The thermocouple wire or cable is mainly used in the manufacturing of thermocouple probes in order to sense temperature by utilizing the point of cold junction compensation (cjc end).
A thermocouple is a sensor that is used for measuring temperature, consisting of two dissimilar metals that are connected together at the sensing end. Different thermocouple types (e.g. J or K type thermocouple wire) utilize different mixtures of metals inside the cable. At the cjc end, the millivolt value given by the thermocouple shows the difference in temperature at the sensing end to that of the cjc end (also known as the reference end).
Thermocouple probes vs. Thermocouple wire?
It is crucial to keep in mind that the only temperature that a temperature sensor can measure is its own temperature. Nevertheless, the selection of a wire style sensor vs. a probe style sensor is just a matter of finding the best way to get the thermocouple junction to the process temperature that you are trying to measure.
For instance, take measuring the temperature of fluid in a container, partially, or fully enclosed pipe. Utilizing a wire style sensor could be fine if the fluid doesn’t attack the conductor or insulation materials, if the fluid is nearly or completely at rest, and the temperature is within the capacity of the materials. But, say that the fluid is under high pressure, high temperature, flowing through a pipe, or corrosive then a probe style sensor, perhaps even with a thermowell, could be a better selection.
It all comes down to finding the best way to get the thermocouple junction to the same temperature as the material, or process, you’re trying to measure the temperature of, in order to get the information you need.
How are insulated thermocouple wires identified?
To help with identification, insulation on thermocouple wire is color coded. According to common guidelines the negative lead in insulated thermocouple wire is red in color. The positive lead takes the color of the thermocouple which is also the overall color of insulated extension grade wire. The outer jacket of thermocouple grade wire is usually brown in color. In case of high temperature wire, it is customary to include a color coded tracer thread in the white material.
What is the difference between Standard and Special Limits of Error (SLE) wire?
SLE (special limits of error) wire vs Standard wire has to do with accuracy of the wire. SLE wire is similar to standard wire with the extra feature of having a little improved accuracy spec. Accuracy of thermocouples changes with the type of thermocouple being used. e.g. for the lower temperature ranges type T, made of copper wire in the positive, lead and copper-nickel mixture (constantan) for the negative, has better accuracy specifications.
What is the difference between Extension grade and Thermocouple grade wire?
Thermocouple grade wire is the wire that is utilized in making of the probe part (or sensing point) of the thermocouple. Extension grade wire is only utilized to extend a thermocouple signal being sent from a probe back to the instrument that is reading the signal. The extension grade wire will usually have a lower ambient temperature limit. It may also pass a signal that represents a higher temperature as received from the probe, but the wire might not be exposed to higher temperatures physically. Thermocouple wire may be utilized as extension wires, but extension grade wire cannot be utilized in the probe part (or sensing point) of the thermocouple. Part numbers for extension wire usually start with an “EX” prefix.
What is the maximum length of thermocouple wire?
There are many different factors that could affect the usable length of a thermocouple. As a recommendation, less than 100 feet with 20 AWG or thicker wire in an area that is free of electromagnetic interference is generally fine. Two important factors for determining usable thermocouple length are preventing electrical noise getting into the signal, and total loop resistance. As different thermocouple wires are made using different materials, the resistance will differ depending on the type as well as the length and diameter of the wire.
The allowable loop resistance could get affected by the input resistance of the amplifier circuit that it is attached to. But, as a recommendation, usually the aim is to keep the total loop resistance under 100 ohms. Loop resistance is calculated by multiplying the length in feet with the resistance per double feet.
In addition to the wire length, don’t forget to include the probe (when used/applicable) in your calculations. The second crucial factor for running a thermocouple wire is to keep it far away from any type of electromagnetic field. Thermocouple wire generates a low voltage signal and should not be run near motors, power wires, etc. To help reduce noise, a twisted shielded wire or metal over braid is commonly used.
Heatcon Sensors is an ISO 9001:2015 certified company and has been in operation for 30 years. Heatcon has received accreditation from certifying bodies like TUV SUD, DRDO CEMILAC, NSIC, and NABL. Production of heat sensing devices like Thermocouples and RTDs has been our forte! We also produce special sensors which find application in numerous industries including the ones specializing in aeronautics, railways, turbines, nuclear plants, and research labs.
With over three decades of expertise in temperature sensor and heater manufacturing, Heatcon Sensors provides a high level of quality, accuracy, and an unbeatable commitment to customer support.
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