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Resistance temperature device (RTD) applies the concept that electrical resistivity of any element has a direct variation with its thermal energy. The relationship between sensible heat in the environment and resistivity of the elements can be easily predicted. RTD devices are permanently replacing the use of thermocouple thermometers in several industrial applications that operate below 600 degrees Celsius. This is due to their repeatability and higher accuracy.
Various materials are used in constructing the devices so as to achieve different relationship between resistivity and thermal energy. Thermal sensitive materials used in constructing the devices include nickel, platinum, and copper. Platinum is the most commonly used material. The materials also offer good tolerance classes and nominal resistivity at zero degrees Celsius.
Platinum is the best metal for RTDs because of its chemical inertness. The thermal coefficient of resistivity is called alpha. Different alpha values for platinum are particularly achieved through doping process; involving introduction of impurities into the platinum. The introduced impurities during doping become embedded within the crystal lattice of the platinum resulting in improved stability.
Apart from the different materials, RTD can be made in two configurations: thin film and wire wound. A wire wound configuration shows an outer wound or an inner coil RTD. Inner coil construction is made up of a resistive coil that runs through an opening in a ceramic, whereas an outer wound consists of winding the resistive material around a glass cylinder or ceramic having a glass dollop.
Thin film RTD consists of a thin layer of resistive substance deposited on a ceramic by a process called deposition. A resistant meander is then etched onto the detector, and lesser trimming then applied in achieving the required nominal value of its sensor. The resistive substance is then guarded with a thin layer of glass. Lead wires are also welded to form pads with the detector and then covered using a glass dollop.
Thermometers are constructed using RTDs in various forms to offer greater accuracy, stability, and repeatability in some cases compared to thermocouples. Thermometers made using RTDs use the concept of electrical resistance and require a constant power source for effective operation. The opposition to flow of current in these devices ideally varies linearly with amount of heat sensible in their surroundings.
To ensure effectiveness and stability of platinum detecting wires are not interfered with, the wires must be kept free from any foreign contamination. Commercial platinum grades are produced which exhibit a coefficient of resistivity of 0.00385 degrees Celsius. RTD devices are however less sensitive to small changes in internal thermal energy as compares to thermistors.
Any appliances made from a resistance temperature device are not suitable for industrial applications that operate above six hundred degrees Celsius. This is because the platinum becomes contaminated with impurities produced from the metal sheath of the thermometer. The appliances are however suitable for precision applications and have wide operating range.
Various materials are used in constructing the devices so as to achieve different relationship between resistivity and thermal energy. Thermal sensitive materials used in constructing the devices include nickel, platinum, and copper. Platinum is the most commonly used material. The materials also offer good tolerance classes and nominal resistivity at zero degrees Celsius.
Platinum is the best metal for RTDs because of its chemical inertness. The thermal coefficient of resistivity is called alpha. Different alpha values for platinum are particularly achieved through doping process; involving introduction of impurities into the platinum. The introduced impurities during doping become embedded within the crystal lattice of the platinum resulting in improved stability.
Apart from the different materials, RTD can be made in two configurations: thin film and wire wound. A wire wound configuration shows an outer wound or an inner coil RTD. Inner coil construction is made up of a resistive coil that runs through an opening in a ceramic, whereas an outer wound consists of winding the resistive material around a glass cylinder or ceramic having a glass dollop.
Thin film RTD consists of a thin layer of resistive substance deposited on a ceramic by a process called deposition. A resistant meander is then etched onto the detector, and lesser trimming then applied in achieving the required nominal value of its sensor. The resistive substance is then guarded with a thin layer of glass. Lead wires are also welded to form pads with the detector and then covered using a glass dollop.
Thermometers are constructed using RTDs in various forms to offer greater accuracy, stability, and repeatability in some cases compared to thermocouples. Thermometers made using RTDs use the concept of electrical resistance and require a constant power source for effective operation. The opposition to flow of current in these devices ideally varies linearly with amount of heat sensible in their surroundings.
To ensure effectiveness and stability of platinum detecting wires are not interfered with, the wires must be kept free from any foreign contamination. Commercial platinum grades are produced which exhibit a coefficient of resistivity of 0.00385 degrees Celsius. RTD devices are however less sensitive to small changes in internal thermal energy as compares to thermistors.
Any appliances made from a resistance temperature device are not suitable for industrial applications that operate above six hundred degrees Celsius. This is because the platinum becomes contaminated with impurities produced from the metal sheath of the thermometer. The appliances are however suitable for precision applications and have wide operating range.
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