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.
RTD is usually manufactured using a pure material, mainly platinum, copper or nickel. The material used always has a predictable variation of resistivity as its internal energy changes. It is this predictable change that is applied to determine its thermal energy changes. Platinum is a noble metal having the most stable conductivity versus resistivity relationship within a range of different thermal conductivity range. Platinum is also the best material for RTDs since it follows a linear relationship in a highly repeatable manner.
Even though any value is achievable for nominal resistivity, the most common is platinum 100 ohm. Tolerance class also determines the accuracy of the sensor. Different industry standards have also been put in place to ensure accuracy is achieved. In addition, using values of tolerance and nominal resistivity, the functional features of their sensor can be defined.
The major categories of RTDs include: strain free elements, thin film elements, coiled elements, wire-wound elements and strain free elements. Wire wound elements have great accuracy, particularly over wide temperature ranges. The diameter of the coil also provides a compromise between mechanical stability and also permitting expansion of the wire to reduce mechanical strain and consequential drift. The detecting wire is wound around an insulating core or mandrel.
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.
Thin film elements have detecting components that is formed by depositing a relatively thin layer of resistive substance, usually platinum, on a ceramic substance. This layer is always below a hundred angstroms. The thin film deposit is coated with glass or epoxy to protect it from contamination and also to act as strain relief for foreign lead-wires. This type of RTD are however not as stable as the coiled or wire wound types.
Platinum detecting wire should be kept free from any contamination so as to ensure it retains its stability. A platinum film or wire should be supported on a former so that it gets minimal strains from its former or minimal differential expansion, even though it is often resistant to vibration. RTD assemblies manufactured using iron or copper are also used in number of applications.
In industries where operations take place beyond 660 degrees Celsius, RTDs are not usually used as they get uncontrollably contaminated. Their resistivity is essentially zero at three Kelvins, therefore rendering them useless. Compared to thermistors, RTDs have slower response time and are less sensitive to small temperature changes. A resistance temperature device is, however, used to make thermometers which have low drift, high accuracy and wide operation range.
RTD is usually manufactured using a pure material, mainly platinum, copper or nickel. The material used always has a predictable variation of resistivity as its internal energy changes. It is this predictable change that is applied to determine its thermal energy changes. Platinum is a noble metal having the most stable conductivity versus resistivity relationship within a range of different thermal conductivity range. Platinum is also the best material for RTDs since it follows a linear relationship in a highly repeatable manner.
Even though any value is achievable for nominal resistivity, the most common is platinum 100 ohm. Tolerance class also determines the accuracy of the sensor. Different industry standards have also been put in place to ensure accuracy is achieved. In addition, using values of tolerance and nominal resistivity, the functional features of their sensor can be defined.
The major categories of RTDs include: strain free elements, thin film elements, coiled elements, wire-wound elements and strain free elements. Wire wound elements have great accuracy, particularly over wide temperature ranges. The diameter of the coil also provides a compromise between mechanical stability and also permitting expansion of the wire to reduce mechanical strain and consequential drift. The detecting wire is wound around an insulating core or mandrel.
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.
Thin film elements have detecting components that is formed by depositing a relatively thin layer of resistive substance, usually platinum, on a ceramic substance. This layer is always below a hundred angstroms. The thin film deposit is coated with glass or epoxy to protect it from contamination and also to act as strain relief for foreign lead-wires. This type of RTD are however not as stable as the coiled or wire wound types.
Platinum detecting wire should be kept free from any contamination so as to ensure it retains its stability. A platinum film or wire should be supported on a former so that it gets minimal strains from its former or minimal differential expansion, even though it is often resistant to vibration. RTD assemblies manufactured using iron or copper are also used in number of applications.
In industries where operations take place beyond 660 degrees Celsius, RTDs are not usually used as they get uncontrollably contaminated. Their resistivity is essentially zero at three Kelvins, therefore rendering them useless. Compared to thermistors, RTDs have slower response time and are less sensitive to small temperature changes. A resistance temperature device is, however, used to make thermometers which have low drift, high accuracy and wide operation range.
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