Thermocouple Types

Open Wire - Temprel T52 
This style is the most basic and economical.  It consists of bare or normally insulated wire with a TIG welded measuring junction and a form of connection to extension wire.  It is available in gauge sizes 20 thru 36 with a wide choice of insulation based on the application.

Sheathed - Temprel T54

This style is similar to the open wire type with the wire slid into a tube and the measuring junction exposed or welded into the end of the tube.  This allows the probe to be mounted in a compression fitting and can be liquid tight.  It is still limited to the temperature of the wire insulation.  Sizes are normally .125” diameter thru .250” diameter and the tube material can be stainless steel, glass, armor or a tightly wound spring (Boden Cable).

Sheathed MgO Filled - Temprel T21, T23, T24, T25, T26

This group shares a common probe style of a metal sheath normally .020” thru .250” diameter with compacted Magnesium Oxide insulation around the thermo-elements.  Most common sheath materials are Inconel 600; 304, 310 or 316 Stainless Steel; or Hastalloy X in single, dual or triple elements.  Since the thermo-elements are very fine and fragile, a transition to a robust termination is required, either a quick disconnect plug (T21); or a potted fitting (T25), a potted fitting with strain relief spring (for high vibration applications) (T26) to flexible 20 or 24 awg lead wires.  Special mounting fittings such as a pipe plug or oil drain plug can be welded to the sheath (T23).  Temperature limits are normally the upper operating range of the thermocouple type based on calibration and diameter (see attached chart).  The thermo-elements can also be welded into the end cap (grounded junction), exposed or electrically isolated from the sheath (ungrounded junction).

Any of the above types can be terminated with a molded on standard or mini plug 2 0r 3 prong or left open.  Open wire styles can include washers or any other type of brazed or adhesive backed mounting method. 

Standard sheath diameters             Suggested Upper Temperature Limits

                    .032”  +.001 -.0005”                            1290°F (700°C)

        .040”  +.001 -.0005”                            1290°F (700°C)

        .063”  ±.001”                                       1690°F (920°C)

        .090”  ±.001”                                       1830°F (1000°C)

        .125”  +.002 -.001”                              1960°F (1070°C)

        .188”  +.002 -.001”                              2100°F (1150°C)

        .250”  +.003 -.001”                              2100°F (1150°C)

If you don't see what you are looking for here, or if you have additional questions please feel free to contact Temprel at 1-800-582-5098.

What is a Thermocouple?

A thermocouple is a temperature measurement device based on the Seebeck Effect.  This is defined as two dissimilar metals, joined at both ends.  When one end is hotter than the other, a small continuous EMF is generated.  Another EMF develops along the temperature gradient of a single homogeneous wire.  This is the Thompson Effect. 

It is important that each section of wire in a given circuit be homogeneous, since with no change in composition, the circuit EMF depends only on the alloy and the temperature of their junctions.  If both junctions are held at the same temperature, the metal does not contribute to the net EMF of the circuit.  The law of intermediate metals points out that the introduction of a third metal into the circuit will have no effect upon the EMF generated as long as the junctions of the third metal with the other two alloys are at the same temperature.

In industry today, standard homogeneous alloys are employed for the various calibrations most widely used.  These are types K, J, T, and E thermocouples.  Others such as R, S, B, N, and W thermocouples are also used for the process and metals industries.

Type     Useful Range       Special limits of error 

   T          0 to 350 C              +/- .5 C or .4% of reading

     J          0 to 750 C              +/- 1.1 C or .4% of reading

    K          0 to 1250 C            +/- 1.1 C or .4% of reading

     E          0 to 900 C              +/- 1.0 C or .4% of reading

The calibration used is not as dependant on the output as it was in the past.  With the advent of the microchip, electronics can filter out noise and linearized the curve of any type thermocouple with cold junction compensation to 0 C over a broad range to allow one type to be a standard for all measurements.  That is why type K thermocouple is the most often specified.

If you don't see what you are looking for here, or if you have additional questions please feel free to contact Temprel at 1-800-582-5098.