Expert: cleggsan - 9/9/2009

Question
How to identify J K type thermocouple? are there any testing methods?

Answer
J

Type J (iron–constantan) is less popular than type K due to its limited range (−40 to +750 °C). The Curie point of the iron (770 °C) causes an abrupt change to the characteristic and it is this that provides the upper temperature limit. Type J thermocouples have a sensitivity of about 55 µV/°C.[2]

K
K type thermocouple.
S and K type thermocouples, the S one is partially sheathed with an alundum tube.

Type K (chromel–alumel) is the most common general purpose thermocouple. It is inexpensive and available in a wide variety of probes. They are available in the −200 °C to +1350 °C range. The type K was specified at a time when metallurgy was less advanced than it is today and, consequently, characteristics vary considerably between examples. Another potential problem arises in some situations since one of the constituent metals, nickel, is magnetic. One characteristic of thermocouples made with magnetic material is that they undergo a deviation in output when the material reaches its Curie point; this occurs for type K thermocouples at around 150 °C. Sensitivity is approximately 41 µV/°C. For K type thermocouples, the chromel conductor is positive and the alumel conductor is negative[5].

Above from Wikipedia page:

http://en.wikipedia.org/wiki/Thermocouple

Don't know your measurement skills and equipment, but maybe easiest way to detect thermocouples it to measure the sensitivity.  A J is 55 and the K is 41 microvolts per degree C.

Does this help?

C