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    Thermocouples

    Thermocouples are temperature sensors, which work on principles for flow of electron in closed circuit as described below. Whenever two dissimilar material are joined at ends and the two ends are subjected to temperature differential, electrons flow in such closed circuit and there is potential difference between cold and hot junction. This potential difference is proportional to the temperature differential and is dependent on the metals joined. Any two dissimilar metals can be used for this purpose, however there are many standard combinations on metals, which are internationally used and designated for use as thermocouple elements. Most commonly used thermocouple elements are broadly divided in two categories:
    A) Base metal thermocouples ( ) and 
    B) Precious metal thermocouples ( ). J, K, T, and E type R, S and B type

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    Thermocouple Types:
    Given below is general description of commonly used thermocouples.

    IEC 584-1, PART1: S:
    This thermocouples can be used in oxidising or inert  atmosphere at temperatures described above. Recrystallised Alumina beads and protective sheaths are generally used.
    However, for lower temperatures upto 1100 Deg C metal sheaths with porcelain beads can be used. Please note that continuous use at elevated temperature may cause fusing of Rhodium in pure Platinum causing reduction in output emf.

    IEC 584-1, PART2: R:
    This thermocouples can be used in applications similar to S type thermocouples. zhe advantage being slightly higher output at same temperature and better stability. This type is preferred over S type thermocouples.

    IEC 584-1, PART3: J:
    This thermocouples are commonly known as iron/constantant thermocouples and can  be used in reducing or inert atmosphere at temperatures described above. Restrict use in oxidizing atmosphere to temperatures up to 400 deg C, otherwise degradation will be rapid. Avoid using for application where condensation temp is often reached otherwise Fe {+leg} will rust rapidly.
     
    IEC 584-1, PART4: K:
    This is most commonly used thermocouple ind is are commonly known as chromel/Alumel thermocouple

    These are designed primarily for use in Oxidising atmosphere at temperatures described above. Great care should be taken when using for any other atmosphere. These thermocouple will get oxidised when used continuously at temp above 1000 Deg C and cause drift.

    For temp range from 250 to 600 Deg C stability is suspect. Other base metal thermocouples are better bet at  this temp range.

    Cyclic :
     temperature changes can cause error of several degrees at these temperatures.

    IEC 584-1, PART5: T:
     
    This is commonly used in laboratory applications where sub zero temperatures are often maintained. Repeatability at -200 + 200 Deg C is excellent. However at temp above this Copper arm rapidly oxidizes causing drift. Note that the negative arm of T and J type are not same and are not interchangeable.

    IEC 584-1, PART6: E and IEC 584-1, PART7: B
    These thermocouples are not in much use presently. E type is more stable in -250 + 200 Deg Range than K type and can be preferred over K in this range. Type B thermocouple can be used at temperature upto 1600 Deg c. Interesting point is that in this type cold junction compensation is not normally required as emf change from 0 to 50 is negligible.
     
     
    Styles Of Thermocouple Assemblies :
     
      Basic element in all common types with welded junction in various wire gauges and lengths
         
      Basic element with ceramic twin bore insulators suiotable for replacement for industrial / high temp thermocouples.
         
      Suitable for general application up to 400 Deg C. J, K,T, E type element in Seamless SS316 sheath with extension lead wire. Grounded/ungrounded junction as required.
         
      Suitable for extruder nozzle, motor and pipe temperature measurement. Available in J and K type element embedded in any standard bolt size with extension lead wire. Suitable for Temp up to 225 Deg C.
         
      Washer type Suitable for surface temperature measurement. Available in J and K type element embedded in any standard washer size with extension lead wire. Suitable for Temp up to 325 Deg C.
         
      Bayonate type suitable for plastic machinery, bearing chokes etc.Available in J and K type element with SS 316 sheath and extension lead wire. Suitable for Temp up to 650 Deg C.
         
      Self adhesive patch thermocouples are suitable for flat or curved surfaces. Available in J and K type element with Teflon insulation. Suitable for Temp up to 200 Deg C.
         
      Adjustable ring thermocouples are suitable for large pipe surface temp measurement. Available in J and K type MI element with SS armour over Fibreglass cable. Suitable for Temp up to 500 Deg C.
         
      Suitable for moving surfaces such as calender rolls or motor shafts upto 225 Deg C and 200 MPM speeds. Available with spring loaded detection head extension leads and mounting brackets. Available in J and K type element.
         
      Heavy duty metal sheathed industrial thermocouples are suitable for most arduous industrial environment suchas furnaces, kilns, ovens Zinc Baths etc Available in J, K, R & S type element with variety of sheaths such as SS 316, Inconell 600, and SS 310.
         
      High temperature ceramic sheathed thermocouples are suitable for temperatures upto 1600 Deg C Available in K, R & S type element with variety of sheaths such as SS SS 310, Porcelain, Ceramic and Recrystallised Alumina.
         
      Mineral insulated metal sheathed thermocouples are suitable for spaces which are difficult to reach. Available in J and K type element with variety of sheaths such as SS 316, Inconell 600, and SS 310.
         
      L shape thermocouples are suitable for molten metal baths. Hot are is manufactured from solid barstock for better life. Available in K type element with variety of sheaths such as Inconell 600, and SS 446.
     
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