Also in this series:
RTD (Resistive Temperature Device) technology and the quality of its measurement are constantly improving. To obtain an accurate and high quality temperature measurement system, the choice of RTD element is crucial.
The RTD is a resistive element constructed from metals, such as platinum, nickel or copper. The metals that are chosen have a predictable variation of their resistance with temperature. In addition, they must possess physical properties that allow an easy assembly and a temperature coefficient of resistance big enough to make measurable changes in temperature.
Other temperature sensing devices, such as thermocouples, fall short of giving the designer a result sufficiently linear with temperature. So, the linear relationship between resistance and temperature of RTD simplifies the implementation of signal conditioning circuits.
Changes in resistance with temperature for each type of RTD is shown in Figure 1. As you can see, the platinum RTD (PRTD) is the most precise and reliable of the three types mentioned.
Of all types of materials, platinum RTDs are best suited for high precision applications where the absolute accuracy and repeatability are critical.
The platinum material is also less susceptible to environmental contamination, while copper is subject to corrosion causing problems for long-term stability. Finally, nickel RTDs have a good tolerance to environmental conditions, but, however, they are limited to more restricted temperature ranges.
The PRTD has nearly linear thermal response, good chemical inertia and is easy to produce in the form of small diameter wires or films. As shown in Figure 1, the resistivity of platinum is superior to other metals, thus making smallest the physical size of the element.
Thermal responsiveness of an RTD influences the measurement time. It also depends on the material of the package and the overall size of the element. Small items can be housed in smaller packages. Also, since RTDs are typically smaller, their thermal response time may be shorter than the Silicon temperature sensors. The value of 0° C of the element is available in a wide range of resistances and can be specified by the user. For example, the standard resistance of PRTD is 100Ω, but there are also available as 50, 100, 200, 500, 1000 or 2000Ω elements.
As mentioned earlier, the RTD are absolute temperature sensing devices, in contrast to the thermocouple, which detects the relative temperatures. Therefore, the use of additional temperature sensors would not necessarily improve the accuracy of the system.
In most applications the linearization is unnecessary. Figure 2 shows the temperature in relation to the resistance for a 100Ω platinum RTD.
With such a PRTD, a temperature change from 0° C to 100° C changes the resistance of:
The accuracy of the PRTD in temperature range is also shown in terms of changes in Δ°C from the ideal.
Of the temperature sensors discussed in this series, the RTD is the most linear with only two coefficients in the linearization equation, for temperatures 0 °C to 859 °C:
and three coefficients for temperatures -200°C to 0°C:
where is the resistance of the RTD at measurement temperature, t is the temperature being measured, is the magnitude of the RTD at 0°C. A,B and C are calibration coefficients derived from experimentation.
These equations can be solved with five iterations making it possible to resolve to ±0.001°C of accuracy.
RTD error analysis
In addition to the initial errors indicated in Figure 2, there are other causes of errors that affect the overall accuracy of the temperature sensor.
Mechanical faults, as bending the wires, shocks, constriction of the element during the thermal expansion and vibration, can have a long-term effect on the repeatability of the sensor.
On the other hand, if mechanical stress can harm their long-term stability, even the electric design used for conditioning, raising the gain and digitize the RTDs output can affect their overall accuracy.
A source of error of this kind is the self heating of the RTD element as a result from the required current excitation. A current excitation is needed to convert the resistance of RTD into a voltage.
It would be desirable to have a high current circulating through the resistive sensor in order to maintain the output voltage above the system noise levels. However, the downside of this approach is that the element will self-heat as a result of the higher current.
The current, in fact, going through the resistive element will generate heat and the heat generated by the power dissipation of the element artificially increases the resistance of the RTD.
The error contribution generated by power dissipation can be calculated given the thermal resistance of the package (), the amplitude of the excitation current and the value of the RTD resistance (RRTD).
For example, if the is 50°C/W, the RTD’s nominal resistance is 250Ω, and the excitation current amplitude is 5mA, the artificial increase in temperature (Δ °C) as a result of self heating is:
This example shows the importance of keeping the magnitude of current excitation as low as possible, preferably less than 1mA.
A second source of error resulting from the electrical design comes from the lead wires to and from the sensing element. The way to connect the RTD to the rest of the circuit can be a critical issue.There are three possible wire configurations when connecting the element to the remainder of the circuit. The 2-wire configuration, in Figure 3a, is by far the least expensive but the excitation current flows through the wires as well as the resistive element. This way, a portion of the wires is exposed to the same temperatures as the RTD.
The effects of the fact that wire resistance change with temperature can become a critical issue. For example, if the lead wire is constructed of 5 gage copper leads that are 50 meters long (with a wire resistance of 1.028Ω/km), the contribution of both wires increases the RTD resistance by 0.1028Ω. This translates into a temperature measurement error of 0.26°C for a 100Ω @ 0°C RTD. This error contributes to the non-linearity of the overall measurement.
The least accurate of configurations shown in Figure 3 is the 2-wire. Circuits can be configured to effectively use the 3-wire and 4-wire configuration to remove the error contribution of the lead wires completely.