July 2003 1 Thermocouples by Greg Passler What is a thermocouple and how does it work? Why do we use thermocouples? What is thermocouple extension cable? These commonly asked questions about thermocouples exist due to the seemingly mysterious nature of these widely used temperature sensing devices. The discussion that follows will dispel some of that mystery and provide a fundamental un- derstanding of thermocouples. Table 1 compares the more common methods of temperature measurement against certain criteria. The devices range from the basic thermometer, which is a liquid expansion device, to more complex methods such as the use of a hand held infrared temperature sensing unit. Each device has applications for which its use is most appropriate, but thermocouples tend to be among the most popular, especially in industrial applications. Device Low Cost Passive Electric Signal Accurate Robust Wide Range Thermocouple
Transcript
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Figure 4 depicts a typical example of a thermocouple in use in an industrial application. Shown is a plastics extruder that is applying a jacket to an armoured cable as it passes through the cross head. The cross head has two thermocouples installed in it to measure and monitor the melt temperature of the plastic (note the arrows).
Most thermocouples have some form of mechanical protection, as shown in Figure 5. The tip of one of the thermocouples that has been pulled from the cross head is protected by a stainless steel sheath and the lead wires are armoured along their length also with stainless steel.
Table 3 contains eight different materials that are used to protect thermocouples and their maximum operating temperatures and typical applications are indicated. With the stainless steel material used in our example, we can see that it is suitable up to 980°C and may be used in corrosive environments. More exotic materials are used for protection at higher tempera-tures.
Figure 4 - Thermocouples monitor the crosshead temperature of this plastics extruder
Type
Maximum Temperature
(°C) ApplicationWrought Iron 650 General purpose except corrosive environmentsCast Irom 815 Acidic and alkaline solutionsStainless Steel 980 Corrosive environmentsNickel 1090 Special chemical applicationsInconel 1200 Used in place of nickel when sulphur presentCorundum 1650 Steel industry, high thermal shock presentCarbofrax 1650 Applications with high thermal and mechanical shockChronium and Aluminum Oxide 1650 Used in brass and bronze foundries
Table 3 - A variety of materials are used to protect thermocouples from their operating environments
Figure 5 - A stainless steel sheath and armour protect the thermocouple and its lead wireFigure 5 - A stainless steel sheath and armour protect the thermocouple and its lead wire
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The size of the thermocouple wire is also an important consideration. Usually the higher the operating tempera-ture, the bigger the gauge size to slow the effects of cor-rosion. Table 4 demonstrates an example of this concept. For a J thermocouple to operating at its highest rated tem-perature, it is recommended that an 8 AWG conductor be selected, whereas much small conductors can be used at lower temperatures.
The materials used to cover thermocouple wire are usually rated for high temperatures and include fluoropolymers, and glass and fibre braids. (see Figure 6).
In general, the materials used to insulate and jacket thermocouples are expensive relative to conventional wire and cable materials such as PVC and polyethylene. This is necessitated by their relatively high temperature ratings.
Another circuit diagram is shown in Figure 7 to demonstrate the use of thermocouple extension cable. Similar to before, thermocouple wire is used at the sensing junction, but this time, the thermocouple lead wires are connected to ther-mocouple extension wire which is connected to the monitoring device.
The reason we use thermocouple extension cable is to reduce costs on long cabling runs. Thermocouple extension cables, because they aren’t in contact with the temperature being measured (like the thermocouple itself), do not need to be de-signed for the same extremes in operating temperature. This equates to less expensive insulation and jacket materials. In addition, even though the extension cable uses the same metal as the thermocouple wire itself, the alloys are often a lower grade that only need to be accurate over a much smaller temperature range.
It should be pointed out that different metals altogether are used for Types S, B, and R extension wire due to the very high cost of the platinum used in these thermocouple types. The extension wire for these thermocouples have similar electro-thermic properties as the thermocouple itself, but over a more limited temperature range. This kind of cable is called a “compensating cable”.
Extension cable is differentiated from thermocouple cable through the use of a “X” suffix, for example “JX”. Compensating cable has a “C” suffix, for example “SC”.
ANSI Type J Thermocouple
Gauge (AWG)Maximum Operating
Temperature (°C)
814202428
760540480370370
Table 4 - Thermocouple wire size is often determined by the maxi-mum expected operating temperature
Thermocouple Insulation/Jackets
• Usually rated for high temperatures - Tefzel (150°C) - Teflon (205°C) - Kapton (316°C) - Glass Braid with Silicome (482°C) - Ceramic Fibre (1400°C)
• Expensive relative to conventional wire and cable materials
Figure 6 - High temperature materials are usually chosen for thermocouple coverings
Figure 7 - Thermocouple extension cable is used to reduce costs on long cabling runs.
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Other than the conductors, a thermocouple extension cable isn’t any different from a standard instrumentation cable with paired conductors, except for the fact that solid conductors, rather than stranded are typically used and it has a special colour code. Fig-ure 8 is a picture of ShawFlex JX, KX and EX thermocouple exten-sion cables (Purple = EX, Yellow = KX, and Black = JX).
Although thermocouple extension alloy types have been standardized internationally, their colour codes have not. The most signifi cant difference is that in North America, a red con-ductor is used to identify the negative polarity wire, whereas other countries usually use the red to identify the positive wire. This can cause problems when wiring equipment manufactured outside of North America. International colour codes are shown in Table 5.
Getting back to the extruder example, we can see in Figure 9 a thermocouple lead wire ex-tending from the bottom of the cross head and into a connector (note arrow). The other side of the connector is attached to a thermocouple extension cable, which is JX, as the jacket is coloured black. The extension wire then runs up to an electrical control panel where it is ter-minated with fi ve other thermocouple extension cables. Figure 10 depicts the control panel.
Figure 8 - Examples of thermocouple extension cable
Tabe 5 - Colour codes for thermocouple extension cable vary internationally
Figure 9 - A connector joins the thermocouple wire to an extension cable Figure 10 - Thermocouple extension cable is typically terminated in an electrical control panel.
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The signal is then sent to a programmable logic con-troller and computer for processing. Finally, the tem-perature information is projected onto a screen for the operator of the extrusion line to view (Figure 11).
Numerous other examples of devices that use thermo-couples exist. They include hand held thermometers, ovens, freezers, test equipment and many kinds of manufacturing equipment. Despite the variety, they all work on the same general concepts described here.
Greg Passler is an Electrical Engineer with 13 years of experience in the Wire and Cable Industry. He currently works for ShawFlex, a manufacturer of Control, Instrumentation and Power Cables, located in Toronto, Ontario Figure 11 - Temperature data is transferred to a computer control screen
