By Tina / 2019年7月9日
Thermocouple wire or cable is used within a thermocouple that senses temperature using the point of cold junction compensation. Thermocouple wire is used to control instrumentation and for building the sensing point in a thermocouple and is made out of two dissimilar metals.
Overmolded Thermocouple cable
There are many factors that impact the usable length of a thermocouple. The two main factors that affect the usable length of thermocouple wire are total loop resistance and electrical noise.
Thermocouple wires are made of different conductor materials. Because of this, the total loop resistance varies from based on the type as well as the wire diameter and length. However, the main goal for a usable length of thermocouple wire is to keep the loop resistance under 100 ohms.
With the other factor, the longer the wire, the higher the possibilities of it getting influenced by electrical/radiated noise. This long length of the wire will then affect the performance of the wire. As such the usable length of thermocouple wire is recommended not to run or exceed a minimum of 10 feet.
And in case the wire is extended it is recommended to use a shield to reduce the noise interference to an acceptable level. For this purpose, a metal over brad or twisted shielded wire can be used. At the same time avoid running wires closer to sensitive areas like motors and power wires.
All in all, the recommended goal for a usable wire length is to keep the wire reduced to less than 100 feet with 20 AWG or thicker wire in an area free of electromagnetic interference. Now with the recommended lengths defined, let’s get into the types of thermocouple wires available and how they differ.
The are many different types of thermocouples that are available to suit different measuring applications such as industrial, scientific, food temperature, medical research, etc. It is, in fact, the different types of thermocouple wires that enable thermocouples to suit different applications.
So to understand this better the thermocouple wire types are listed below with the positive electrode first, followed by the negative electrode.
Made out of Chromel (Nickel-Chromium Alloy) / Alumel (Nickel-Aluminium Alloy), type K thermocouple wires are the most commonly used for general purposes. It has a temperature range of −200 °C to +1200 °C and a sensitivity of 41 μV/°C. It is also comparatively inexpensive to other types of thermocouple wires.
K type Fiber glass braided thermocouple wire
Made out of Chromel / Constantan (Copper-Nickel Alloy), type E thermocouple wire has a high sensitivity of (68 μV/°C). This type is well suited for cryogenic use (Those that has to deal with very low temperature). Cryogenic uses include food preservation, rocket fuels, and transport of liquefied gases and so on.
Made out of Iron / Constantan, this type is used in old equipment that finds it difficult to accept modern thermocouples. It has a temperature range of limited −40 to +750 °C with a sensitivity about 52 μV/°C. This type when used above 760 °C which can cause permanent de-calibration and can also prevent its use in some applications.
Type J thermocouple wire
Made out of Nicrosil (Nickel-Chromium-Silicon Alloy) / Nisil (Nickel-Silicon Alloy, this type has high stability and resistance to high-temperature oxidation. This makes type N suitable for high-temperature measurements. Their temperature range can rise above 1200 °C with a sensitivity of about 39 μV/°C at 900°C.
Type N thermocouple wire
Made out of Copper / Constantan, type T has a temperature range of −200 to 350 °C with a sensitivity of 43 μV/°C. Both the conductors in type T are non-magnetic. Because of this, type T is an ideal choice for applications like electrical generators that has strong magnetic fields.
Type T thermocouple wire
Made out of Platinum 30% Rhodium / Platinum 6% Rhodium, type B thermocouple wires are suitable for high-temperature measurements up to 1800 °C. Because of their temperature-voltage curve, type B thermocouple wires has given its output at 0 °C and 42 °C.
Made out of Platinum 13% Rhodium / Platinum, type R thermocouple wires are also suitable for high-temperature measurements up to 1600 °C. However, they are expensive and have a low sensitivity of about 10 μV/°C. This makes them unsuitable for general purpose use.
Made out of Platinum 10% Rhodium / Platinum, Type S is also suitable for high-temperature measurements up to 1600 °C. Like type R, type S also has a low sensitivity of about 10 μV/°C and is not suitable for general purpose use. Type S is however used as is used as the standard of calibration for the melting point of gold (1064.43 °C), because of its high stability.
Made out of Tungsten 5% Rhenium / Tungsten 26% Rhenium, type C has a temperature range of 32 to 4208°F ((0 to 2320°C). Type T is highly suitable for measurements of high temperature such as for vacuum furnaces at extremely high temperatures. However, type C must never be used around oxygen at temperatures above 500°F.
Made out of Nickel Alloy 19 / Nickel-Molybdenum Alloy 20, with a temperature limited to 2500°F (~1400°C). This type is suitable to be used in the vacuum furnaces.
Clearly, the types of thermocouple wires directly decide the ability and performance of a thermocouple. In simple terms, the type of thermocouple wire decides how well a thermocouple can measure the temperature depending on your expectations.
And to sum up, let’s looks at some uses of thermocouple wires.
As we said thermocouple sensors are used to measure the temperature in various environments. So it’s important for an engineer to understand the differences between these thermocouple wires and their characteristics appropriately.
Only the wire will decide how effectively the sensor can be used to measure the temperature. It is, in fact, the wire and the material it is made out of that gives the sensor the ability to be used appropriately in different environments.
On a single view, the uses of thermocouple sensors are directly the uses of the thermocouple wires. This is because thermocouple wires are the essential element that enables a thermocouple sensor to work the way they do.
However specifically, thermocouple wires are used in many industrial, scientific and OEM applications. They’re also used in all industrial market such as Power Generation, Oil/Gas, Pharmaceutical, BioTech, Cement, Paper & Pulp, etc. Thermocouple wires are also used in daily appliances like toasters, stoves, and furnaces.
So thermocouple wires are continuing to deliver many uses from our work to daily lives applications. These wires have also simplified the use of sensing devices, mainly by matching different types of wires to best suit the need or expectation.
We hope this has provided a good overview of Thermocouple wire. If you have any questions about thermocouple wires, their types and their uses, or need assistance with anything injection molding related projects you have in mind, then please contact us for more help and information.