Temperature Sensing Devices
Overview of Platinum RTD's
in Temperature Characteristics
Characteristics of interest in choosing the proper resistance element are briefly outlined as follows:
Temperature Coefficient of Resistance The change in element resistance for unit change in temperature, generally stated for the temperature range from 0 to 100oC (0 to 100oC).
Resistance A function of material, diameter of wire, and its length. A common resistance value for platinum RTD elements is 100 ohms at 0oC.
Element Material Tensile Strength and Ductility Determines minimum wire size available.
Stability The ability of the element to withstand a variety of conditions such as vibration, shock, temperature cycling and time.
Linearity The relationship between element resistance and temperature should be of a linear, or straight line nature over the temperature range of interest.
Platinum has become the general choice for element material for numerous industrial applications. Even though platinum has become a general choice for industrial RTD's based on all of the above considerations, because of its inherent material characteristics, all platinum RTD elements are not manufactured to have the same temperature coefficient of resistance. Some of the reasons for the variation in temperature coefficient are metal purity, degree of anneal, and strain on the element.
The higher the purity of the platinum in the element, the higher the temperature coefficient of resistance. Highest purity platinum in a strain free, fully annealed element has a temperature coefficient (T.C.) of 0.003926 ohm/ohm/oC; that is, its resistance at 100oC is 1.3926 times its resistance at 0oC. As the purity of platinum decreases the T.C. also decreases. It takes platinum of 99.999% purity to get a T.C. of 0.003926; platinum with a "controlled degree of impurity" is regularly used to give a T.C. of 0.00385 in the strain free, fully annealed state.
Drawing of the platinum wire and strain induced in the manufacture of the element reduces the T.C. of even the highest purity platinum so that annealing is mandatory in the manufacture of a repeatable, temperature stable resistance element. Generally, the platinum element is embeded in a ceramic or glaze material which also reduces the T.C. of the element even when starting with highest purity, strain free, fully annealed platinum. Variations in the temperature coefficient attributable to the above mentioned factors undoubtedly have created the differing resistance-temperature characteristics published by platinum RTD Element manufacturers.
elements having the same nominal temperature coefficient for the normally
defined range of 0 to 100oC from different manufacturers
are not identical outside of that range. The standard temperature
coefficient for the elements used in Nutech Engineers's sensors has
a T.C. of 0.00385 for the range of 0 to 100oC.
RTD sensors are supplied with stated accuracies of 0.1% or 0.25% at
a temperature of 149oC because most of our customers' usage
will be at or near this temperature. Most other manufacturers
specify a stated percentage of accuracy at 0oC. Our
choice of 149oC means we generally offer elements having
a tighter accuracy at a temperature closer to that which our customer
is interested in, then those where accuracy is stated at 0oC.
Another area for concern in the use of RTD Assemblies is whether or not the lead resistance variation, as a function of temperature, will affect the accuracy of the element reading. Two-wire element construction is used for total assembly length up to 48 inches. Above 48 inches, combined lead and element length requires an increase of ohm tolerance of 0.03 ohm for each foot above 48 inches. Three-wire and four-wire element construction is used to allow lead resistance variation cancellation by proper bridge wiring. Three wire construction has found acceptance in industrial applications. Four wire construction is generally used in laboratory situations. because two separate readings are required at each point.
RTD's are used in bridge networks. The flow of bridge current through the RTD Element can cause a temperature rise in the element above the value that is being measured. The self-heating error depends on the magnitude of drive current, head dissipative capacity of the sensor, and conditions of medium flow past the sensor. Self heating is generally specified by giving the change in indicated temperature per milliwatt or dissipated power under a given condition of temperature and medium flow.
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