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.
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.
Calibration may be performed beyond a hundred degrees Celsius or below zero degrees Celsius. In this case, the comparison method or the fixed point method may be used. Fixed point calibration uses the best accuracy calibration by using freezing point, triple point or melting point of pure substances such as zinc, argon and tin to generate a known repeatable fixed point temperature.
Thin film elements have a detecting device which is formed by depositing a relatively thin layer of resistive substance, usually platinum, on a ceramic substrate. The main disadvantage of this type is that they are less stable compared to the wire-wound and coiled counterparts. They also have different expansion rates brought about by the substrate deposited that creates a strain gauge effect.
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.
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.
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.
Calibration may be performed beyond a hundred degrees Celsius or below zero degrees Celsius. In this case, the comparison method or the fixed point method may be used. Fixed point calibration uses the best accuracy calibration by using freezing point, triple point or melting point of pure substances such as zinc, argon and tin to generate a known repeatable fixed point temperature.
Thin film elements have a detecting device which is formed by depositing a relatively thin layer of resistive substance, usually platinum, on a ceramic substrate. The main disadvantage of this type is that they are less stable compared to the wire-wound and coiled counterparts. They also have different expansion rates brought about by the substrate deposited that creates a strain gauge effect.
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.
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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