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Temperature / thermal sensors

Measuring temperature - from Galileo Galilei to us

The human perception of temperature allows us to distinguish between warm and cold, but it is very imprecise and subjective. This is why, at the end of the 16th century, Galileo Galilei set about measuring temperatures. He experimented with the expansion of liquids under the influence of heat and cold and built the first instrument for measuring temperature. He is therefore accredited with being the inventor of the first thermometer. Many different instruments followed, all of which work according to the same principle, albeit without a standardised unit of scale and using different liquids.

The Fahrenheit scale

It was not until 1715 that the Gdansk glass blower Daniel Gabriel Fahrenheit created a mercury thermometer and defined a scale using three fixed points.

  • An ice, water and salt mixture served as the zero point. He used this to avoid negative temperatures (approx. -17 °C)
  • The freezing point of water is 32° Fahrenheit (0 °C)
  • The temperature of the human body is 100° Fahrenheit (approx. 37 °C)

These resulted in reproducible fixed points that could be used to create a standardised scale. The Fahrenheit scale is still used today, for example in the USA. Most countries use the scale devised by Anders Celsius. He defined the scale named after him in 1742 using two fixed points: the melting point of water, 0° Celsius, and the boiling point of water, 100° Celsius.

Kelvin and absolute zero

William Thomson, later to become Lord Kelvin, defined the temperature scale that bears his name in the 19th century based on absolute zero. This is -273.15 °C (0 K). At absolute zero, molecular movement ceases. It is the lowest temperature achievable. The second fixed point was defined in 1954 as the triple point of water, 273.16 K (0.01 °C). At the ‘triple point’, all three physical states – solid, liquid and gas – occur together. Further fixed points are defined using the solidification points of pure metals. One Kelvin corresponds to one degree Celsius.

1 K = 1 °C

Kelvin has been the legally defined SI unit for temperatures since 1968. Its fixed-point temperatures were defined in the IPTS-68 temperature scale. This was replaced in 1990 by the ITS-90 scale, which still applies today, since it allowed the fixed points to be defined with greater precision.

Seebeck and Siemens

In 1821, Thomas Johann Seebeck discovered a thermo-electric effect (Seebeck effect) in metal conductors. He used this to build the first thermocouple. 50 years later, Karl William Siemens unveiled a thermometer that used changes in resistance to measure temperature. The first resistance thermometer he built used the linear, temperature-dependent resistance of the platinum built into it (Pt100/Pt1000). It is on the basis of these discoveries that both resistance thermometers and thermocouples have continued to be improved and optimised over the years.

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