Thermography: what it is and where to use it in industry

Thermography is measuring surface temperature at a distance, from the infrared radiation a body emits. Every body above absolute zero radiates; the hotter it is, the more it radiates. The camera captures that radiation and turns it into an image in which each pixel is a temperature.

Because almost every industrial problem turns into heat before it turns into downtime, thermography is one of the most cost-effective techniques in maintenance. This article explains the principle, what the camera sees and what it does not, where to apply it in the plant and when periodic inspection is not enough. For the specific case of panels, with numerical decision criteria, see thermography on electrical panels.

How the camera works

The sensor in most industrial cameras is an uncooled microbolometer, sensitive to long-wave infrared, roughly 8 to 14 micrometers: the band in which bodies near ambient temperature radiate the most, and in which the atmosphere is reasonably transparent.

Three numbers describe a camera:

• Detector resolution (320 by 240, 640 by 480 pixels). It determines how many pixels fall on the target. A small terminal seen from far away has no reliable temperature: the reading becomes an average with whatever is behind it.
• Field of view and distance to target. There is a maximum distance for each target size. Measuring from too far underestimates a small hot spot.
• Thermal sensitivity (NETD). The smallest temperature difference the camera can distinguish. Current cameras reach a few tens of millikelvin, more than enough for maintenance.

What the camera actually measures

It measures apparent temperature, not true temperature. Three effects come between the surface and the detector:

• Emissivity. How much the surface radiates compared with an ideal blackbody. Polished metal, bare copper and bare aluminum have low emissivity: they reflect a lot and look colder than they are. Matte paint, rubber, plastic and oxide are easy to measure. A field trick: stick a piece of matte electrical tape on the spot and measure on the tape.
• Reflected apparent temperature. What the target reflects from its surroundings enters the calculation, and it matters enough to have its own standardized method for measuring and compensating it. Lamps, the sun and the inspector’s own body become false hot spots.
• Atmosphere and obstacles. Distance, humidity, dust and steam attenuate the signal. And ordinary glass is opaque to long-wave infrared: you can only measure through a purpose-made infrared inspection window.

Hence the rule that is worth more than any table: compare the suspect with an identical neighbor, of the same material, in the same image, under the same load. The difference between them is reliable even when the absolute value is not. And the limit: thermography sees the surface. It does not see inside a closed component or through a steel door.

Where to use it in industry

Electrical panels, MCCs and busbars. The classic application: loose or oxidized connections, unbalanced phases, circuit breakers with worn contacts, undersized cables. Heat grows with the square of the current, so the inspection is only valid under load, with each phase’s current recorded alongside the image.

Motors, bearings, gearboxes and belts. A housing running hotter than the machine’s own normal pattern points to overload, blocked ventilation or a winding problem; at the bearing, friction and poor lubrication show up as heat before noise. On rotating machines thermography works alongside vibration: heat tells the friction story, vibration tells the mechanical one.

Thermal insulation, furnaces and refractory. A hot spot on the shell of a furnace, boiler or oven reveals cracked refractory or insulation saturated with moisture. Here thermography measures energy loss.

Steam: traps, valves and lines. A trap stuck open dumps steam; stuck closed, it floods the line. The image settles the question without touching anything.

Transformers, inverters and photovoltaic systems. An asymmetric pattern between phases, windings or modules; a heated cell or string points to a faulty diode, connection or module.

Who does it, and under what conditions

For machine condition monitoring, ISO 18434-1 sets out the general procedures for thermography, and good practice separates the responsibilities of the end user from those of the thermographer. Personnel qualification is covered by the ISO 18436 series.

Conditions that distort an inspection: load too low, direct sun on the target, a wet surface, strong wind in outdoor inspections (which cools the hot spot and hides it) and too great a distance. Record load, time, distance, emissivity and ambient temperature with each image, or the comparison with the next inspection will not hold up.

Safety is not a detail: panel inspection is done with the equipment energized, and opening the door means exposure to live parts. The person who opens it must be a qualified professional, with a risk assessment and suitable PPE, under your local electrical-safety rules.

Periodic inspection or fixed monitoring

A periodic inspection, carried out by a thermographer once or twice a year, finds whatever is already hot that day. It is cheap, covers the whole plant and is the right starting point for almost everyone. What it does not see is the evolution: a terminal 2 degrees above its neighbors in March and 8 degrees in June is telling a story, and it is that story that lets you schedule the repair.

Fixed monitoring solves this where it pays off: a camera aimed at the panel, the busbar or the bearing records the pattern at regular intervals, and the platform compares each image with the history of the same point. A new hot spot becomes an alert; a stable one is logged. It is justified on assets whose unplanned stop is expensive, on panels inspected frequently, where opening the panel is risky (combined with an infrared inspection window) and on equipment that is hard to reach. It is not justified for the rest of the plant, which is well served by the annual route with a handheld camera.

What to do now

1. List your assets by the consequence of a stop and mark which ones have a failure mode that shows up as heat.
2. Do the first inspection with the plant at representative load, recording load, distance and emissivity.
3. Classify the findings and schedule repairs by severity.
4. Choose two or three critical points for fixed monitoring and keep the rest on a route. Always compare with the previous image of the same point: a growing delta is the priority.

If you want to know what your plant needs, talk to one of our engineers.

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Sources: ISO 18434-1, Condition monitoring and diagnostics of machines — Thermography — Part 1: General procedures; ISO 18436-7 (qualification of personnel in thermography for condition monitoring); NFPA 70B (inspection under load); technical literature on uncooled microbolometers in the 8 to 14 micrometer band.