The thermocouple refers to a temperature-measuring device. It is used in various types of devices and applications. It transforms the change of temperature in an electrical voltage. However, the thermocouple operates over the thermoelectric effect working principle. This article explains the diagram, circuit, advantages, disadvantages, and application of thermocouples.
A thermocouple also refers to an Active Transducer. A thermocouple does not need any outer power supply to operate or work. It generates a voltage on its own, regarding the fact that it has a small magnitude. However, thermocouple cost less, construct easily, and have the ability to measure large amounts of temperature.
Moreover, a thermocouple contains two wires of metal that join with each and make a junction point. When that specific junction feels any type of temperature change, it creates voltage. Typically, thermocouples are specially designed to measure or sense the difference in temperature as compared to the normal temperature of the environment. This implies that it uses the temperature of the environment as a reference temperature. However, thermocouples use two junctions in which one has a cold temperature.
Principle Of Thermocouple Circuit
A thermocouple working principle follows three distinct types of effects.
· Seebeck Effect
This type of effect happens between two distinct metals. However, when one metal goes through heat, it starts flowing the electron from the heated metal to a cold metal. This induces the direct current in a circuit.
In simple words, the Seebeck effect refers to a process in which the difference in temperature between two distinct metals induces the difference potential between them. This effect creates a small amount of voltage per kelvin temperature.
· Peltier Effect
The Peltier effect simply refers to an inverse effect of the Seebeck effect. It represents that one can easily create a temperature difference between two distinct conductors by just providing a potential difference between both conductors.
· Thompson Effect
Thompson’s effect represents that whenever two distinct metals join with each and create two junctions, then due to the Tg or temperature gradient, the voltage may induce the entire conductor’s length. The Tg or temperature gradient refers to a physical term. It represents the direction along with the rate of temperature change over a specific location.
Construction Of Thermocouple Circuit
The thermocouple contains two distinct metals. Both metals weld together at a point of junction. However, this junction serves as a measuring point. These junction points have three categories.
· Ungrounded Junction
This type of ungrounded junction contains conductors totally isolated from a protective sheath. However, it is mainly used in applications that work under pressure. Moreover, its main goal is to reduce the stray magnetic field effect.
· Grounded Junction
These types of grounded junctions weld the protective sheath and metals together. Moreover, it helps in temperature measurement in corrosive areas. Furthermore, it provides noise resistance.
· Exposed Junction
These types of exposed junctions are only used in specific places where one requires a fast response. Moreover, exposed junctions also help in measuring the gas temperature.
Also, the selection of thermocouple material depends on the temperature measuring range.
Measurement Of Thermocouple Circuit Output
One can easily measure the thermocouple EMF output through various methods. Some of them include:
· Multimeter
Using a multimeter proves to be one of the simplest methods to measure the thermocouple output EMF. The thermocouple connects the multimeter with its cold junction. Moreover, a multimeter pointer deflection falls equal to the amount of current flowing through it.
· Potentiometer
A DC potentiometer can also help the thermocouple to measure the output.
· Amplifier
Thermocouple uses an amplifier to amplify the output. Then it sends the amplified output to an indicating or recording instrument.
Advantages Of Thermocouple Circuit
Thermocouples give various benefits; therefore, they are used in almost all transformers. The transformer contains a hot junction, while the externally mounted meter contains a cold junction.
This simple installation of a meter and transformer helps the meter to read the rise of temperature in the oil compared to the surrounding location. Moreover, due to their low cost and rugged installation, a turbine hall can use thermocouples around them exclusively. Also, a thermocouple can measure a great range of temperatures as compared to an RTD.
Disadvantages Of Thermocouple Circuit
Apart from giving benefits, it does have some disadvantages too. Thermocouple requires expensive extension-level compensating cables or thermocouple wire if it is present at a distance from a measuring device. Moreover, thermocouple does not work in regions of high radiation, such as reactor vaults. Also, radioactive radiation can add voltage inside thermocouple cables.
As the thermocouple releases a signal in the form of voltage, any additional voltage can cause an error in the output of the temperature transmitter. Furthermore, it gives a slow response as compared to RTDs. If it has remote control logic along with a temperature transmitter, then it may cause power failure. Thus resulting in faulty readings.
Failure Modes of Thermocouple Circuit
Thermocouple also has failure modes. If the thermocouple device has an opening circuit, then it does not allow the current flow. This results in off-scale or low readings of temperature. Moreover, other than open circuits, short circuits also cause off-scale readings of temperature. A short circuit allows the leakage of current to GND and small voltage.
Applications of Thermocouple Circuit
Thermocouples use in a variety of devices and applications. It has a vast range of industrial use like iron and steel industries. They use thermocouples to measure the temperature of production. Moreover, it uses in HVAC systems.
It helps in monitoring the room temperature, ambient temperature, etc. it also uses in devices of temperature control. These devices include refrigerators, air conditioners, and heat pumps. Here, thermocouples control the temperature system automatically.
