ResistanceTemperatureThermometer_RTD.ppt
- 1. Presently Used Industrial Thermometers.
- 2. Resistance Temperature Detectors P M V Subbarao Professor Mechanical Engineering Department Active Temperature Detectors……
- 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.
- 5. 0 50 100 150 200 250 300 350 400 -300 -100 100 300 500 700 900 t, o C R, Ohm Platinum RTD Copper RTD
- 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 • Meyersʼ 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 Meyersʼ design. • This is the bird-cage element proposed by Evans and Burns.
- 9. The Bird-cage RTD Stainless Steel open ended fluid probe is ideal for protecting the temperature sensor at the same time of reducing thermal mass
- 10. COMPARISON OF ELEMENT TYPES 0 50 100 150 200 250 300 350 400 -300 -100 100 300 500 700 900 t, o C R, Ohm Platinum RTD Copper RTD
- 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.
- 12. Calendar—Van Dusen equation 3 0 01 . 0 1 01 . 0 01 . 0 1 01 . 0 1 T T T T T R R • Where, , and are calibration constants, dependent on the purity of the platinum which is country dependent. •The dominant constant is which has a value of either 0.003921/°C for the so-called U.S. calibration curve, or • 0.003851/°C for the "European" calibration curve. •RTD sensors corresponding to either curve are available. •For the U.S. calibration curve, 1.49, 0 for T < 0 and = 0.11 for T>0. 4 3 2 0 100 1 T C T C T B T A R R