RTDs or Resistance Temperature Detectors are the simplest way to measure temperature. RTD sensors work on the principle that a metal’s electrical resistance changes with temperature. For instance, the electrical resistance of pure metals typically increases with an increase in temperature, that is, they exhibit a positive temperature coefficient. RTDs operate over a huge temperature range, starting from -200 °C, right up to +850 °C. They offer excellent long-term stability, high accuracy, and repeatability.
An RTD sensor, being a passive device, does not produce a signal by itself. An electronic circuit is necessary to send an excitation current through the sensor. This produces a voltage across the RTD, proportional to the excitation current and the resistance of the RTD. Further electronic circuitry amplifies the voltage across the RTD and delivers it to an analog-to-digital converter, whose output produces a digital output, a representative of the temperature of the RTD.
The electronics in an RTD circuit have some basic trade-offs. For instance, the excitation current must be small enough to prevent self-heating in the RTD element. Any excitation current produces Joule or I2R heating in the RTD. This self-heating effect can raise the sensor’s temperature to a value higher than that of the environment that the RTD is measuring. By keeping the excitation current low, it is possible to keep the self-heating low to a great extent. Moreover, the amount of self-heating also depends on the medium surrounding the RTD sensor, and how effectively it allows heat to accumulate. For instance, placing the RTD element in still air produces a more pronounced self-heating effect than immersing it in moving water.
System noise, offsets, and drift of different system parameters also affect the minimum detectable change in temperature. Therefore, the RTD voltage must be large enough to overcome them. As the excitation current must be low enough to prevent self-heating, it is necessary to use an RTD sensor with sufficiently large resistance, so that it will produce a relatively large voltage. Although it is necessary to use a large RTD resistance to reduce measurement errors, it is not advisable to arbitrarily increase the resistance. This is because a large RTD resistance leads to an increase in the response time.
Theoretically, any metal should work for constructing an RTD. In fact, Siemens used copper wire for constructing the first RTD in 1860. However, he soon discovered that by using platinum, he could produce RTDs that were more accurate over a wider temperature range.
Precision thermometry typically uses platinum RTDs as the temperature sensor. This is because platinum has a linear resistance-temperature relationship, higher repeatability, and a wider temperature range. Moreover, platinum does not react with most of the contaminant gases in the environment. However, the industry also uses two other materials for making RTDs: nickel and copper. Among the three metals, copper offers the highest linearity and the lowest cost. However, as copper has the highest conductivity of the three, it offers a lower resistance. A copper RTD, therefore, produces a relatively lower voltage and can be difficult to use for measuring small temperature changes.