Diese Präsentation wurde erfolgreich gemeldet.
Wir verwenden Ihre LinkedIn Profilangaben und Informationen zu Ihren Aktivitäten, um Anzeigen zu personalisieren und Ihnen relevantere Inhalte anzuzeigen. Sie können Ihre Anzeigeneinstellungen jederzeit ändern.

RTD-RESISTANCE TEMPERATURE DETECTOR

esistance thermometers, also called resistance temperature detectors (RTDs), are sensors used to measure temperature. Many RTD elements consist of a length of fine wire wrapped around a ceramic or glass core but other constructions are also used. The RTD wire is a pure material, typically platinum, nickel, or copper. The material has an accurate resistance/temperature relationship which is used to provide an indication of temperature. As RTD elements are fragile, they are often housed in protective probes.
Resistance thermometers are constructed in a number of forms and offer greater stability, accuracy and repeatability in some cases than thermocouples. While thermocouples use the Seebeck effect to generate a voltage, resistance thermometers use electrical resistance and require a power source to operate. The resistance ideally varies nearly linearly with temperature per the Callendar–Van Dusen equation.

The platinum detecting wire needs to be kept free of contamination to remain stable. A platinum wire or film is supported on a former in such a way that it gets minimal differential expansion or other strains from its former, yet is reasonably resistant to vibration. RTD assemblies made from iron or copper are also used in some applications. Commercial platinum grades exhibit a temperature coefficient of resistance 0.00385/°C (0.385%/°C) (European Fundamental Interval).[7] The sensor is usually made to have a resistance of 100 Ω at 0 °C. This is defined in BS EN 60751:1996 (taken from IEC 60751:1995). The American Fundamental Interval is 0.00392/°C,[8] based on using a purer grade of platinum than the European standard. The American standard is from the Scientific Apparatus Manufacturers Association (SAMA), who are no longer in this standards field. As a result, the "American standard" is hardly the standard even in the US.

Lead-wire resistance can also be a factor; adopting three- and four-wire, instead of two-wire, connections can eliminate connection-lead resistance effects from measurements (see below); three-wire connection is sufficient for most purposes and is an almost universal industrial practice. Four-wire connections are used for the most precise applications.

  • Als Erste(r) kommentieren

RTD-RESISTANCE TEMPERATURE DETECTOR

  1. 1. RESISTANCE TEMPERATURE DETECTOR Karolinekersin.E Assistant professor
  2. 2. RTD • RTD stands for Resistance Temperature Detector. • RTDs are sometimes referred to generally as resistance thermometers. • The American Society for Testing and Materials (ASTM) has defined the term resistance thermometer as follows: • Resistance thermometer, n. - a temperature- measuring device composed of a resistance thermometer element, internal connecting wires, a protective shell with or without means for mounting a connection head, or connecting wire or other fittings, 2
  3. 3. PRINCIPLE • An RTD is a temperature sensor which measures temperature using the principle that the resistance of a metal changes with temperature. • In practice, an electrical current is transmitted through a piece of metal (the RTD element or resistor) located in proximity to the area where temperature is to be measured. • The resistance value of the RTD element is then measured by an instrument. • This resistance value is then correlated to temperature based upon the known resistance characteristics of the RTD element 3
  4. 4. • TDs work on a basic correlation between metals and temperature. • As the temperature of a metal increases, the metal's resistance to the flow of electricity increases. • Similarly, as the temperature of the RTD resistance element increases, the electrical resistance, measured in ohms (Ω), increases. 4
  5. 5. COMMON COMPONENTS OF RTD • RTD platinum resistance element • RTD Tubing Material • RTD Process Connection • RTD Wire Configuration • RTD cold end termination 5
  6. 6. RTD PLATINUM RESISTANCE ELEMENT: • This is the actual temperature sensing portion of the RTD. Elements range in length from 1/8″ to 3″. • The standard temperature coefficient is an alpha of .00385 and the standard resistance is 100 Ω at 0° C. 6
  7. 7. RTD OUTSIDE DIAMETER • The most common outside diameter is ¼” in the US or 6mm (.236″) for non-US applications. • However, outside diameters range from .063″ to .500″ 7
  8. 8. RTD PROCESS CONNECTION: Process connection fittings include all standard fittings used with thermocouples (i.e. compression, welded, spring-loaded, etc.). 8
  9. 9. RTD WIRE CONFIGURATION • RTDs are available in 2, 3 and 4 wire configuration. • 3 wire configurations are the most common for industrial applications. • Teflon and fiberglass are the standard wire insulation materials. • Teflon is moisture resistant and can be used up to 400° F. Fiberglass can be used up to 1000° F. 9
  10. 10. RTD COLD END TERMINATION • RTDs can terminate on the cold end with plugs, bare wires, terminal heads and any of the reference junctions common to thermocouples. 10
  11. 11. RTD ELEMENTS • RTD elements are commonly specified according to their resistance in ohms at zero degrees Celsius . • The most common RTD specification is 100 Ω, which means that at 0o C the RTD element should demonstrate 100 Ω of resistance. • Platinum is the most commonly used metal for RTD elements due to a number of factors, including its • Chemical inertness, • linear temperature versus resistance relationship, • Temperature coefficient of resistance that is large enough to give readily measurable resistance changes with temperature and • Stability (in that its temperature resistance does not drastically change with time). • Other metals that are less frequently used as the resistor elements in an RTD include nickel, copper and Balco. 11
  12. 12. CONFIGURATION OF RTD Thin film Wire wound Coiled element 12
  13. 13. CONFIGURATION OF RTD RTD elements are typically in one of three configurations: • A platinum or metal glass slurry film deposited or screened onto a small flat ceramic substrate known as "thin film" RTD elements, and • Platinum or metal wire wound on a glass or ceramic bobbin and sealed with a coating of molten glass known as "wire wound" RTD elements. • A partially supported wound element which is a small coil of wire inserted into a hole in a ceramic insulator and attached along one side of that hole. Of the three RTD elements, the thin film is most rugged and has become increasingly more accurate over time 13
  14. 14. PLATINUM RTDS • Platinum RTDs are the most common type of RTD used in industrial applications. • This is because platinum has excellent corrosion resistance, excellent long-term stability, and measures a wide range of temperature, (-200…+850°C). 14
  15. 15. NICKEL RTDS • Nickel RTDs are less expensive than platinum and have good corrosion resistance. • However, nickel ages more rapidly over time and loses accuracy at higher temperatures. • Nickel is limited to a measurement range of -80…+260°C. 15
  16. 16. COPPER RTDS • Copper RTDs have the best resistance to temperature linearity of the three RTD types, and copper is a low cost material. • However, copper oxidizes at higher temperatures. • Copper is limited to a measurement range of -200…+260°C 16
  17. 17. WIRE WOUND RTD • In a wire wound RTD, a resistance wire is wound around a non-conducting core, which is usually made of ceramic. • The sensor maker carefully trims the length of resistance wire to achieve the specified resistance at 0°C. This is called the “R0” resistance. • Next, lead wires are attached to the resistance wire, and then a glass or ceramic coating is applied over the wire for protection. • As temperature increases, the length of resistance wire increases slightly. 17
  18. 18. • Care must be taken in the design to ensure that the resistance wire does not twist or otherwise deform as temperature increases. • This is because mechanical strain causes a change in wire resistance. • Laboratory-grade RTDs used by calibration and standards laboratories eliminate this source of error by loosely winding resistance wire around a non-conducting support structure. • This type of RTD can be extremely accurate, but is fragile and not suited for most industrial applications. 18
  19. 19. COILED ELEMENT RTD • In a coiled element RTD, the resistance wire is rolled into small coils, which loosely fit into a ceramic form that is then filled with non-conductive powder. • The resistance wire is free to expand and contract as temperature changes, minimizing error caused by mechanical strain. 19
  20. 20. • The powder increases the rate of heat transfer into the coils, thereby improving the response time. • Coiled element RTDs are usually protected by a metal sheath and are used in industrial applications 20
  21. 21. THIN FILM RTDS • Thin film RTDs are mass-produced and cost less than the other RTD types. • They are smaller, and have a faster response time than the others, which is desirable in many applications. • They are made by depositing a thin pathway of platinum on a ceramic base. • The manufacturer adjusts the resistance at 0°C by opening parallel shunts in the pathway with a laser beam. • The more shunts are opened, the higher is the resistance at 0°C 21
  22. 22. Thin film RTDs are not as accurate as the other types because: The R0 resistance cannot be adjusted as precisely as in the other types. The ceramic base and platinum coating have slightly different expansion rates. This creates a strain error at higher temperatures. Because thin film RTDs are smaller, the RTD excitation current causes a slightly higher error due to RTD self-heating. 22
  23. 23. RTD RESISTANCE RATIO • The term “resistance ratio” describes the average slope of temperature vs. resistance as the RTD temperature changes from 0°C to +100°C. The expression for resistance ratio is (R100-R0) / R0 Where: • R100 RTD Resistance at 100°C. • R0 = RTD Resistance at 0°C. Resistance ratio is affected by the type and purity of the metal used to make the RTD 23
  24. 24. CHARACTERISTICS OF RTD ELEMENT Temperature Coefficient Nominal Resistance Temperature Range of Application Physical Dimensions or Size Restrictions Accuracy 24
  25. 25. MATERIAL OF RESISTANCE ELEMENT • Several metals are quite common for use in resistance elements and the purity of the metal affects its characteristics. • Platinum is by far the most popular due to its linearity with temperature. • Other common materials are nickel and copper, although most of these are being replaced by platinum elements. • Other metals used, though rarely, are Balco (an iron-nickel alloy), tungsten and iridium 25
  26. 26. TEMPERATURE COEFFICIENT • The temperature coefficient of an element is a physical and electrical property of the material. • This is a term that describes the average resistance change per unit of temperature from ice point to the boiling point of water NOMINAL RESISTANCE Nominal Resistance is the prespecified resistance value at a given temperature 26
  27. 27. TEMPERATURE RANGE OF APPLICATION • Depending on the mechanical configuration and manufacturing methods, RTD’s may be used from -270oC to 850oC. • Specifications for temperature range will be different, for thin film, wire wound and glass encapsulated types 27
  28. 28. PHYSICAL DIMENSIONS The most critical dimension of the element is outside diameter (O.D.), because the element must often fit within a protective sheath ACCURACY IEC 751 specifications for Platinum Resistance Thermometers have adopted DIN 43760 requirements for accuracy 28
  29. 29. APPLICATIONS OF RTD • RTD sensor is used in automotive to measure the engine temperature, an oil level sensor, intake air temperature sensors. • In communication and instrumentation for sensing the over the temperature of amplifiers, transistor gain stabilizers, etc • RTD is used in power electronics, computer, consumer electronics, food handling and processing, industrial electronics, medical electronics, military, and aerospace. 29
  30. 30. APPLICATIONS OF RTD Power electronics, Computer, Consumer electronics, Food handling and processing, Industrial electronics Medical electronics Military Aerospace. 30
  31. 31. THERMISTORS • Thermistor is special type of resistor, whose resistance varies more significantly with temperature than in standard resistors. • Generally, the resistance increases with the temperature for most of the metals but the thermistors respond negatively i.e. the resistance of the thermistors decrease with the increase in temperature. • This is the main principle behind thermistor. • As the resistance of thermistors depends on the temperature, they can be connected in the electrical circuit to measure the temperature of the body. 31
  32. 32. • Thermistors are mainly used as temperature sensors, inrush current limiters, self- resetting over-current protectors and self-regulating heating elements. • A thermistor is made from a semiconductor material. • It is shaped into a disc, a rod or a bead. • Bead thermistors may be only a few millimetres in diameter. • Some bead thermistors have the bead enclosed in a glass capsule. 32
  33. 33. TYPES OF THERMISTORS Positive-temperature coefficient (PTC) Negative-temperature coefficient (NTC) 33
  34. 34. • In PTC type thermistor, resistance increase with increase in temperature. Whereas in NTC thermistor, resistance decrease with increase in temperature. • Pure metals have positive temperature coefficient (PTC) of resistance, alloys have nearly equal zero temperature coefficient of resistance and semi conductors have negative temperature coefficient (NTC) of resistance. • PTC thermistors can be used as heating elements in small temperature controlled ovens. NTC thermistors can be used as inrush current limiting devices in power supply circuits. 34
  35. 35. FEATURES OF THERMISTOR • Thermistors are at least 10 times as sensitive as the platinum Resistance temperature detector (RTD). • This high sensitivity of thermistors is very useful for precision temperature measurement, control and compensation. • Although thermistors are very sensitive but on the other hand, it exhibits highly non-linear characteristics of resistance versus temperature. • Thermistors are available in variety of sizes and shapes • Thermistors are compact and rugged in construction. • It is widely used in the application where temperature measurements ranges -60 °C to 15 °C. 35
  36. 36. THERMISTOR CHARACTERISTICS • Resistance increase with increase in temperature for PTC and Resistance decrease with increase in temperature for NTC. • The thermistor exhibits a highly non- linear characteristic of resistance vs temperature. 36
  37. 37. CONTD.. • There are two fundamental ways to change the temperature of thermistor internally or externally. • The temperature of thermistor can be changed externally by changing the temperature of surrounding media and internally by self-heating resulting from a current flowing through the device. 37
  38. 38. THERMISTOR APPLICATIONS • Thermistors are used in circuits to control temperature. • They are used to compensate for the effects of temperature on conductor or circuit performance. • It is used in the measurement of power at high frequencies. • Thermistors are used to provide time delay in circuits. • It is used in the measurement of thermal conductivity. • It is used in the measurement of composition of gases. • Thermistors are used for the measurement of level, flow, and pressure of liquids. 38
  39. 39. CONTD.. • PTC thermistors were used as timers in the degaussing coil circuit of most CRT displays. • A degaussing circuit using a PTC thermistor is simple, reliable (for its simplicity), and inexpensive. • PTC thermistors used as heater in automotive industry to provide additional heat inside cabin with diesel engine or to heat diesel in cold climatic conditions before engine injection. • PTC thermistors used as current-limiting devices for circuit protection, as replacements for fuses. • NTC thermistors used to monitor the temperature of an incubator. • Thermistors are also commonly used in modern digital thermostats and to monitor the temperature of battery packs while charging. 39
  40. 40. CONTD.. • NTC thermistors are used in the Food Handling and Processing industry, especially for food storage systems and food preparation. Maintaining the correct temperature is critical to prevent food borne illness. • NTC thermistors are used throughout the Consumer Appliance industry for measuring temperature. Toasters, coffee makers, refrigerators, freezers, hair dryers, etc. all rely on thermistors for proper temperature control. • Hot ends of 3D printers; they monitor the heat produced and allow the printer’s control circuitry to keep a constant temperature for melting the plastic filament. 40
  41. 41. CONTD.. • NTC thermistors are used as resistance thermometers in low- temperature measurements of the order of 10 K. • NTC thermistors can be used as inrush-current limiting devices in power supply circuits. 41
  42. 42. THANK YOU 42

×