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.
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.
Apart from the different materials, RTD can be made in two configurations: thin film and wire wound. A wire wound configuration shows an outer wound or an inner coil RTD. Inner coil construction is made up of a resistive coil that runs through an opening in a ceramic, whereas an outer wound consists of winding the resistive material around a glass cylinder or ceramic having a glass dollop.
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 made using RTDs have improved accuracy, repeatability and stability in most cases unlike the thermocouple types. To measure their opposition to flow of current, a small current has to be passed through the device being tested. This results in resistive heating, resulting in significant loss of accuracy if the design of does not adequately consider the heat path, or the limits set by the manufacturer are not adhered to. For most precise applications, four wire connections are often used.
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.
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.
Apart from the different materials, RTD can be made in two configurations: thin film and wire wound. A wire wound configuration shows an outer wound or an inner coil RTD. Inner coil construction is made up of a resistive coil that runs through an opening in a ceramic, whereas an outer wound consists of winding the resistive material around a glass cylinder or ceramic having a glass dollop.
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 made using RTDs have improved accuracy, repeatability and stability in most cases unlike the thermocouple types. To measure their opposition to flow of current, a small current has to be passed through the device being tested. This results in resistive heating, resulting in significant loss of accuracy if the design of does not adequately consider the heat path, or the limits set by the manufacturer are not adhered to. For most precise applications, four wire connections are often used.
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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