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Presently Used Industrial Thermometers.

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Presently Used Industrial Thermometers.

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    1. Presently Used Industrial Thermometers.

    2. Resistance Temperature Detectors P M V Subbarao Professor Mechanical Engineering Department

    3. History The same year that Seebeck made his discovery about thermoelectricity, Sir Humphrey Davy announced that the resistivity of metals showed a marked temperature dependence. Fifty years later, Sir William Siemens proffer the use of platinum as the element in a resistance thermometer. His choice proved most propitious, as platinum is used to this day as the primary element in all high-accuracy resistance thermometers. In fact, the Platinum Resistance Temperature Detector,15 or PRTD, is used today as an interpolation standard from the oxygen point (-182.96°C) to the antimony point (630.74°C).

    4. BASIC THEORY The electrical conductivity of a metal depends on the movement of electrons through its crystal lattice. Due to thermal excitation, the electrical resistance of a conductor varies according to its temperature and this forms the basic principals of resistance thermometry. The effect is most commonly exhibited as an increase in resistance with increasing temperature, a positive temperature coefficient of resistance. When utilising this effect for temperature measurement, a large value of temperature coefficient is a deal. Stability of the characteristic over the short and long term is vital if practical use is to made of the conductor in question. The relationship between the temperature and the electrical resistance is usually non-linear and described by a higher order polynomial.

    7. The Birth of RTD The classical resistance temperature detector (RTD) construction using platinum was proposed by C.H. Meyers in 1932. He wound a helical coil of platinum on a crossed mica web and mounted the assembly inside a glass tube. This construction minimized strain on the wire while maximizing resistance.

    8. Laboratory Standard Meyers' design produced a very stable element. The thermal contact between the platinum and the measured point is quite poor. This results in a large thermal response time. The fragility of the structure limits its use today primarily to that of a laboratory standard. Another laboratory standard has taken the place of Meyers' design. This is the bird-cage element proposed by Evans and Burns.

    9. The Bird-cage RTD

    10. COMPARISON OF ELEMENT TYPES

    11. Platinum Sensing Resistors Platinum, with its wide temperature range and stability, has become the preferred element material for resistance thermometers. Platinum sensing resistors are available with alternative Ro values, for example 10, 25 and 100 Ohms. A working form of resistance thermometer sensor is defined in IEC and DIN specifications. This forms the basis of most industrial and laboratory electrical thermometers. The platinum sensing resistor, Pt100 to IEC 751 is dominant in many parts of the world. Its advantages include chemical stability, relative ease of manufacture, the availability of wire in a highly pure form and excellent reproducibility of its electrical characteristic. The result is a truly interchangeable sensing resistor which is widely commercially available at a reasonable cost.

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