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TEMPERATURE MEASUREMENT
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.zaV
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
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.
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,
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. 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.
Evolution
of
temperature
measurement
Steve Edwards, R&C Instrumentation
• 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.
Electricity+Control
March ‘16
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