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You are here → Home > Guide to Thermocouple and Resistance Thermometry > Thermocouples - The Basics

Frequently Asked Questions About Thermocouples and RTD Pt100 Sensors

Contents

  1. Thermocouples – Frequently Asked Questions
  2. RTD Pt100 – Frequently Asked Questions

Thermocouples – Frequently Asked Questions

  1. What is a thermocouple and how does it work?
    A thermocouple is a temperature sensor formed by joining two dissimilar metals. When the junction is heated or cooled, it produces a voltage proportional to the temperature difference between the measuring and reference junctions.
  2. Which thermocouple type should I use?
    Different types (K, J, T, N, etc.) are suited for different ranges and environments. Type K is common for general use; Type T for cryogenics; Type N for high-temperature stability; and Type J for oxidizing/reducing atmospheres.
  3. What is the temperature range of a thermocouple?
    This depends on the type. For example, Type K typically measures from -200°C to +1370°C. Always consult manufacturer specifications.
  4. How accurate are thermocouples?
    Accuracy varies by type and calibration standard, but typical error margins range from ±1 to ±3°C. Special tolerances are available.
  5. What is cold junction compensation?
    It corrects for the reference junction not being held at 0°C. Electronic compensation is built into most modern instruments.
  6. What does "thermocouple break protection" mean?
    It detects open circuits and alerts the user, sometimes driving the reading to a defined value (e.g., upscale or downscale).
  7. How do I wire a thermocouple correctly?
    Use the correct thermocouple extension wire (same type) and ensure polarity is not reversed. White is usually negative for IEC colour codes.
  8. What causes thermocouple drift over time?
    High temperatures, oxidation, and contamination can degrade accuracy. Periodic calibration or replacement is recommended.
  9. Can I extend thermocouple cables?
    Yes, but use matching thermocouple extension wire to maintain accuracy. Avoid junctions with different materials.
  10. What is the difference between grounded, ungrounded, and exposed junctions?
    Grounded: fast response but less isolation. Ungrounded: isolated from sheath, slower. Exposed: very fast but fragile.
  11. Can thermocouples be used in corrosive or high-vibration environments?
    Yes, with appropriate sheath materials (e.g., Inconel, ceramic) and construction. Thermowells may be used for protection.
  12. How do I troubleshoot a faulty thermocouple?
    Check for open circuits, correct polarity, consistent readings, and physical damage. Use a calibrator to simulate input. See our Toubleshooting Guide for more info.
  13. Do thermocouples need calibration?
    Yes, especially for critical processes. Factory calibration or field verification against a reference standard is common.
  14. What is the maximum distance I can run thermocouple wiring?
    Usually up to 30-100 meters with proper shielding. Beyond that, signal conditioners or transmitters are recommended.
  15. What’s the difference between a thermocouple and a thermopile?
    A thermopile is a series of thermocouples connected in series or parallel to increase output voltage, often used for energy harvesting or flame sensing.

RTD Pt100 – Frequently Asked Questions

  1. What is an RTD and how does it differ from a thermocouple?
    An RTD (Resistance Temperature Detector) measures temperature by correlating resistance with temperature. It offers higher accuracy and stability than thermocouples but has a smaller temperature range.
  2. What does Pt100 mean?
    Pt = platinum; 100 = 100 ohms at 0°C. It’s the most common RTD type.
  3. What’s the difference between 2-wire, 3-wire, and 4-wire RTDs?
    2-wire: simplest, but affected by lead resistance. 3-wire: compensates for lead resistance. 4-wire: most accurate, cancels resistance fully.
  4. How accurate is a Pt100 sensor?
    Class A and Class B tolerances define accuracy. Class A is ±0.15°C at 0°C, Class B is ±0.3°C at 0°C. However, higher accuracies where the element is ±0.03°C at 0°C are available.
  5. What is the temperature range of an RTD?
    Typically -200°C to +600°C, depending on design and construction.
  6. Can RTDs be used in industrial applications?
    Yes, especially where precision is needed. They are widely used in process industries, labs, and HVAC systems.
  7. How do I wire a Pt100 sensor correctly?
    Match the number of wires to your instrument’s input type. Use good-quality, low-resistance cable. See our RTD Wiring Guide for more info.
  8. What kind of cable should I use for RTD extension?
    Use shielded copper cable with low resistance. For 3- or 4-wire RTDs, maintain wire symmetry.
  9. How do RTDs respond to vibration and thermal cycling?
    High-quality RTDs are designed for durability, but repeated thermal cycling or vibration can cause mechanical stress or wire breakage.
  10. Can I bend RTD probes during installation?
    Only if the design allows. Some are flexible or spring-loaded; others must remain straight to avoid damage.
  11. What are the common causes of RTD failure?
    Wire breakage, insulation failure, and mechanical damage are typical. Increased resistance is a warning sign.
  12. How do I calibrate or test an RTD?
    Use a precision resistance meter or compare it to a reference standard in a known-temperature bath.
  13. Are RTDs intrinsically safe or suitable for hazardous areas?
    Yes, when used with appropriate barriers, transmitters, or intrinsically safe systems.
  14. How do RTDs compare to thermistors?
    RTDs are more linear and stable over time. Thermistors are cheaper and more sensitive but have a narrower range.
  15. Why does the resistance not match exactly 100 ohms at 0°C?
    Manufacturing tolerances or drift over time can affect resistance. Check the class tolerance (A or B) to understand allowable deviation.

Note: The information in this guide is provided for general informational and educational purposes only. While we aim for accuracy, all data, examples, and recommendations are provided “as is” without warranty of any kind. Standards, specifications, and best practices may change over time, so always confirm current requirements before use.

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