Thermocouples. Thermocouple

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Thermocouple Nimish Shah Thermocouples Most frequently used method to measure temperatures with an electrical output signal. Outline What Is A Thermocouple Sensor?

1 Thermocouple Nimish Shah Thermocouples Most frequently used method to measure temperatures with an electrical output signal. Outline What Is A Thermocouple Sensor?? Basic Working Principle Practical Thermocouple Construction Thermocouple Materials Thermocouple Color Codes Characteristics Major Specifications Capabilities and Limitations Selecting A Temperature Sensor Comparisons What Is A Thermocouple Sensor?? Thermocouple comprises of at least two metals joint together to form two junctions: Hot junction & Cold junction hot or measuring junction : connected to the body What Is A Thermocouple Sensor?? cold or reference junction: connected to the body of known temperature. Thus the thermocouple enables measuring the unknown temperature of the body with reference to the known temperature of the other body. Working principle: thermocouple Principle: A circuit made by connecting two dissimilar metals produces a measurable voltage (emf-electromotive force) when a temperature gradient is imposed between one end and the other.

2 Thermocouples Two alloys joined at one end and free at the other. At the open end, the emf is a function of temperature T 1 at closed end As T rises, emf increases T 2 must be kept at a standard temp (0 C) Thermocouples Junction composition does not affect thermocouple action T 1 must be kept constant throughout the junction Junction material must be electrically conductive Basic Working Principle Seeback effect Thomson effect Peltier effect Thermocouples Background Seebeck 1822 Current flow is proportional to temperature difference Peltier 1834 If battery inserted in the circuit, flowing current will cause heat to be absorbed & liberated Thomson analyzed effect thermodynamically Seeback Effect In, 1821 T. J. Seebeck observed the existence of an electromotive force (EMF) at the junction formed between two dissimilar metals (Seebeck effect). Seebeck effect is actually the combined result of two other phenomena, Thomson and Peltier effects. Thermocouples Principle of Operation Thomson observed the existence of an EMF due to the contact of two dissimilar metals at the junction temperature. Peltier discovered that temperature gradients along conductors in a circuit generate an EMF. The Thomson effect is normally much smaller than the Peltier effect.

insulated from each other long the length.

3 Practical Thermocouple Construction Practical Thermocouple Construction A thermocouple construction consist of two conductors, welded together at the measuring point and insulated from each other long the length. It will usually have an outer protection sheath. Practical Thermocouple Construction Practical Thermocouple Construction Thermocouple Materials

4 Which materials should be used? Thermocouple Material Vs EMF Depends on requirements: Temperature range Required accuracy Chemical resistance issues Abrasion or vibration resistance Installation requirements (size of wire) Thermal conduction requirements Thermocouple Materials Most commonly used three materials: Iron-Constantan (Type J), Copper-Constantan (Type T), and Chromel-Alumel (Type K). 1-) The first named element of the pair is the positive element. 2-) The negative wire is color coded red. Iron-Constantan: (Type J) color code: white and red generates about 50 µv/ C (28 µv/ F). The Iron wire is magnetic. Junctions made by welding or soldering with commonly available solders and fluxes. can generate a galvanic EMF between the two wires and should not be used in applications where they might get wet.

5 Major Specifications Type J: Iron / Constantan: Useful range of temperature is -300F to 1200F. Maximum temperature 1600F. Possible problems: Oxidizes rapidly due to the iron wire. preferred over the beaded bare wire style of thermocouple. Chromel Alumel: (Type K) color code: yellow and red generates about 40 µv/ C (22 µv/ F). The Alumel wire is magnetic. Junctions made by welding or soldering, but high temperature silver-solders and special fluxes must be used. generate electrical signals, while the wires are being bent, and should not be used on vibrating systems, unless strain relief loops provided. Major Specifications Type K: Chromel / Alumel Useful range of temperature is -300F to 1800F. Maximum temperature 2300F. Type E: Chromel / Constantan Useful range of temperature is -300F to 1800F. Maximum temperature 1000F. Copper-Constantan: (Type T) color coded blue and red generates about 40 µv/ C (22 µv/ F). Neither wire is magnetic. Junctions made by welding or soldering with commonly available solders and fluxes. Are very susceptible to conduction error due to the high thermal conductivity of the copper, and should not be used unless long runs of wire (100 to 200 wire diameters) can be laid along an isotherm. Major Specifications Type T: Copper / Constantan Useful range of temperature is -300F to 700F. Maximum temperature 700F. Thermocouple Color Codes

6 Thermocouple Color Codes Thermocouple wiring is color coded by thermocouple types. Different countries utilize different color coding. Jacket coloring is sometimes a colored stripe instead of a solid color as shown. Characteristics Thermocouples have non-linear characteristics given by an approximating polynomial. For example for type J (range 1, -210 to 760 C) the characteristic is given by Capabilities and Limitations Capabilities: - Wide Range - Fast Response - Passive - Inexpensive where V is in and T in C The output signal from a thermocouple is up to 50mV. Advantages & Disadvantages Thermocouple can be used over great distances since emf is a function of T1only These long leads can cause problems There could be a break in the circuit or a reversal of leads Induced voltage from nearby A/C magnetic field Limitations: - Non-Linear Selecting A Temperature Sensor

7 Comparisons Thermocouple Resistance Thermometer (RTD) Thermistor Stability (Drift) Reasonable for limited lifetime Good Good Good Repeatability Reasonable Good Good Good Hysteresis Excellent Good Good Good Vibration Very Resistant Less Resistant Whole Good Tolerant Measurement Area Single Point Whole RT Element bead (Small) Varies Diameter Small Sizes (to 0.25mm) Larger (3.0mm min) (0.5mm min) Varies Linearity Not Linear Reasonably Linear Not Linear Reasonably Linear Reference Junction Required Not Required Not Required Infrared Not Required Lead Wire Resistance No Problem Must be Considered No Problem Not Required Contact Required Yes Yes Yes No Response Fast Slower Medium Fast

Thermometer Physical Principles. Thermal Expansion Electrical Resistance Thermoelectric Radiation Change-of-State. Temperature Sensors

Thermometer Physical Principles Thermal Expansion Electrical Resistance Thermoelectric Radiation Change-of-State Temperature Sensors Fluid-Expansion o Household thermometer o Mercury, Alcohol, gas Bimetalic