Temperature Sensor Thermocouple and RTD


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Temperature Sensor Thermocouple and RTD

  1. 1. TemperatureSensor RTD and Thermocouples<br />Rashid Akbar<br />Reg # 10A -002-IC<br />Advanced Instrumentation<br />Submitted to: Ahmed Ali<br />
  2. 2. Contents<br />What is Temperature Sensor<br />Types Of Temperature Sensors<br />Used in Applications <br />Advantages/Disadvantages<br />
  3. 3. What is Temperature<br />A scalar quantity that determines the direction of heat flow between two bodies<br />A statistical measurement<br />A difficult measurement<br />A mostly empirical measurement<br />
  4. 4. Types Of Temperature Sensors<br />Thermocouples<br />RTD<br />
  5. 5. Thermocouples<br />A thermocouple is constructed by twisting or welding together at one end, two conductors made from dissimilar metals.<br />The generated voltage is non-linear.<br />Thermocouples are designated as classes such as type E, K, J, N, or T based on the composition of the two materials used to form the junction. Each type has a different temperature range/voltage response curve.<br />
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  7. 7. Thermocouple Applications<br />Fuel manifold temperatures.<br />Compressor discharge temperature.<br />Combustor gas path temperatures.<br />Thermocouple assy<br />with connection head<br />
  8. 8. Advantages/Disadvantages<br />Sometimes it is necessary to extend the length of the thermocouple wire. <br />In a factory environment it is impractical to try to pull a very small diameter thermocouple wire through a conduit, so a thermocouple extension wire is used in its place. <br />The extension wire exhibits a thermocouple curve similar to that of the measurement thermocouple wire; however, it is also a possible source of error. Since the extension wire cannot exactly match the characteristic curve of the thermocouple wire, it is best to keep the extension/thermocouple junction near room temperature. <br />
  9. 9. Continued..<br />The thermocouple is useful in more types of atmospheres and over wider temperature ranges. Since you do not have to put current through the thermocouple to measure it, it has no self-heating and this can be extremely important.<br />Wide variety<br />Cheap<br />Wide T. range<br />No self-heating<br />Hard to measure<br />Relative T. only<br />Nonlinear<br />Special connectors<br />Needs a reference point<br />
  10. 10. RTD<br />An RTD (resistance temperature detector) is a resistor with an electrical resistance that changes in proportion to changes in the temperature of the resistor.<br />Typical three wire RTD input Circuits<br />Wire Wound RTD<br />
  11. 11. Continued..<br />The measured resistance value is typically linearized by a specialized integrated circuit, or in microcomputer based systems, by software algorithms.<br />
  12. 12. RTD Applications<br />Lube oil temperatures.<br />Bearing temperatures.<br />Enclosure temperatures.<br />Ambient temperature.<br />
  13. 13. Advantages/Disadvantages<br />Most accurate <br />Most stable<br />Fairly linear<br />Expensive<br />Slow<br />Needs I source<br />Self-heating<br />Small change in resistance<br />Current source required<br />
  14. 14. When to used..<br />Factors which defined when to used RTD or Thermocouple<br />Temperature, time, size, and overall accuracy requirements.<br />What are the temperature requirements? <br />If process temperatures fall from -328 to 932°F (-200 to 500°C), then an industrial RTD is the preferred option. Thermocouples however have a range of -300°F to 2300°F, so for extremely high temperatures they may be the only choice.<br />What are the time-response requirements?<br />If the process requires a very fast response to temperature changes--fractions of a second as opposed to seconds (i.e. 2.5 to 10 sec)--then a thermocouple is the best choice. Keep in mind that time response is measured by immersing the sensor in water moving at 3 ft/sec with a 63.2% step change.<br />
  15. 15. Continued..<br />What are the size requirements? <br />A standard RTD sheath is 0.125 to 0.25 in. dia., while sheath diameters for thermocouples can be less than 0.062 in.<br />What are the overall requirements for accuracy? <br />If the process only requires a tolerance of 2°C or greater, then a thermocouple is appropriate. If the process needs less than 2°C tolerance, then an RTD is the only choice. Keep in mind, unlike RTDs that can maintain stability for many years, thermocouples can drift within the first few hours of use.<br />
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