Resistance Temperature Detector (RTDs)

In 1821 Sir Humphrey David discovered that the resistance of some metals changed when heated. This was approximately the same time that Thomas Seebeck discovered that voltage is produced when two different metals are joined to form a junction and the junction is heated. Since the amount of change in resistance was small and not uniform, Sir Humphrey David's discovery was not evaluated completely until 1871 when Sir William Siemens decided to use platinum as the metal. He found that platinum has a positive coefficient of resistance when heated. This means that the resistance of platinum increased as its temperature increased. He also found that this change was rather linear. In 1932 C. H. Meyers developed platinum into the sensor that we now know as the resistance temperature detector (RTD). Early versions of this sensor used a platinum wire that was wrapped around a ceramic stem. Today platinum wire is wound around a glass stem or ceramic bobbin to make the RTD. After the wire is wound onto the element, the entire sensor is sealed in glass in the shape of a bulb so that it's durable and can withstand higher temperatures. The glass bulb can be used as an exposed sensor or it can be shaped so that it will easily fit into a metal sleeve to make a probe. ill. 1 shows an example of a typical RTD, which is also called a resistive temperature detector.

Above: ill. 1 Example of a general-purpose resistance temperature detector (RTD).

The latest RTDs use a platinum or metal glass slurry to create a thin film that is mounted on a flat ceramic substrate. The thin-film detector (TFD) can be manufactured to be used as a small glass bead or it can be sealed in a metal element or in plastic such as Teflon. The main feature of these RTDs is that they are miniature in size, which makes them useful in temperature measurement applications where there is not a lot of room for a full-size sensor. ill. 2 shows several examples of these smaller RTD sensors. It should be noted that since the material for the RTD is wound on a stem, the entire shaft of the RTD is temperature sensitive, whereas only the tip of a thermocouple is sensitive.

Above: ill. 2 Examples of miniature RTD sensors made from thin film technology. Notice the relative size of the RTD sensor as compared to a standard pencil.