The ‘Type K’ thermocouple

The thermocouple junction is housed at the end of the 200 x 3 mm AISI 310 stainless steel sheath (also known as Chronium Nickel Steel). It has a one metre long cable that terminates in a mini plug colour coded green to indicate thermocouple ‘type K’ (the International IEC colour code for K Type thermocouples is green).

Theory

The simplest thermocouple has two wires made of different metal alloys (nickel chromium (+) and nickel aluminium (-) in type K). When the ends of these two different wires are connected or twisted together, if one end is heated or cooled while the other end remains at a constant temperature, a small potential or voltage is created. This small voltage difference is called the Seebeck Effect; it is amplified and used as a way of measuring temperature difference.

Further information

  • The Thermocouple sensor can use any industry standard ‘K type’ thermocouple. Limit the working range to that of the thermocouple attached e.g. a welded tip PTFE insulated ‘K’ type thermocouple may have an operating range of only –50 ̊C to +200 ̊C.
  • The old colour codes for thermocouples were phased out and replaced with a new specification code, IEC 5843 (for international use). The IEC colour code at present for a ‘K’ type thermocouple is green.
  • The metal sheath of the type K thermocouple can withstand temperatures up to 1,200 ̊C, however the PVC insulation on the connecting cable has a working range of -30 ̊C to 90 ̊C, so keep this cable away from the source of heat e.g. by clamping the Sensor in position. The metal sheath is filled with a silica compound to keep the junction away from the walls of the sheath.
  • Stainless steel 310 (Chromium Nickel Steel) was selected as the material for the sheath for its properties as a poor conductor; heat should not transfer along the full length of the sheath as long as it is not insulated. 
  • The thermocouple junction is insulated from the stainless steel sheath. If un-insulated thermocouples are used, they must be insulated from each other or incorrect readings will result.
  • The sensor’s microchip converts thermocouple EMF to degree Celsius with integrated cold - junction compensation using temperature correction coefficents derived from the National Institute of Standards and Technology thermocouple database.
  • Carbon deposits are best wiped off using a dry cloth.
  • The adaptor body is not waterproof. It may be cleaned using a damp cloth. Do not immerse in water or detergent. Do not place the adaptor in an environment in which high humidity levels are possible as this may result in damage or malfunction.
  • When working from high to lower temperatures, allow the Sensor time to cool and stabilize before taking a new reading.

SAFETY:

If the metal rod of the thermocouple is very hot, allow it to cool in air for while before plunging into cold water.  

Allow the thermocouple rod time to cool before handling. 

Follow health & safety procedures if working with extremely hot or cold objects.


Conditions to avoid:

  • Do not put the sensor housing part into liquids, chemicals, ovens, or fire at any time.
  • Do not let the flame from a Bunsen, candle or similar make contact with the thermocouple’s PVC insulation. Only the tip of the metal thermocouple probe should be in contact with the flame.
  • Do not place the sensor housing part inside a freezer, only thermocouple probe portion should be placed in the freezer.