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The Frenchman Alexis-Marie de Rochon and the Italian Marsilio Landriani had noticed, in the second half of the eighteenth century, that not all the components of light, which Newton's prism had decomposed a hundred years earlier, carried the same amount of heat but it took almost another century before William Herschel provided a systematic explanation to the phenomenon. Herschel was looking for a way to observe the sun through a telescope, experimenting with filters - coloured glass - and when he moved the thermometer he was using to measure the heat of different regions of the visible spectrum, in the dark region beyond the red end of the spectrum, he discovered that the heat, instead of decreasing, continued to increase. 

The German musician who had emigrated to the England of George III and the self-taught astronomer, Herschel, to whom we owe the discovery of Uranus and, following his appointment as the royal astronomer, made a number of very important contributions in the description of the Milky Way; in fact he had just discovered infra-red (which he called "thermometric spectrum"). It might be assumed that thermography found its origins here. In reality, stressing the concept a little, the first uses of the heat emitted by a body - to understand how this was working - date back to ancient Egypt; already in 400 BC, doctors used to spread a thin layer of mud over a patient's body and would then observe the different drying rates.

From doctors in the shadow of the pyramids to the discovery of Hershel, not only were the measurement techniques refined, but also the diagnostic capacity, managing to relate the heat detected to the cause of the heat itself. The first half of the twentieth century saw noticeable improvements in infra-red imaging thanks to the introduction of infra-red-sensitive photographic films. During the Second World War and the Korean War infra-red was used for a variety of military applications, such as detecting troop movements. Companies such as Texas Instruments, Hughes Aircraft and Honeywell developed detectors for the US military, but these were extremely expensive. 

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The evolution of thermal imaging cameras

The pyroelectric tube in vidicon was developed in the 1970s by Philips and EEV and was used for the first time by the Royal Navy for fire-fighting on board ships. The vidicon was based on the variation of the electric resistance according to the light radiation that struck the sensor (a series of electrodes contained in a glass tube, under high vacuum). 

The evolution in this sector was in particular linked to sensors and substances that made them sensitive to heat. In 1978, Raytheon, then part of Texas Instruments, patented the iron-electric infra-red detectors that used barium strontium titanium or BST.

Honeywell later developed the vanadium oxide (VOx) microbolometric technology, patented in 1994. US federal programs have provided substantial funding to develop military thermal imaging technologies. The transit to the civil sector owes a great deal to the use of thermography in clinical medicine, and to the advent of computer science, which has made available the computing capacity necessary to allow the sector to take a qualitative leap. Bullard introduced his first specially designed fire-fighting thermo-camera in 1998.

The diffusion of thermography for maintenance purposes

At the beginning of the 2000s, the prices of infra-red cameras began to fall, allowing the thermographic survey to also become one of the most widespread non-destructive inspection systems in the civil sector in general and in the industrial one in particular. In those same years, industry standards began to emerge which concerned the technologies, diagnostic methods and the qualifications of the personnel involved in the use of these devices.

One of the fundamental advantages of thermography is that, unlike many other diagnostic systems that require stopping of the operation of the system to be investigated, and therefore of production in the context of industrial equipment, this allows non-invasive in-line controls during operation.

An abnormal increase in temperature is often a symptom of a malfunction or even of a near fault. A further advantage of thermography, compared to the normal non-predictive periodic maintenance, is that it is used to intervene on the components that present critical issues without needing to replace others, perhaps still in a good condition, only because they have passed a number of statistically critical cycles.

Thermographic survey procedure

Depending on the diagnosis needs, the thermographic survey can be passive, detecting the heat emitted by the apparatus and obtaining the appropriate deductions in terms of operating mode, or active, heating the target (through lamps, hot air or other) and detecting the ability to accumulate energy as a function of the specific heat and density of a material, the times of exposure and phase, accumulation or thermal release, deducing the state of structures hidden from view.

Correct thermography must take into due consideration the climatic and environmental conditions of the place being surveyed. Natural or artificial sources of heat, dissipation mechanisms that can alter the results but also the conditions of the object (whether they are contingent - wet surface - or structural characteristics of emissivity) are elements that an expert technician must be able to consider added to whether they should consider the need to compensate or eliminate.

The standardization of thermography

The training and qualification of the operator is regulated by the UNI EN ISO 9712:2012 Non-destructive tests - Qualification and certification of personnel involved in non-destructive testing, in the industrial field and includes three levels of skills: the execution of basic measures, the development of investigation programs and in-depth diagnostic skills.

Applications of thermography to fire protection systems

With regard to a fire prevention system the main components that can be verified by infra-red thermography are:

  • Motors
  • Pumps
  • Heat exchangers
  • Flexi joints and drives
  • Bearings
  • Electrical connection points
  • Electrical equipment inside panels

With the thermographic scan it is possible to identify slight variations in temperature and, from these, to deduce the causes, such as:

  • Ineffective cooling due to poor flow of the cooling system
  • Problems on the insulation of electric motor windings
  • Misalignment of drive shafts or joints
  • Excessive bearing wear, misalignment or inadequate lubrication
  • Problems related to the status of electrical connections
  • Imbalance or overload of the power supply

 

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