“Assuming that the temperature T is an intensive quantity, i.e., a single-valued, continuous and differentiable function of three-dimensional space (often called a scalar field), i.e., that:
Where x, y and z are the coordinates of the location of interest, then the temperature gradient is the vector quantity defined as:
…” 1
Temperature changes with increasing distance from the source.
Thermal gradients are an inherent part of thermography. Seldom, if ever, are you viewing the source of heat caused by a fault. Every thermographer needs to have a clear understanding of thermal gradients and how they can lead to a false criticality prediction. At the same time, the thermographer must also understand thermal gradients can be a beacon showing the way to the fault.
There have been many discussions about the pitfalls of thermal gradients and how they can lead an infrared technician to fear them. I have a different opinion of the dreaded thermal gradient.
I have always tried to live an optimistic lifestyle. I try to accept those things I cannot change and convert them into productive assets. Contrary to what some people may lead you to think, thermal gradients are anything but a negative in an IR inspection. If the heat of a fault never left the fault area we, as thermographers, would seldom find any problems using an infrared camera.
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| Image 1 |
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| Image 2 |
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Image 3 |
My thoughts here are geared (pun intended!) to mechanical infrared inspections. Mechanical anomalies can have huge thermal gradients. In image 1 you can see a pattern across the surface of a conveyor belt from a fault in image 2 that was close to one half mile away. There are many cases where the heat detected by the thermographer is found far from the heat source.
Underground steam leaks can be located on the surface of the ground even though the leak is many feet below the surface. Faults that are hydraulic in nature can be detected hundreds of feet from the source (such as faulty, pumps, valves, heat exchangers or leaks).
Heat exchangers can be examples of a large thermal gradient. Heat exchangers often can be hundreds of feet from the machine or process for it is regulating. A heat exchanger that is not operating at normal temperatures isn't always the fault of the exchanger. Improper exchanger temperatures can be a result of some sort of failure in the machine or process it is trying to regulate.
Faulty lubrication systems can be found by inspecting the plumbing for abnormal temperatures. Cool piping can lead you back to a stuck valve or faulty pump. Abnormal lubrication temperatures on the return side of the lubrication plumping can be caused by a failure in the system that is being lubricated.
In many cases you will find direct inspection of a component isn't possible and you will need to rely on thermal gradients to find a problem. In image 3 the shaft is hotter near the coupling and getting cooler towards the motor. So, even though the coupling cannot be inspected directly, a coupling problem can be detected using an abnormal thermal gradient. The proper heat signature would be the shaft warmer nearest the motor and getting cooler towards the coupling.
So I say, “Long live thermal gradients!” Without them, we would all be out-of-work thermographers.
References:
1 .) Temperature gradient. (2012, November 18). In Wikipedia, The Free Encyclopedia. Retrieved 17:55, November 27, 2012, from http://en.wikipedia.org/w/index.php?title=Temperature_gradient&oldid=523708580