Thermocouple probes are created utilising thermocouple cables to measure temperature using the point of cold junction correction. A thermocouple is a temperature sensor made of two different metals fused at the sensing end. Various thermocouple types employ various metal cable combinations. The cable used to attach a thermocouple to the temperature transmitter is known as a thermocouple extension cable. The thermocouple and extension cables are made of the same substance. Thus, only iron and constantan will be used to make extension cables. The thermocouple extension cable is identified by appending the letter X to the end of the thermocouple type, such as a K-type thermocouple, and the letters KX for the extension cable.
Here you can see the difference between Thermocouple and an Extension wire:
What is a thermocouple extension cable?
The base metal type thermocouple extension cable is made of the same materials as the equivalent thermocouple wire. Due to the high cost of the noble material, alternative compositions are employed for noble metal compositions. Also, colour-coded to match the calibration are jacks and connectors. Wires can be stranded rather than solid for simpler installation. Use protected thermocouple wires when putting them next to other electrical conduits. The accuracy of your temperature measurements may decrease with increased wire length, therefore, you may want to think about thermocouple wire with specific length restrictions.
How are insulated thermocouple wires identified?
The thermocouple wire’s insulation is colour labelled for identification. As a general rule, insulated thermocouple wire has a negative red lead. The thermocouple and the general colour of extension-grade insulated wire are both on the positive lead. The thermocouple grade wire and thermocouple extension cable often have a brown outer coating. It is typical to have a colour-coded tracer thread in the white material for high-temperature wire.
What is the thermocouple wire’s maximum length?
The usable length of a thermocouple might vary depending on various circumstances. Whole loop resistance and preventing electrical noise from contaminating the signal are key elements in establishing a usable thermocouple length. The resistance will vary depending on the kind, diameter, and length of the wire because different thermocouple wires are constructed of other materials.
The input resistance of the amplifier circuit to which the loop is linked impacts the permissible loop resistance. Usually, the goal is to keep the entire loop resistance around 100 ohms. One can get the loop resistance by dividing the length in feet by the resistance per double foot. Remember to add the probe to the wire length when performing your calculations. Avoiding electromagnetic fields is the second most important consideration while conducting a thermocouple wire. It is not advisable to run thermocouple wire next to power lines, motors, etc., since it produces a low voltage signal. Metal over the braid or twisted shielded wire is frequently used to reduce noise pickup.
Thermocouple wire vs. extension cable
While utilising a thermocouple, it is critical to realise that the signal might originate anywhere along the conductors, from the junction to the end of the wires, instead of only at the junction. So, the accuracy of the signal will depend on how well the thermocouple materials work and how the temperature varies along its length.
There are three types of thermocouple wire such as standard limits, special limits, and extension. The temperature changes from 370C at the tip to 200C at the connector to an extension cable on a thermocouple probe manufactured of special precision Type T (0.5C or 0.4%) wire. Your output will be accurate within specific limits up to this point. A specific limit of error cable, which will observe a temperature change from 200C to 50C, is attached here. The output accuracy of your total measurement will continue to meet the Special limits of error because both thermocouple conductor lengths are special limits of error.
Now extend that illustration using an extension grade wire for the thermocouple extension cable. This will provide you with a combined special limit of error from 370C to 200C and standard limits of error from 200C to 50C for your overall accuracy. Wire of the extension grade is suitable if that is acceptable. Since extension grade wire is only tested and certified to 200C, any usage above that temperature cannot be assumed to fulfil standard limits unless the wire is calibrated and the actual tolerance is used to calculate the measurement accuracy.
Parting words
Thus, the above details are about the difference between Thermocouple and Extension wire. Choose the thermocouple or extension wire that best suits your needs because they serve the same purpose with little differentiation.