Today I wanna share some information of thermocouple wire with guys.
A Thermocouple is a sensor used to measure temperature of industrial field, Thermocouples consist of two wire legs made from different metals. The wires legs are welded together at one end, creating a junction. This junction is where the temperature is measured. When the junction experiences a change in temperature, a voltage is created. The voltage can then be interpreted using thermocouple reference tables to calculate the temperature.
Usually thermocouple wire is used in the following fields:
Heating - Gas burners for ovens
Cooling - Freezers
Engine protection - Temperatures and surface temperatures
High temperature control - Iron casting
Hope more and more people know about thermocouple wire and thermocouple sensor.
>When the junction
>experiences a change in temperature, a voltage is created.
This statement is not quite correct. The junction will produce a voltage proportional to the temperature of the junction. The temperature does not have to change to produce the voltage. For example, a Type K thermocouple will produce a voltage of 12.21 millivolts at 300 degC. If the temperature does change, the voltage will change - at 310 degC, the voltage will be 12.63 millivolts.
Strangely, a wide range of literature available about thermocouple describes its working in a wrong way. Like, "temperature causes a voltage to appear at the junction".
This statement is highly misleading. First of all, voltage does not appear at the junction. It simply cannot. The junction is merely a point or node which cannot experience a voltage difference.
The voltage difference is observed at the open ends of the thermocouple which is normally called the cold junction. The voltage produced is a function of the temperature difference between hot and cold ends of the thermocouple. The relationship between temperature and voltage is mostly linear in the normal working range of the thermocouple.
When two ends of a single wire are at a temperature gradient, a voltage difference is created between the two ends of the wire due to Seebeck effect.
In a thermocouple, two different wires are used, and they are tied together at the hot end to create the hot junction. Each wire experiences the Seebeck effect independently and a voltage is created between hot and cold ends of both wires due to temperature difference. The voltage across each wire is different because both wires are made from different alloys. Each wire acts as a voltage source having voltage v1 and v2. Since hot end of both wires is tied together, the voltage difference measured at the cold end is equal to difference of voltages produced across each wire individually due to Seebeck effect (v1-v2).
Note that unlike RTDs or thermistors, thermocouple is a relative measuring device. It does not measure the temperature absolutely, but only tells the temperature difference between hot and cold junctions. The measuring device, e.g. a DCS card, measures the voltage difference at the cold junction and converts that voltage to a temperature gradient using an internal lookup table. Finally, it adds the ambient temperature at the cold junction (measured using an RTD or thermistor) to find the process temperature at the hot junction.