Tuesday, November 5, 2013
Impacts Of Resistance Temperature Device In Industries
By Allyson Burke
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.
The significant behavior of the metals used in manufacturing resistive elements is the ability to approximate their resistivity versus thermal energy relationship ranging from zero to a hundred degrees Celsius. Industrial standards have also been established so as to ensure the elements meet the required standards and accuracy. Functional characteristics of the sensors can also be found by applying values of nominal resistivity and tolerance.
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.
RTD can also be made inform of thin film or wire wound. Wire wound show an external winding or an internal coil. Inner coil comprises of a resistive coil that passes through a cavity in a ceramic, while an outer wound comprises of windings of the resistive substance element around a ceramic or a glass cylinder. Wire wound elements exhibits excellent accuracy, especially over varying thermal energy in materials.
Thermometers are constructed using RTDs in various forms to offer greater accuracy, stability, and repeatability in some cases compared to thermocouples. Thermometers made using RTDs use the concept of electrical resistance and require a constant power source for effective operation. The opposition to flow of current in these devices ideally varies linearly with amount of heat sensible in their surroundings.
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.
The significant behavior of the metals used in manufacturing resistive elements is the ability to approximate their resistivity versus thermal energy relationship ranging from zero to a hundred degrees Celsius. Industrial standards have also been established so as to ensure the elements meet the required standards and accuracy. Functional characteristics of the sensors can also be found by applying values of nominal resistivity and tolerance.
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.
RTD can also be made inform of thin film or wire wound. Wire wound show an external winding or an internal coil. Inner coil comprises of a resistive coil that passes through a cavity in a ceramic, while an outer wound comprises of windings of the resistive substance element around a ceramic or a glass cylinder. Wire wound elements exhibits excellent accuracy, especially over varying thermal energy in materials.
Thermometers are constructed using RTDs in various forms to offer greater accuracy, stability, and repeatability in some cases compared to thermocouples. Thermometers made using RTDs use the concept of electrical resistance and require a constant power source for effective operation. The opposition to flow of current in these devices ideally varies linearly with amount of heat sensible in their surroundings.
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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