Insulated Thermocouple Wire Considerations

There are many different types of thermocouple wire insulation based on the specific application required for each use.  Generally, the larger the wire diameter, the less flexible it will be.  The smaller the diameter flexibility increases but at the cost of strength.  The most common sizes are 20 and 24 awg solid or stranded.  30 and 36 awg sizes are also used for special applications.  Stranding will increase the flexibility with a minimal increase in overall diameter.   As you may assume the better the insulation is for temperature, moisture and abrasion resistance, the higher the cost.

Thermocouple Time Response

Thermocouple time response is one of the questions we get all the time from customers and it is sometimes misunderstood. 

The chart below shows the average time response for the first of 7 time constants to achieve 0 to 100% step change.  It does not matter if the change is from 0 to 100 C or 100 to 1000 C the time response is the same. The data shown is for the 1st time constant or 2/3’s step change. Each of the remaining time constants are the same, 2/3’s of the remaining change. You can see that since we are looking at increments of 2/3’s, we achieve approximately 98% of the step change rather quickly. Obviously, size is a determining factor as the more mass, the more time to see the temperature change. Keep in mind that the smaller the probe, the faster the time response at the expense of being a more robust design. Too small and the probe could fail early in a severe application so a balance must be drawn between time response and probe life.

Approximate response time in seconds.

Values are for 2/3 of total temperature change from 0 to 100% as measured in water.

20-awg bare wire junction measured in gas.

Outside         Approximate        Approximate        Grounded        Ungrounded

Diameter      Wire Gauge          Wall thickness      Junction           Junction

.020                     38                       .003                     .02                      .03
.040                     32                       .006                     .04                      .13
.062                     28                       .009                     .22                      .40
.090                     25                       .012                     .33                      .68
.125                     22                       .017                     .50                    1.10
.188                     19                       .025                   1.00                    2.30
.250                     16                       .033                   2.20                    4.10
20 awg bare wire junction .45

Additional Time Response Information can be found here http://www.temprel.com/time-response-information-42/ or by calling Temprel at (800) 582-5098.

Thermocouple Repair or Replacement Options

Temprel, Inc. can examine your old and/or used thermocouples and will quote you a price to provide the necessary repairs to your sensor.  Temprel will also quote you the price for a comparable replacement if it is beyond repair.  No matter which way you choose to go, you can still take advantage of their one year warranty.  Call Temprel at 1-800-582-5098 for more details.

Specialty Thermocouple Designs

There are many different thermocouple configurations in addition to Temprel's standard design. Here are some specialty thermocouple designs that may help your selection:



Reduced Tip

The mechanical strength of the .125 inch diameter probe with the time response of the .062 inch. The thermocouple tip is swaged down for ¾ inch to achieve this design. This is available on all sheath materials and is commonly carried in Hastalloy (specify H for sheath material).



Venturi Tip

In some applications, the thermocouple reading can be affected by radiant energy from heat sources close to the desired gas stream reading.  This design provides a shield from these heat sources while providing for a flow path to measure the desired gas stream and protect the smaller tip.

3-prong standard plug

For applications requiring continuation of the circuit ground, we off a molded on 3-prong standard plug. The grounding wire is brazed to the third copper pin to ensure continuity of the connection. Specify S3 in the connector option of the part number.


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.

Measuring Junctions Time Response

Grounded Junctions -The thermo-elements are welded into the end cap using the same weld rod as the sheath material. Fast time response. Recommended for high-pressure applications.

Ungrounded Junctions - The thermo-elements are welded together and are electrically isolated from the sheath. Recommended for applications where stray EMF’s would affect the reading.

Exposed Junctions -The thermo-elements are welded together outside of the sheath. This provides the fastest time response but exposes the elements to contamination. Electrically isolated from the sheath.  

Approximate response time in seconds.


Values are for 2/3 of total temperature change from 0 to 100% as measured in water.


20-awg bare wire junction measured in gas.


Outside        Approximate          Approximate              Grounded           Ungrounded
Diameter       Wire Gauge         Wall thickness              Junction               Junction
.020                    38                    .003                             .02                          .03
.040                    32                    .006                             .04                          .13
.062                    28                    .009                             .22                          .40
.090                    25                    .012                             .33                          .68
.125                    22                    .017                             .50                        1.10
.188                    19                    .025                           1.00                        2.30
.250                    16                    .033                           2.20                        4.10


20 awg bare wire junction                                                                              .45

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.

Thermocouple Wire Selection Guide

Insulation Thermocouple and Thermocouple Extension Wire Selection Guide.

Type	Flexibility	Moisture Resistance	Abrasion Resistance	Continuous Temp Rating	Single Reading Temp Rating
Fiberglass Code 302, 305	Good	Fair	Good	900F (480C)	1000F (540C)
High Temp Fiberglass Code 321	Good	Fair	Fair	1300F (700C)	1600F (870C)
PVC Code 502, 505	Very Good	Excellent	Very Good	-15 to +220 F -26 to +105C
Teflon FEP (Extruded) Code 506, 507, 509	Good	Excellent	Excellent	-90 to +400 F -67 to +205 C	500 F (260C)
Teflon TFE Tape wrap Code 508	Good	Excellent	Very Good	-90 to + 500 F -67 to + 260 C	600F (315C)
Teflon PFA Extruded Code 516	Good	Excellent	Excellent	-90 to +500 F -67 to +260 C	550F (290C)
Kapton Tape Wrap Code 511, 513	Good	Excellent	Excellent	-90 to +600 F -67 to +315 C	800F (430C)

Fiberglass insulation is impregnated with a modified resin to enhance abrasion resistance and prevent fraying. It is retained up to 400F (205C).

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.

Compacted MgO Thermocouples

All Temprel MgO insulated thermocouples are made using the highest purity MgO for temperatures up to 2300F (1260C). The thermo-elements are all ANSI special limits of error to give your measurements the best possible results. The various sheath materials are dependent on the application and the following will help you make the best selection.
304 SS - Maximum temperature of 1650F (900C) and is the most widely used low temperature sheath material. It offers good corrosion resistance but is subject to carbide precipitation in the 900F to 1600F (480 to 870C) range.

310 SS - Maximum temperature of 2100F (1150C) and offers good mechanical and corrosion resistance similar to 304 SS. Very good heat resistance. Not as ductile as 304 SS.

316 SS - Maximum temperature of 1650°F (900C) and has the best corrosion resistance of the austenitic stainless steels. Subject to carbide precipitation in the 900F to 1600F (480 to 870C) range.

Inconel - Maximum temperature 2150°F (1175°C) and is the most widely used thermocouple sheath material. Good high temperature strength, corrosion resistance and is resistant to chloride-ion stress corrosion, cracking and oxidation. Do not use in sulfur bearing environments.

Hastelloy X – Maximum temperature 2200F (1205C) widely used in aerospace applications. Resistant to oxidizing, reducing and neutral atmospheric conditions. Excellent high temperature strength.

Standard sheath diameters .032” +.001 -.0005" .040” +.001 -.0005” .063” ±.001” .090” ±.001” .125” +.002 -.001” .188” +.002 -.001” .250” +.003 -.001”

Suggested Upper Temperature Limits 1290°F (700°C) 1290°F (700°C) 1690°F (920°C) 1830°F (1000°C) 1960°F (1070°C) 2100°F (1150°C) 2100°F (1150°C)

All MgO compacted thermocouples can be bent on a radius of twice the sheath diameter.

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.

Thermocouple Wire Insulation Types

There are many different types of thermocouple wire insulation based on the specific application required for each use.  Generally, the larger the wire diameter, the less flexible it will be.  The smaller the diameter flexibility increases but at the cost of strength.  The most common sizes are 20 and 24 awg solid or stranded.  30 and 36 awg sizes are also used for special applications.  Stranding will increase the flexibility with a minimal increase in overall diameter.   As you may assume the better the insulation is for temperature, moisture and abrasion resistance, the higher the cost.  This list shows the most common insulations starting with the least expensive.   

 

 

Insulation         Upper Temp                        Resistance to                       Typically

Type                        Limit                        Moisture     Abrasion                 used for
 

PVC              105 C (221 F)              Excellent        Good            Extension

Ripcord         Same as PVC                                                        T/C

Glass Braid     540 C (1000 F)             Good            Fair                T/C

High Temp

Glass Braid     870 C (1600 F)             Good            Good              T/C

Teflon FEP     260 C (500 F)              Excellent        Excellent          T/C

Extruded

Teflon TFE     316 C (600 F)              Excellent        Good              T/C

Tape wrap

Kapton          427 C (800 F)              Excellent        Excellent          T/C

Wire preparation also increases in difficulty as Glass braids, Teflon TFE and Kapton are harder to strip as compared to PVC. Thermal strippers can help but Teflon FEP is known to creep when heated.

Stainless Steel wire overbraid is also added to the wire for increased abrasion resistance and to help in high vibration applications. Stainless Steel interlocking armor is also used for applications where the wire may be subjected to extreme abuse.

Part number breakdown, K24-2-305

K – Calibration
24 – Gauge size
2 – Solid wire, – 4 stranded, thermocouple grade
6 – Solid wire, – 8 stranded, extension grade

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.

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.