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Variations
in Temperature Characteristics
The use of elements that change their resistance in a predictable
and repeatable manner as a function of temperature has become increasingly
popular in numerous industrial applications. Many materials have
found acceptance for use in the manufacture of the element whose predictable
resistance change over a temperature differential can be used for temperature
indication and/or control in an industrial process. Some of these materials
are platinum, copper, nickel and 70% nickel-30% iron.
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.
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Even
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.
Accuracy/Interchangeability
Another are of confusion when specifying RTD Assemblies is that
of accuracy and interchangeability. It is normal practice to specify
and state the accuracy of an RTD Assembly as a percentage of the element's
resistance at a specific temperature. It is not correct
to assume that the stated percentage accuracy applies over a range of
temperatures or at another temperature. Normally, a manufacturer
will furnish a table showing the tolerance of interchangeability in
both ohms and degrees at various temperatures. It is best practice
to specify the accuracy at the specific temperature or range which is
of concern. Specifying unneeded accuracy, or a tight accuracy
over a range not being used, will increase sensor cost unnecessarily.
Nutech
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.
Other
Considerations
RTD elements differ from thermocouple sensors in that the RTD elements
produce a signal that averages the temperature over the length of the
platinum elements. So-called tip sensitive RTD assemblies are
available by insulating the sides of the element from the protective
tube housing, and using a highly thermal conductive button at the sensing
end of the element.
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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