Through the ages, various techniques and devices have been used in an effort to measure and compare temperature conditions: for example, the reference points of boiling water and melting ice. In the early days of ceramics manufacture, the craftsman used materials which indicated through deformation that certain (higher) temperatures had been reached. Bakers on the other hand used a piece of paper, the quicker it became brown the hotter the oven.
It was known that specific materials changed state at set temperatures and this formed the basis of temperature measurement. The disadvantage of these techniques was that they were not reversible. In addition, the accuracy of the results was very dependent on the user and their experience.
Thermocouples make things easier
It was not until the discovery of thermometers, a little over 400 years ago, that actual temperature conditions could accurately be measured. It was determined that a probe made of two different conductors forming a junction at one end had certain temperature dependent voltage producing properties – the thermoelectric effect. This voltage is proportional to the temperature difference between the hot and cold junctions. 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 difficult to obtain.
The emergence of infrared
The discovery of infrared radiation by the physicist Wilhelm Herschel at the beginning of the 19th century opened up new possibilities for measuring temperature – without contact and thus without affecting the object being measured and the measurement device itself.
Compared to early infrared temperature measurement devices, which were heavy, 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 offerings is vast, with a variety of accessories and software for the collection and analysis of measurement data provided by the majority of suppliers.
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 photo-sensitive layer (retina) via the atmosphere. This is converted into a signal that is sent to the brain. Simple single point IR temperature measurement has evolved into units that now display thermal images used in applications that monitor on-line real-time molten steel temperature, to hot-spot technology that helps umpires decide if a batsman is out or not after a catch has been taken.
The advantages of IR thermometers include:
• Fast measurements (milliseconds).
• Measurement on moving and difficult to reach objects.
• Hot objects (> 2000°C).
• Feedback-free measurement.
Advantages in single detectors and thermal imaging continue to grow and Fluke Process Instruments (formally Raytek, Ircon and Datapaq) remains a market leader in IR temperature measurement devices.
For more information contact R&C Instrumentation, 086 111 4217, [email protected], www.randci.co.za
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