With these general guidelines
to the basic function, performance, and recognized standards for RTD's, anyone can specify
the right device for the application.
Setting the specifications for any sensor or instrument can be a difficult process, and
RTD's (resistance temperature detectors) are no exception. No one can be expected to be an
expert in all fields, and frankly, no one needs to be. With these general RTD guidelines,
along with a little common sense and background information on the application, you will
successfully detail the specifications of an RTD that will satisfy your requirements.
THEORY OF OPERATION
A basic physical property of a metal is that its electrical resistivity changes with
temperature. All RTD's are based on this principle. The heart of the RTD is the resistance
element. Several varieties of semi-supported wire-wound fully supported bifilar wound
glass, and thin film type elements are shown here.
Some metals have a very predictable change of resistance for a given change of
temperature; these are the metals that are most commonly chosen for fabricating an RTD. A
precision resistor is made from one of these metals to a nominal ohmic value at a
specified temperature. By measuring its resistance at some unknown temperature and
comparing this value to the resistor's nominal value, the change in resistance is
determined. Because the temperature vs. resistance characteristics are also known, the
change in temperature from the point initially specified can be calculated. We now have a
practical temperature sensor, which in its bare form (the resistor) is commonly referred
to as a resistance element.
Through years of experience, the characteristics of various metals and their alloys have
been learned, and their temperature vs. resistance relationships are available in look-up
tables. For some types of RTD's, there are also equations that give you the temperature
from a given resistance. This information has made it possible for instrument
manufacturers to provide standard readout and control devices that are compatible with
some of the more widely accepted types of RTD's.
RTD SPECIFICATIONS
Eight salient parameters must be addressed for every RTD application to ensure the desired
performance. Many will be specified by the manufacturer of the instrument to which the RTD
will be connected. If it is a custom circuit or special OEM application, the designers
must make all the decisions. The four specifications dictated by the instrumentation or
circuitry are: sensor material, temperature coefficient, nominal resistance, and, to some
extent, wiring configuration. Sensor Material Several metals are quite common for use in
RTD's, and the purity of the metal as well as the element construction affects its
characteristics. Platinum is by far the most popular due to its near linearity with
temperature, wide temperature operating range, and superior long-term stability. Other
materials are nickel, copper, balco (an iron-nickel alloy), tungsten, and iridium. Most of
these are being replaced with platinum sensors, which are becoming more competitive in
price through the wide use of thin film-type resistance elements that require only a very
small amount of platinum as compared to a wire-wound element.
Temperature Coefficient
The temperature coefficient (TC), or alpha of an RTD is a physical and electrical property
of the metal alloy and the method by which the element was fabricated. The alpha describes
the average resistance change per unit temperature from the ice point to the boiling point
of water. Various organizations have adopted a number of different TC's as their standards
(see "Temperature Coefficient Standards").
Nominal Resistance
Nominal resistance is the pre-specified resistance value at a given temperature. Most
standards, including IEC-751, use as their reference point because it is easy to
reproduce. The International Electrotechnical Commission (IEC) specifies the standard
based on 100.00 Ohms at 0°C, but other nominal resistance's are quite common. Among the
advantages that thin film technology has brought to the industry are small, economical
elements with nominal resistance's of 500, 1000, and even 2000 ohms.
Wiring Configuration
The wiring configuration is the last of those parameters typically specified by the
instrument manufacturer, although the system designer does have some control based on the
application. An RTD is inherently a 2-wire device, but lead wire resistance can
drastically reduce the accuracy of the measurement by adding additional, uncompensated
resistance into your system. Most applications therefore add a third wire to help the
circuit compensate for lead wire resistance, and thus provide a truer indication of the
measured temperature.
Four-wire RTD's provide slightly better compensation, but are generally found only in
laboratory equipment and other areas where high accuracy is required. When used in
conjunction with a 3-wire instrument, a 4-wire RTD will not provide any better accuracy.
If the fourth wire is not connected, the device is only as good as the 3-wire RTD; if the
fourth wire is connected, new errors will be introduced. Connecting a 3-wire RTD to a
4-wire instrument can cause serious errors or simply not work at all, depending on the
instrument circuitry. A 2-wire RTD can be used with either a 3 or a 4 -wire instrument by
jumping the appropriate terminals, although this defeats the purpose and reintroduces the
un compensated resistance of the leads. To get the optimum performance, it is generally
best to specify the RTD according to the instrument manufacturer's recommendations.
Two other parameters are more application dependent;
the temperature range of the application; and,
the accuracy.
Temperature Range
According to the ASTM, platinum RTD's can measure temperatures from -200°C to 650°C.
(IEC says -200°C to 850°C).
You must consider the temperature limitations of all the materials involved, where they
are applied, and the temperatures to which each will be exposed.
A few quick examples to illustrate this point:
TFE Teflon should not be used for wire insulation if it will be exposed to temperatures
above 200°C (250°C for some).
Moisture proof seals are commonly made with various types of epoxy that generally have
limits below that of the Teflon insulation.
Many wire insulating materials become brittle at subzero temperatures and therefore should
not be used for cryogenic work.
So state the intended temperature range right up front and let the applications engineer
assist you, especially since it may affect the materials chosen for internal construction
of the probe.
Accuracy
You are probably wondering why accuracy was not the first topic covered, because RTD's are
generally known for their high degree of accuracy and it is typically one of the first
specifications laid out. Well, the subject is not quite that simple, and it requires a bit
of discussion. First, we must establish the difference between accuracy, precision, and
repeatability. In the case of temperature, accuracy is commonly defined as how closely the
sensor indicates the true temperature being measured, or in a more practical sense, how
closely the resistance of the RTD matches the tabulated or calculated resistance of that
type RTD at that given temperature.
Precision, on the other hand, is not concerned with how well the RTD's resistance matches
the resistance from a look-up table, but rather with how well it matches the resistance of
other RTD's subjected to that temperature. Precision generally refers to a group of
sensors, and if the group has good precision at several temperatures, we can also say that
they are well matched. This is important when interchangeability is a concern, as well as
in the measurement of temperature gradients. Repeatability can best be described as the
sensor's ability to reproduce its previous readings at a given temperature.
Here's an example. An ice point reading is done with an RTD that is then used to take
readings at 100°C, 150°C, 37°C, and again at 0°C.
A comparison of the first and last ice point readings will give you an indication of the
sensor's repeatability under those conditions. A note of caution, however: an RTD's
repeatability is very application-dependent. So when you get right down to it, accuracy
without repeatability is worthless. If you start with a sensor that is ±0.03°C at 0°C
but is found to have repeatability only around ± 0. 5°C, what you have is a sensor whose
readings are far less reliable than a standard-accuracy probe with good repeatability. A
high-accuracy RTD installed in a field application also does not ensure that you will be
getting a highly accurate signal back at the control room.
Most 4-20 mA transmitters and many display units and controllers have adjustable zero and
span controls that if improperly adjusted will destroy the high accuracy of the RTD
signal.
The best solution for applications of this type is to have both the RTD and the
transmitter, or display, or whatever, calibrated as a unit by a certified calibration
laboratory.
Fortunately, the requirements for this degree of accuracy best solution for applications
of this type are few and far between. For more on this subject see, Accuracy Standards.
Our final two parameters are application dependent and vary from the specification of a
bare resistance element to a large industrial assembly with thermowells, connection heads,
and possibly field -mounted transmitters. We will discuss only the most basic areas:
physical dimensions and size restrictions, and material compatibility.
Dimensions and Size
The physical dimensions and size requirements can be more complicated than you might
think. On the low end, a resistance element to be used in the construction of a sheathed
RTD generally requires only that the element is small enough to fit into the desired
sheath ID. For cylindrical elements, such as wire-wound units, this is obvious-just don't
forget to allow for the wall thickness of the sheath. For thin film-type elements, we must
apply the Pythagorean theorem; we need to know the width of the element, w, and the
thickness of the element at its largest point, t. Then the minimum ID of the sheath will
be given by; ID > (w2 + t2).
When we begin to discuss RTD probes and assemblies, the subject becomes more demanding. We
need to examine the mounting arrangement: will it be used for direct immersion or with a
thermowell? Or will it be something special, like an exposed airflow probe or surface
mount sensor? Probe designs are endless in their configurations, and it seems that most
applications have some unique requirements that make this a rather creative field in
itself.
In many applications, the probe is immersed in a small vessel or piping system. Dimensions
here are generally limited to sensor diameter (which affects response time); immersion
depth into the fluid; and the mounting arrangement, i.e., will the sensor be screwed into
a threaded port, typically with ANSI tapered threads, or will it be used in con-junction
with a fluid seal already in place? Or will some other special considerations need to be
made? There may be other variables, such as pressure limitations or high flow, depending
on the complexity of the application. It is always best to look at the whole picture. and
then discuss it with your applications engineer.
Thermowells are generally used for larger vessels and systems so that the system will not
have to be drained in the event the sensor requires calibration or changing. Assuming the
thermowell has already been specified, we need only to specify the probe diameter
(typically ¼ in. OD for a 0.260 in. bore well), the depth of the thermowell bore, and how
the RTD will be secured into the well (typically spring-loaded through a ½ in. NPT nipple
or hex-nipple).
Material Compatibility
Most people specifying RTD probes have to pay attention only to the chemical compatibility
that will prevent corrosion. This is generally straightforward and guidelines can be taken
from other materials used in the system in which the RTD will be installed. If the piping
system is constructed of 316 S.S., then the probe probably should be also. But always
check a corrosion guide for corrosion rates and material recommendations if you have the
slightest doubt.
For applications involving thermowells, the thermowell will carry the burden of corrosion
protection. However, be sure to protect the connecting wires and any terminals or plugs
from possible corrosion caused by splash or corrosives in the atmosphere.
SUMMARY
There are quite a few things to be considered when specifying an RTD probe or even
resistance elements. But it's just a matter of applying a bit of common sense and using
information from the application environment to set down a clear set of requirements. And
if there is something you are uncertain about, get your background information together
and call that applications engineer. We can't all be experts at everything.
Reprinted from SENSORS, October 1995 - Copyright © 1995 by Helmers Publishing, Inc. 174
Concord St., Peterborough, NH 03458 - All Rights Reserved.
(ASTM) American Society for Testing Materials® (IEC) International Electrotechnical
Commission ® THE AUTHOR
David J. King
BSME with Thermometrics since 1991
For more information, please contact Dave
King in Vermont at
(802) 236-5893 (cell phone), or (802) 948-2858 (phone or fax)
Email Dave@RTDsource.com
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