Thermographic inspection
What is a thermographic inspection: thermography explained, and the misreading that causes most of the trouble
A thermographic inspection uses an infrared camera to measure the thermal radiation leaving the surface of a target and convert it into a surface temperature map, so that a condition beneath or behind that surface can be inferred from the pattern it produces. Also called infrared thermography or an IR survey, and abbreviated IRT in the condition monitoring standards.
ISO 18434-1 puts the mechanism plainly: infrared thermography is based on measuring the distribution of radiant thermal energy emitted from a target surface and converting this to a map of radiation intensity differences, that is, a surface temperature map or thermogram (ISO 18434-1:2008).
Thermography does not see through walls
This is the single most useful thing to get straight, because a great deal of bad practice follows from getting it wrong. An infrared camera cannot see through a wall, a panel, a coating or a cover. Infrared radiation in the bands these cameras use is stopped at the surface. What the camera reads is the face of the object, and the fault beneath is inferred from how that face behaves thermally: a loose electrical connection heats the terminal in front of it, a missing insulation batt cools the plasterboard over it, a wet roof insulant holds heat and releases it more slowly than the dry insulant beside it.
Everything thermography claims about what is inside is an inference from that surface signature. Strong inference, often, but an inference.
How does the physics constrain the reading?
- Apparent temperature is not temperature. ISO 18434-1 defines apparent temperature as the uncompensated reading from the camera containing all radiation incident on the detector, regardless of its source. Reflections included.
- Emissivity governs the whole measurement. The standard is explicit that infrared thermography is limited because radiometric sensing is susceptible to unacceptable error when used on most low emissivity surfaces. Bare copper busbar, polished stainless and galvanised sheet are exactly those surfaces.
- Reflected apparent temperature must be measured and compensated for, not guessed. ISO 18434-1 carries a normative annex on field measurement of both reflected apparent temperature and emissivity.
- Attenuating media sit in the path: viewing windows, lenses, atmosphere, steam and dust all reduce what reaches the detector.
- Load and conditions must be right. An electrical fault under 10 percent load may be invisible. A building envelope survey needs a temperature differential across the fabric. A roof survey depends on solar gain during the day and radiative cooling at night.
Where thermographic inspection earns its place
- Electrical distribution: loose or corroded connections, imbalanced phases, overloaded conductors, all surveyed live and without contact.
- Rotating and mechanical plant: bearing condition, coupling misalignment, lubrication problems and friction losses, feeding predictive maintenance programmes.
- Building fabric: missing or displaced insulation, thermal bridging and air leakage paths across an envelope.
- Roofs and moisture: locating suspected wet insulation, which is where a damp survey and a thermal survey usually meet.
- Process equipment: refractory degradation, tank and vessel levels, steam trap performance, blocked or bypassing lines.
The common misuse
The recurring failure is treating a thermal anomaly as a diagnosis. It is a temperature difference on a surface, and temperature differences have many causes: variation in material thickness, a change in construction, a reflection off a nearby hot object, recent solar loading, an emissivity change where paint has worn through.
The standards say so themselves. ASTM C1153, the practice for locating wet insulation in roofing systems by infrared imaging, provides for verification of infrared data using invasive test methods, and states that it does not provide methods to determine the cause of moisture or its point of entry (ASTM C1153). The camera finds the area. A core sample or a moisture meter confirms it, and someone else works out where the water is getting in.
The second misuse is a competence problem. Anyone can hold a camera and produce a colourful image. Interpreting one requires knowing the equipment, the process and the physics, which is why ISO 18436-7 exists as the personnel qualification standard for thermography within the condition monitoring series.
Thermographic inspection: a practical example
A switchboard survey finds one terminal on an outgoing way reading 38 K above its neighbours at 70 percent load. The thermographer records load current, emissivity used, reflected apparent temperature and distance, and grades it against the site's severity criteria. It is not called a fault on the image alone. The panel is taken out of service, the connection is checked and found loose, and it is remade and re-surveyed under similar load to confirm the anomaly is gone. That last re-survey is the step most often skipped.
What it is commonly confused with
Thermography is not the same as spot pyrometry, which measures one point without spatial context. It is not non destructive testing in the volumetric sense, since it finds no flaws inside material, only their thermal shadow on the surface. And a thermal image on its own is not a report: without emissivity, reflected temperature, load, ambient conditions and distance recorded alongside it, the temperature figure printed on the image is not defensible.
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