Thermocouple Notes PDF

# Thermocouple Thermocouples are temperature sensors used to measure temperature variations. They sense the temperature and convert the non-electrical quantity (temperature) into voltage (electrical quantity). - Thermocouples are **active transducers** because they do not require an external powe...

# Thermocouple Thermocouples are temperature sensors used to measure temperature variations. They sense the temperature and convert the non-electrical quantity (temperature) into voltage (electrical quantity). - Thermocouples are **active transducers** because they do not require an external power source to operate. - They are constructed from two wire leads made from different metals. - The wire leads are welded together to create a junction. - As the temperature changes from the junction to the ends of the wire leads, a voltage develops across the junction. - Combinations of different metals create a variety of voltage responses. - This leads to different types of thermocouples used for different temperature ranges and accuracies. - Choosing a thermocouple often is a function of the measurement temperature range required in the application. - Other considerations include the temperature accuracy, durability, conditions of use, and the expected service life. ## Thermocouple Working Principle The thermocouple working principle is based on the Seeback Effect. This effect states that: - When a closed circuit is formed by jointing two dissimilar metals at two junctions, and junctions are maintained at different temperatures then an electromotive force (*e.m.f.*) is induced in this closed circuit. - The amount of induced *e.m.f.* is different for different metal combinations and is proportional to the temperature difference of the junctions. - This is the basic thermocouple working principle. ## Typical Circuit Diagram - Two dissimilar metals 'A' and 'B' are joined at the two junctions 'P' and 'Q'. - The 'P' junction is the measuring junction or hot junction. - The 'Q' junction is the reference junction or cold junction. - A PMMC (permanent magnet moving coil) meter instrument is connected in this arrangement. - **Hot junction (P)** is exposed to the heat source. - **Cold Junction (Q)** is maintained at a constant temperature. ## E.M.F. Generation - When the junctions are kept at different temperatures, generally cold junction is kept at 0 °C, and the measuring junction is kept at an unknown temperature. - This temperature difference causes an *e.m.f* to be generated in the circuit. - The *e.m.f* is measured with a PMMC instrument. - When both the junctions are at the same temperature, *e.m.f.* is zero. - The *e.m.f.* generated in a thermocouple is given by: - E = a(ΔΘ) + b(ΔΘ)^2 - ΔΘ = difference in temperature between two junctions (in °C) - a, b = constants: Seebeck constants for the thermocouple. - Generally, 'a' is very large compared to 'b', so 'b' can be neglected. -Therefore E = a(ΔΘ) - ΔΘ = Ε/a ## Thermocouple Metals - Theoretically, any pair of dissimilar materials can be used as a thermocouple. - In practice, only a few materials have found applications for temperature measurement. - The choice of materials is influenced by several factors: - sensitivity, - stability in calibration, - inertness in the operating atmosphere - reproducibility. - Table-I shows the common types of thermocouples, their types, composition, range, sensitivity, etc. - The upper range of the thermocouple is normally dependent on the atmosphere where it has been put. ### Thermocouple Materials and Characteristics The table below shows some common thermocouple types and their characteristics, including: - Thermocouple Type - Lead Metal A (+) - Lead Metal B (-) - Temperature Range (°C) - EMF over Temperature Range (mV) - Seebeck Coefficient (µV/°C at 0°C) | Thermocouple Type | Lead Metal A (+) | Lead Metal B (-) | Temperature Range (°C) | EMF over Temperature Range (mV) | Seebeck Coefficient (µV/°C at 0°C) | |---|---|---|---|---|---| | J | Iron | Constantan | -210 to 1200 | -8.095 to 69.553 | 50.37 | | K | Chromel | Alumel | -270 to 1370 | -6.458 to 54.886 | 39.48 | | T | Copper | Constantan | -200 to 400 | -6.258 to 20.872 | 38.74 | | E | Chromel | Constantan | -270 to 1000 | -9.385 to 76.373 | 58.70 | | S | Platinum and 10% Rhodium | Platinum | -50 to 1768 | -0.236 to 18.693 | 10.19 | ## Thermocouple Measurement Sensitivity - Figure 2 illustrates the output voltage versus temperature for the various types of thermocouples. ## Thermocouple Construction - Thermocouples come in several different construction types as shown in Figure 3. - Thermocouple leads are protected by a layer of insulation and often have a protective sheath at the thermocouple junction tip to protect the sensor element. - **Exposed Thermocouple**: This type has no protective sheath, enabling fast sensor response but making it susceptible to damage. - **Grounded Thermocouple**: It has a protective sheath welded to the sensor, providing extra protection, but also allowing for electrical contact and potential ground loops. - **Ungrounded Thermocouple**: This type has the slowest response because the sensor is isolated from the sheath by a layer of insulation, minimizing electrical contact. ## Advantages of using a Thermocouple: - **Fast Response**: Thermocouples follow the temperature changes with a small time-lag. They are suitable for applications where very rapid changes in temperature take place. - **Point Measurement**: They are convenient for measuring the temperature at one particular point in any apparatus or setup. ## Disadvantages of using a Thermocouple: - **Low Accuracy**: Thermocouples have very low accuracy, making them unsuitable for high-precision measurements. - **Vulnerability to Contamination**: They require protection against contamination to ensure a long life. - **Error due to Distance**: They can have errors if placed a very large distance from the measuring device.