Technology review
Martin Keenan
Technical Director Avnet Abacus
Almost everything we know, whether it is organic or inorganic, is to some extent sensitive to temperature. Our solar system supports life purely because Earth is at the right distance from the burning star at its centre. As a form of energy, heat manifests itself as the movement of atoms and molecules which, if left unchecked, can result in destruction. Primitive man learnt to harness and ultimately control this energy, making it work for him. Mastering that level of control has enabled us to forge tools, build factories and develop highly complex integrated circuits. The importance of temperature is apparent in all things, which may explain why it remains one of the most closely monitored of all parameters.
The Evolution of Sensing
An important part of any control loop is the feedback path, and this is where temperature sensors excel. All sensors are effectively a way of changing one kind of energy into another, and achieving that requires a medium that reacts to both.
Materials including ceramics, polymers and alloys react in useful and predictable ways to changes in temperature. Conductive materials are particularly exploitable and are used to make devices that exhibit either gradual or sudden changes in their resistance,
in either a linear or non-linear way that correspond with changes in temperature.
Researchers are constantly discovering and developing ways to use new or existing materials for measuring temperature. For example, a company in Cambridge, UK, discovered that eddy currents, which are induced in a substrate by passing an AC signal through a nearby coil, exhibit parameters that are temperature-dependent. One breakthrough in technology has led to a new form of non-contact temperature sensing that can be used at extremely high temperatures.
Other research has shown that polymer-derived ceramics, or PDCs, are capable of measuring temperatures in the region of 830°C with excellent repeatability and accuracy, while silicon carbide based sensors could be useful up to temperatures of around 500°C. In another example, researchers at the California Institute of Technology found that mixing carbon nanotubes with pectin – a naturally occurring sugar molecule – produced a new material that displays measurable changes in resistance with variations in temperature in the range of 5°C to 40°C. These are just some examples of how the ways we measure temperature remains an active area of research.
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