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Thermodynamics in Thermography, The Scientists

Updated on January 28, 2015

Equipment Images using a Radiometer


The Four Laws of Thermodynamics

Here are the four laws of thermodynamics that all of science is based on with the exception of Quantum Mechanics, which is based on an error Einstein and other made. (They are just starting to figure out that the rejection of aether was a mistake, and that dark matter and energy are the same thing. They have not yet figured out that light uses these as a medium. Once they do this, quantum mechanics will need to painfully merge with a newer model based on Lorentz. But I digress…a lot…)

The Zeroth Law states that if two bodies are both in thermal equilibrium with a third body, they are necessarily in equilibrium with each other.

The First Law states that the total energy in a system is conserved, that is, not destroyed. While this seems obvious, the caloric theory held that once heat was lost, it was actually destroyed until Joule clarified this.

The Second Law explains what happens to energy not used in the system, it is simply disorganized or becomes less useful, so this law is that entropy always increases.

Lastly, the Third Law states that entropy approaches a constant, that is, zero entropy, as temperature approaches absolute zero.

The question is, how did we get to understand these laws?

Gustav Kirchhoff

It is important to a thermographer to understand laws of thermodynamics, how and why they are applicable to correct inspection and analysis of images generated by a thermal imaging device, or radiometer. So, I have set out to do this with a series of articles discussing these important and early scientists. Of these, Gustav Kirchhoff provided a fundamental understanding of black-body radiation critical to thermography and the understanding of how objects radiate thermal energy. More importantly, he defined the difference between a theoretical black-body which absorbs all energy incident to its surface, but also radiates all thermal energy conducted to its surface. However, to all real objects the additive combination of the emissivity and the surface reflectivity at any temperature and at any wavelength will be the black-body perfect number of one. How does this affect a thermographer?

Of great importance in thermography is Gustav Kirchhoff. He described in 1862 black-body radiative formulae and state the emissivity at any given temperature, that is, the percentage of energy emitted from the surface at any given wavelength, will be the same as its absorptivity. In a black-body, the emissivity is one and the reflectivity is zero, thus the surface passes all energy that reaches it. To restate this, all the energy that strikes the surface by conducting internally, emits from the surface and all the radiation that is radiated to the surface absorbs. He said in 1860:

“For a body of any arbitrary material, emitting and absorbing thermal electromagnetic radiation at every wavelength in thermodynamic equilibrium, the ratio of its emissive power to its dimensionless coefficient of absorption is equal to a universal function only of radiative wavelength and temperature, the perfect black-body emissive power.”

The thermal interaction at the surface is identical to both electromagnetic radiation of a given wavelength incident on the surface and also toward thermal energy conducted to the surface and emitted. This allows us to know their inverse properties by measuring light reflective from the surface and deduce the reciprocal and complimentary functions.

Let’s assume at a given wavelength a body or mass reflects 80% of the energy incident on, or which strikes the surface, this means of course 20% is absorbed. Kirchhoff tells us that if this is so, then at that specific temperature and wavelength, that mass retains 80% of the energy that reaches its surface, or restated, there is 80% resistance to the thermal energy radiating from the surface just as there is resistance at the surface to absorbing the radiation, thus the 80% reflectivity.

This is the reason a thermal imaging device has both emissivity and background radiation settings, so the imager and analysis software know what percent of the IR radiation it detects is emitted from the object and how that needs to be adjusted to determine a correct temperature and then what amount of radiation to subtract from the image because it is a reflection of background infrared energy. How does it do this? It considers the background radiation you have added to the data and multiplies this times its reflectivity and subtracts the appropriate value.

The question remains, what is the background radiation you should use? You should use the radiation from the objects being reflected from the device that is being imaged. This is determined on a flat surface by calculating the angle of reflection, when a light ray strikes a reflective surface, it will reflect at an angle equal to the angle it strikes the surface. The angle of incidence equals the angle of reflection. That is the source of the background radiation for a thermal imaging device.

To take the correct measurement of this radiation, set your imager to an emissivity of one, and enter that temperature, then enter the emissivity of the device you are measuring before taking the reading.

William Thompson, Lord Kelvin

Most people know the Kelvin as the temperature range that starts at absolute zero, was named for Lord Kelvin. Though he was not the first to develop an absolute range did measure absolute zero more accurately than say, Carnot, both are based on a centigrade scale, that is one in which there is 100 gradients between the freezing point of pure water at sea level and the boiling point.

Thompson had taken a professorship at the age of 22 and at 23 sat through James Joule’s lecture at the British Association for the Advancement of Science. Joule attacked the caloric theory of heat trying again to establish his correct theory that heat is largely a mechanical force and the two are interchangeable. History informs us that Thomas Newcomen and John Calley had patented a steam engine used to pump water from mines in 1708, and that they had engines that had been operational for more than 130 years at that time.

The dominant theory of heat at the time was the Caloric Theory which held that heat was a self-repellant gaseous fluid, called caloric, and so pushed itself away from a hot object toward a cooler object through very small pores, however, the heat lost was actually destroyed. Joule objected claiming only the creator could create or destroy energy, thus the First Law of Thermodynamics is credited to him, and Thompson eventually stating that the energy was lost to the use of man, but not lost to the atmosphere.

Thompson started to doubt the caloric theory after listening to Joule, conducted several of his own experiments and by 1851 was convinced Joule had the correct theory of thermal energy, the kinetic theory. They worked together on numerous experiments bolstering Joules kinetic theory into prominence.

Energy always flows down a thermodynamic gradient and entropy, or disorganization always increases, so Thompson, who was a devout Christian, disputed that the sun’s energy could last long enough to accommodate Darwin’s uniformitarian evolution theory.

Thompson also believed the luminiferous aether theory that there was a medium through which electromagnetic waves pass. Einstein disagreed based on his naturalistic theories, and produced similar mathematic results without aether, and so the theory was grossly disrespected. Before Einstein died he realized his general theory missed 96% of the mass of the universe.

Interestingly enough, this theory is gaining momentum again but renamed dark energy and if proved true, which is looking more and more likely, quantum mechanics will eventually fuse into it as explanations of physical interactions between observable and unobservable phenomena. As I explained to a physicist friend recently, quantum mechanics is only there to explain why waves exist in the absence of a medium. Find the medium and the theories change significantly.

To the thermographer, Kelvin created the absolute temperature used in every thermal imaging radiometer to calculate temperature differences used to create the image projected on the screen.

Kelvin significantly helped our understanding of thermal energy.

James Joule

We remember James Joule’s contribution to thermodynamics in unit of energy a Joule. However, we forget he was the person most responsible for correcting the caloric theory of Lavoisier, challenged by Joule with a simple statement: “Believing that the power to destroy belongs to the Creator alone I affirm . . . that any theory which, when carried out, demands the annihilation of force, is necessarily erroneous.”

This represents the first clear definition of the First Law of Thermodynamics, energy is not now being created or destroyed. Since this dealt with the energy of coal used to heat water in a boiler, indicating the thermal energy was lost to the environment as opposed to being used for the general purpose of heating water, inferred the Second Law of Thermodynamics as well.

It was Joule that understood heat as a form of work energy or mechanical energy, which, with the correct process, one could convert one into the other, in other words, a heat engine was possible, and other explanations of this energy had major faults. Joule converted Lord Kelvin to his way of thinking and won the day with a correct understanding of thermal energy.

However, as suggested in other articles I have written, in thermography we need equipment to operate ideally at full power, but at least at fifty percent power. However, when I saw a very small increase in temperature on a large S-Band radar array main power feed, I prioritized it as a problem needing immediate repair because of Joule’s 1841 expression that the power is equal to the current squared times the resistance. Since the current flow is 50% the heat generated from a problem is four times that if the same system at 25% or 1/4 that if it were at full power.

In the above mentioned piece of equipment, having it fail when it is most needed could produce dire consequences for the United States. The power could not be increased beyond 20% while I was present, but I understood Joule’s law, now built into Ohm’s Law, and so applied a much higher repair priority.

Work done by Joule and Kelvin in thermal changes in compressing gases when forced through small apertures received broad acclaim. It was during the experiments in the first of these inquiries that led Joule to appreciate the value of surface condensation which increased the efficiency of the Thomas Newcomen 1708 steam engine. A new form of condenser was tested on the small engine that was later described in an memoir presented to the Royal Society in 1860. Joule’s results, according to Kelvin, led directly and speedily to the present practical method of surface condensation, one of the most important improvements of the steam engine. This was critical in the development of steam engine used for steam ships, which lead to condensing water for use on the ships, which, of course, were the earliest of desalination units, still used in many ships.

Thomas Newcomen and James Watt

In 1712 Thomas Newcomen installed the first practical steam engines, which he called atmospheric engines, to pump water from coal mines. He was not at all the first to attempt this but was the first to succeed at it, and his individual engines were known to work for more than a century.

These were very inefficient but used both coal and water from the mine so no one much cared, they worked! Efficiency increases with their size, and so they tended to make larger and larger engines.

One of the things we Scots do is take other inventions and quite successfully improve them, and so James Watt did just that. Watt was tasked with repairing a Newcomen steam engine operated by Glasgow University he worked at and when he did, he realized the machine wasted about 75% of its energy.

The Newcomen design used the main cylinder both to expand with steam and then to create a vacuum by condensing the steam in the same cylinder wasting all the heat used to heat the cylinder in every cycle. Watt conserved much of this energy by the use of a steam jacket to maintain the heat in the cylinder and extracting the steam to a separate chamber for condensation.

This design allowed for a lot of innovation in the future, including smaller engines which used less coal, and less water, enabling the entire industrial revolution.

Newcomen utilized principles of thermodynamics and engineering that would later be described, but Watt significantly improved these by simply insulating the cylinder and then isolating the condensation.

Twenty eight years ago we were working on diesel powered aircraft carriers both spotting electrical issues, but primarily to resolve heat loss issues. We would identify areas where insulation, or it improvement was needed then calculate the heat loss and savings if the insulation was added, changed, or corrected. The ships wasted heat by it radiating thermal energy from the boilers and pipes, which necessitated its removal in order to make a better working environment, so they paid for the fuel, and then to have the resulting heat removed using large fans to vent the heat into the atmosphere.

This is an interesting parallel to me, with the last name of Newcomb and a Watt as a grandfather.

The US Navy learned the lesson and now pays much more attention to this issue and in the same effort reduces energy loss.

Most thermographic inspections of ships today focus on electrical issues since few ships use much steam today; however, the savings from these inspections have shifted from fuel savings to saving equipment from premature failure.

Have the savings diminished? Not at all, a qualified marine thermographer may inspect 500-900 devices on a ship, reducing maintenance expenses and premature component failures, and prevent several fires each year, and a small fire in the Motor Control Center can cost between one half to one million dollars in damage when all costs are considered returning the investment many times over.


Max Planck

Max Planck was a very important German physicist for thermodynamics and thermography. In 1894 he studied black-body radiation in his work relating to efficiency of electric light bulbs. Planck moved the discussion from experimentation to theory.

Wilhelm Wien proposed his law, and was able with it to predict the output of bodies, in this case metals such as tungsten used in bulbs at high frequencies but not at low frequencies.

This was a difficult problem and took some years to resolve but in 1900 Planck developed the famous Planck black-body radiation law, which added to Kirchhoff’s works, and relied heavily on Boltzmann’s work in thermodynamics and well described the empirical observations of the black-body spectrum. Thermal energy being radiated as Kirchhoff described was in fact part of the increase in Rudolf Clausius’ entropy distributed as Boltzmann described.

In essence, Planck describes a black-body at a constant temperature and the radiation this yields. However, the equations yield lines called Planck’s Curves when plotted out, show why an infrared imaging device can detect very small differences in temperature.

At any given temperature a body can be described as giving off infrared radiation of differing wavelengths. The lower the thermal energy, the longer the wavelengths, and so the higher the thermal energy, the shorter the wavelengths. Visible light is very short ranging between 380 and 740 nanometers, however, the longest IR radiation is up to one millimeter in length, and light in the ultra violet range reaches to the 40nm range.

Plank’s law tells us how much of which frequencies a body gives off. The more thermal energy a body has, the more luminance, and the shorter the peak wavelength given off by the body, the sun, somewhere between 6000-7700 Kelvin gives off its peak radiation in the visible range, how convenient.

But, most equipment thermographers are examining are not giving off visible light, although I have seen this, and this means the temperature of the connector on an aircraft elevator exceeded 900F. We had it shut down immediately. Rarely, however does equipment reach the 300F range without causing significant problems or if left alone, fires.

But I digress, back to Planck.

Since the number of photons emitted changes dramatically by Planck’s Law and increases dramatically with even a one degree rise in temperature imaging devices can easily discriminate between two small areas. By way of example, the difference in luminance per square meter of surface between 30C and 31C is 113,100,548 photons per second according to the Stephan-Boltzmann law, based on Planck’s work.

The larger effect on thermography is in understanding the thermal range of the camera needed to inspect a given piece of equipment, whether near infrared, short-wave, mid-wave, or the more commonly used long-wave cameras according to the thermal range being inspected.

Thermography is the best of the predictive and preventative maintenance tools available, and owes much to the work of Max Planck.

Rudolf Clausius

Rudolf Clausius’ 1850 paper in German, whose translated title is “On the Moving Force of Heat and the Laws of Heat which may be Deduced Therefrom,” laid out the kinetic theory of thermal energy later rigorously defended by Joule, then Lord Kelvin.

He pointed out that Carnot’s theory contained contradictions to the principal of the conservation of energy, what we now call the First Law of Thermodynamics, and restated this along with entropy as contradicting Carnot’s support of the Caloric Theory of heat. His statement of the second law was reasonably close to the actual case when he stated “Heat can never pass from a colder to a warmer body without some other change, connected therewith, occurring at the same time.”

Exact terms are sometimes glossed over for simplification. While it is true that heat always flows down a thermodynamic gradient, I have heard this wrongly expressed as infrared radiation only passes from a hotter body to a cooler body, but this is incorrect. Temperature, that is, a measurement of the heat of two bodies will indicate thermal energy transfers from the warmer body to the cooler body, but radiation itself is constantly exchanged between two bodies of mass, however, more of it transfers from the higher mass into the lower mass cooling the first and warming the second. Clausius understood this and stated:

“The energy of the universe is a constant. The entropy of the universe tends to a maximum.”

These are perfectly adequate statements of the First and Second Law of Thermodynamics, respectively. The laws developed over time, as the principles came to be understood, and so multiple people are credited with each law as we look back at their work and see what they wrote and how they stated each principle.

Where do we see this in thermography? If the energy of the universe were not currently constant, say, energy was being created randomly, then we would not understand the a hot electrical connection was caused by, say, a dirty connection translating electrical resistance into heat, we may think it is merely the point energy is created.

Joule used this when he stated that the energy from coal which was not used to increase the heat on a boiler was not destroyed, as Carnot thought, rather simply lost its organization, that is, entropy increased, the total energy remains a constant, therefore we need to identify where heat is coming from and why it is being generated.

This forms the basis for everything a thermographer does and thinks about when he sees a hot spot on his or her imager. You need to know where the heat is coming from and why it is there, how it arrived at the surface you are looking at and what the actual temperature might be inside the device based on these principles and others we will discuss in other articles.

Robert Boyle

Robert Boyle, the 17th-century natural philosopher, lay theologian, and chemist, is widely regarded as the first modern chemist, moving science away from alchemy toward a more rational view of chemical interactions as the scientific method was developing. He is most famous for Boyle’s law.

Boyle's law informs us of the inversely proportional relationship between the absolute pressure and volume of a gas, assuming constant temperature in a closed system. That is, if the temperature of a system does not change, and pressure is reduced, there will be a corresponding increase in the gas volume within. Conversely, if you increase the pressure, the volume will decrease.

This is non-linear; other factors are known to be in play including the compressibility effect, specific heat of the gas or gas mixture, van der Walls forces, and so forth. This is a very complex issue and there are at least ten different models trying to adequately explain this.

The manner this applies to thermographers is in observing effects with increased heat. This is mostly seen in relief valves in refineries or other manufacturing when increased temperature increases internal pressure.

Engineers are of course well aware of this, but when a thermographer sees a steam trap where most of the heat is on one side and very little is on the other, he can see the valve is plugged and needs maintenance.

Last year, I was inspecting a ship and found an issue with the steam distribution where the insulation was missing at certain locations. This caused the steam to lose energy condense into water which then reheated as steam flow increased and then flashed back into steam causing what is referred to as a steam or water hammer, a loud popping sound. The recommendation was, of course, to insulate those locations to prevent the sudden increase in pressure from damaging the pipes and valves.

Without the change in temperature, there would have been relatively constant pressure. Other laws apply of course, but Boyle’s Law explains the decrease pressure caused by a steam pipe cooling. Steam hammer or water hammers cause significant damage. The forces which cause the noise are real and need to be addressed.

I found the issue by simply listening; the thermal imager helped me define the problem.


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