Electricity and Control March 2016

TEMPERATURE MEASUREMENT

Evolution of temperature measurement

Steve Edwards, R&C Instrumentation

V arious techniques and devices have been used throughout time in an effort to accurately measure and compare temperature conditions. For example: • Fire and ice, hot and cold – elemental extremes • Boiling water is a fixed temperature as is melting ice In the early days of ceramics manufacture the craftsman used melt- able materials which indicated through deformation that certain higher temperatures were reached. A baker on the other hand, used a piece of paper – the quicker it became brown in the oven, the hot- ter the oven was. It was known that specific materials changed state at set tempera- tures and this is what they based their temperature measurement on. The disadvantage of all these techniques was that they were not reversible – cooling could not be determined. Also, the accuracy of the results was very dependent on the user and his or her experience. It was not until the discovery of thermometers, a little over 400 years ago, that actual temperature conditions could be measured exactly. It was determined that a probe made of two dif- ferent conductors forming a junction at one end had

offerings is vast. A variety of accessories and software for the collec- tion and analysis of measurement data are provided with the majority of infrared temperature sensors. An IR thermometer can be compared to the human eye. The lens of the eye represents the optics through which the radiation (flow of photons) from the object reaches the photosensitive layer (retina) via the atmosphere. This is converted into a signal that is sent to the brain. Simple single point Infrared temperature measurement has evolved into units that now display Thermal images, used in appli- cations that monitor on-line real time molten steel temperature to deciding if a batsman is out or not when a catch is taken from the ball hitting his pads. The advantages of IR thermometers are: • Very fast measurements (milliseconds) • Can measure on moving objects

• Difficult to reach objects • Hot objects (> 2 000°C) • Feedback-free-measurement

Conclusion Advantages in single detectors and thermal imaging con- tinue to grow and Fluke Process Instruments (formally Raytek, Ircon and Datapaq) lead the field as the biggest supplier of IR temperature measurement devices.

certain voltage producing abilities. The thermocou- ple produces a temperature dependent voltage known as the thermoelectric effect. This voltage is proportional to the difference between the hot and cold junction. Commercial thermocouples are inexpensive and can be used in a wide range of temperature applications. Their main limitation is accuracy, they have to be in contact with the high temperature and system errors of less than 1°C are dif- ficult to obtain.

• Infrared radiation was discovered by Wilhelm Herschel at the beginning of the 19 th Century. • Early IR thermometers were heavy, awkward and complicated to operate. • Modern IR thermometers are small, ergonomic, easy to operate and can be installed in machinery.

The discovery of infrared radiation by the physicist, WilhelmHerschel, at the beginning of the 19 th Century opened up new possi- bilities for measuring temperature – without contact and thus without affecting the object beingmeasured and themeasurement device itself. Compared to early infrared temperature measurement devices, which were heavy,

take note

Born and educated in Yorkshire, England, Steve Edwards has been involved in rotating equipment monitoring for many years. He joined AECI (South Africa) in the consulting engineering department… and later started R&C Instrumenta- tion, providing industries with industrial instrumentation and consulting ser- vices and applications design in Infrared temperature monitoring and scanning. Enquiries: Tel. 032 946 2805 or email stevee@randci.co.za

awkward, and complicated to operate, the image of such devices today has completely changed. Modern infrared thermometers are small, ergonomic, easy to operate, and can even be installed into machinery. From versatile handheld devices to special sensors for integration into existing process systems, the spectrum of product

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