About Instrumentation

Resistance Temperature Detectors (RTD), as the name implies, are sensors used to measure temperature by correlating the resistance of the RTD element with temperature. Most RTD elements consist of a length of fine coiled wire wrapped around a ceramic or glass core. The element is usually quite fragile, so it is often placed inside a sheathed probe to protect it. The RTD element is made from a pure material whose resistance at various temperatures has been documented. The material has a predictable change in resistance as the temperature changes; it is this predictable change that is used to determine temperature.

Common Resistance Materials for RTDs:

· Platinum (most popular and accurate)

· Nickel

· Copper

· Balco (rare)

· Tungsten (rare)

The resistance is directly proportional to a metal wire's length, and inversely proportional to the cross-sectional area.

R = k L / A (1)

where

R = resistance (ohm, Ω)

k = constant of proportionality or resistivity of the material (ohm, Ω)

L = length of conductor (m)

A = cross sectional area of conductor (m²)

Resistivity and temperature can be expressed as

k_t = k_o [1 + α (t - t_o)] (2)

where

k_t = resistivity at temperature t (ohm, Ω)

k_o = resistivity at standard temperature t_o (ohm, Ω)

α = temperature coefficient of resistance (1/^oC)

t = temperature (^oC)

t_o = standard temperature (^oC)

Combining (1) and (2):

R / R_o = α t + 1 (3)

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.

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, 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 temperature coefficients as their standards .

Material	Temperature Coefficient of Resistance - α - (1/^oC)
Nickel	0.0067
Iron	0.002 to 0.006
Tungsten	0.0048
Aluminium	0.0045
Copper	0.0043
Lead	0.0042
Silver	0.0041
Gold	0.004
Platinium	0.00392
Mercury	0.0009
Manganin	+- 0.00002
Carbon	-0.0007
Electrolytes	-0.02 to -0.09
Thermistor	-0.068 to 0.14

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.


2 wire


3

Four wire RTDs provide slightly better compensation, but are generally found only in laboratory equiptment 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 RTD instument 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 3-4 wire instrument by jumping the appropriate terminals, although this defeats the purpose and reintroduces the uncompensated resistance of the leads. To get the optimum performance, it is generally best to specify the RTD according to the instrument manufacturer's recommendations.

Temperature Range:

According to the ASTM, platinum RTDs can measure temperatures from -200°C to 650°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.

· 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 assit you, espically since it may affectthe materials chosen for internal

construction of the probe.

Accuracy:

You are probably wondering why accuracy was not the first topic covered, because RTDs are generally known for their high degree of accuracy and it is typically on 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 RTDs resistance matched the resistance from a look up table, but rather with how well it matched the resistance of other RTDs 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 reading 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 destory the high accuracy of the RTD signal. The best solution for applications of this type is to have noth the RTD and the transmitter, or display, calibrated as a unit by a certified calibration laboratory.

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 (W) width of the element, and the (T) thickness of the element at its largest point. 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 Compatability:

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.

Article Source: www.omega.com, www.thermometricscorp.com