**

*   **Open Circuit Test:** Set the PR235 to **Ohms (Ω) resistance mode**. Disconnect the thermocouple. Measure across the two TC wires. A reading of **OL (Over Limit) or very high resistance** confirms an open circuit—the thermocouple is broken and must be replaced.

*   **Short Circuit Test:** In the same Ohms mode, a reading of **zero or very low resistance (e.g., 1-2 Ω)** indicates a short circuit between the two wires, often near the sensing tip. This will typically give a reading near ambient temperature.

**Step 4: Check for Insulation Resistance (Megger Test)**

*   A common failure mode is degradation of the insulation between the thermocouple wires and the sheath, especially at high temperatures.

*   Use a dedicated megohmmeter (megger).

*   Connect one megger lead to one thermocouple wire and the other lead to the sheath or the other wire. Apply a test voltage (e.g., 50V DC). A reading **below 100 MΩ** indicates moisture or insulation breakdown, which can cause signal drift and inaccurate readings.

**Step 5: Simulate to Verify the Loop**

*   Use the PR235's **TC source (simulate)** function.

*   Disconnect the thermocouple from the control system and connect the PR235 in its place.

*   Simulate the millivolt signal for the temperature that should be present. If the control system now displays the correct value, the problem is confirmed to be the thermocouple or its wiring. If not, the issue is in the control system input card or wiring.

#### **4. Systematic Diagnosis of RTDs with the PR235**

**Step 1: Visual Inspection** (Same as for TCs)

**Step 2: Measure the Resistance (Ω)**

*   Set the PR235 to the **Ohms (Ω) measurement** function.

*   Measure the resistance across the RTD terminals.

*   **Compare to Expected:** Use a standard RTD table (or the PR235's convert function) to check the measured resistance against the expected resistance for the current ambient temperature (~20-25°C). A Pt100 should read approximately **109-111 Ω at 25°C**.

    *   **OL (Open Circuit):** The RTD wire is broken. Replace the sensor.

    *   **Zero or Very Low Ω:** A short circuit.

    *   **Higher than Expected Resistance:** Often indicates corrosion or partial breakage of the fine wire inside the element.

    *   **Lower than Expected Resistance:** Moisture ingress or insulation failure.

**Step 3: Check for Lead Wire Resistance**

*   A common issue in 2-wire RTD configurations is that the lead wire resistance adds to the measurement, causing a high reading.

*   Measure the resistance of each lead wire from the sensor to the control panel. This resistance should be low and, crucially, **identical for both wires**. A difference indicates a poor connection in one of the wires.

**Step 4: Check Wiring Configuration (2-wire vs. 3-wire vs. 4-wire)**

*   This is critical. The PR235 can be set to match the configuration.

*   **3-Wire RTD:** The most common industrial configuration. It allows the PR235 to compensate for lead wire resistance.

    *   **Test:** Measure resistance between all three wire pairs (A-B, A-C, B-C). The resistance between two pairs should be very similar, and the third pair should be roughly equal to the lead wire resistance. A significant imbalance indicates a faulty wire or sensor.

*   **4-Wire RTD:** Provides the most accurate measurement as it completely eliminates the effect of lead resistance. The PR235 directly measures the resistance of the element itself.

**Step 5: Simulate to Verify the Loop**

*   Use the PR235's **RTD source (simulate)** function.

*   Disconnect the RTD and connect the PR235 in its place, ensuring the calibrator is set to the correct type (Pt100) and wiring (2, 3, or 4-wire).

*   Simulate a resistance value. If the control system displays the correct corresponding temperature, the fault is confirmed to be the RTD. If not, the issue is in the control system or wiring.

#### **Conclusion**

Diagnosing thermocouple and RTD faults with the PR235 is a methodical process that moves from simple checks to complex measurements. By leveraging its functions as a precise ohmmeter, a millivolt meter, and a capable simulator, a technician can isolate virtually any fault in a temperature measurement loop. The key is a systematic approach: always start with a visual inspection, use resistance measurements to identify opens/shorts, and finally, use the simulation function to conclusively determine whether the fault lies with the sensor or the rest of the system. This logical methodology, powered by the versatile PR235, minimizes downtime, prevents unnecessary spending on parts, and ensures the continued accuracy and reliability of critical temperature measurements.