A resistance temperature device (RTD) is a thermal sensor that works on the measurement principle that the electrical resistivity of any material varies directly with its degree of sensible heat energy. The relationship between the environmental temperatures and the resistance of these devices is highly predictable. This allows for a consistent and accurate measuring of the amount of sensible heat in the detector. When the devices are supplied with stable source of current and resultant voltage drop across the resistor measured, its force that opposes flow of current can be computed, and its sensible heat also determined.
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.
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.
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.
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.
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.
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.
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.
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.
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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