Resistive temperature detectors (RTDs) are used whenever high accuracy and repeatability are required.
Typical applications include food and pharmaceutical areas. Several techniques are used to manufacture RTDs.
The most common RTD configuration is a length of platinum wire wound on a glass or ceramic bobbin; afterwards, the RTD is encapsulated in glass or other protective material. A second variety of RTD is constructed by depositing a conductive film on a non-conductive substrate; afterwards, it is encapsulated or coated to protect the film.
Main advantages of RTDs are:
- Linear
- High stability
- Wide operating temperature range
- Interchangeable over wide temperature range
Main disadvantages of RTDs are:
- Small change in resistance versus temperature
- Relatively slow time response
- Expensive
- Low resistance requires three or four wire measurement
- Sensitive to shock and vibration
- Current source required
Operating principle of resistive temperature detectors
RTDs are based on the principle that most metals show an increase in resistance with temperature. The following equation represents the variation of resistance with temperature:
RT = R0 (1 + a1T + a2T2 + … + anTn)
where:
- RT is the resistance at temperature T;
- R0 is the resistance at 0ºC;
- a1 … an are constants.
The number of terms to be considered depends on the material, the temperature range, and the accuracy required.
For limited temperature range applications (0 to 100ºC), the following approximation is, normally, adequate (which means that the relation is considered linear):
RT = R0 (1 + aT)
Tungsten, platinum, nickel, rhodium, and copper are, historically, the most common materials. Originally, copper wire was used in resistive temperature detectors, but due to its low resistivity, a (too) long wire is needed to wind a transducer.
Also, tungsten is fragile and nickel has a non-linear response. On the other hand, platinum is less susceptible to contamination and is highly resistant. Therefore, most general-purpose RTDs are made of platinum wire. At temperature below 20 K, rhodium, which exhibits a higher sensitivity than platinum, is used.
The resistance of platinum RTDs varies from tens of ohms to several thousands of ohms, but most common value is 100O at 0ºC. Depending on the purity of platinum used, the temperature coefficient can be 0.00385O/O/ºC (the European curve) to 0.00392O/O/ºC (the American curve).
This means that, typically, a 100O RTD produces a resistance change of only 0.385O/ºC. In this sense, the resistance of the leads connecting the RTD to the acquisition system is of major importance; for example, 1O is equivalent to a temperature error of about 2.5ºC !
Historically, RTDs have been implemented either as part of a Wheatstone bridge circuit or in a four-wire configuration.
Figure 2.3 Bridge with two-wire RTD
Figure 2.3 presents a basic configuration based on a Wheatstone bridge, where it is necessary to use some additional resistive components and the resistance of the wire leads from the bridge to the RTD must also be taken into consideration. The resistances of the leads are referred by RL in the figure.
On the other hand, Figure 2.4 shows a refinement of the basic configuration, where a three-wire RTD is used, allowing usage of a separate voltage sensing lead from the RTD to the voltmeter, in order to minimize the effects of lead resistance.
Finally, in Figure 2.5 a four-wire RTD is used, providing two separate leads to connect to the voltmeter, and relying on a current source to excite the bridge.
So, RTDs are available in two-, three-, or four-wire devices to support the presented configurations.
Figure 2.4 Bridge with three-wire RTD
Figure 2.5 Four-wire RTD