Resistance temperature device (RTD), are detectors used to record the amount of sensible heat in a substances by correlating the resistivity of the element with its sensible heat energy. Several RTD elements are made of fine wire wound around a glass core or ceramic. The element is normally fragile; therefore, it is placed inside a probe that is sheathed to protect it.
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.
To ensure the stability of platinum wires is retained, they should be kept free from any contamination. When measuring their resistivity, a small current should be passed through the device being tested. Mechanical strain on the thermometers can also lead to inaccuracy. To avoid this, four-wire connections are used for most precise 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.
To ensure the stability of platinum wires is retained, they should be kept free from any contamination. When measuring their resistivity, a small current should be passed through the device being tested. Mechanical strain on the thermometers can also lead to inaccuracy. To avoid this, four-wire connections are used for most precise 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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