Emissivity

 Misleading metal temperatures… (a little more techie)

The emissivity of a surface is the extent to which it radiates heat, as expected for its temperature. A perfect emitter (a ‘black body’) has an emissivity of 1.  Most building materials are pretty good emitters, with emissivities of about 0.95, which is the normal setting for thermal imaging cameras.

Bare metals (and glass to a lesser extent) have low emissivities and so give false temperature readings on a thermal camera set for the normal 0.95 emissivity. Low emissivity also goes with more thermal reflection.
Hot Kettle_2

This hot  kettle shows a temperature (Sp2) of 32.4°C, even though it’s just boiled!  Its stainless steel surface has a low emissivity and so radiates less and also reflects more of the relatively cool surroundings, giving a false, low temperature.

Some PVC electrical insulating  tape (emissivity=0.95) has been stuck on, near the bottom-right of the kettle. It appears bright because it’s radiating, as expected for its temperature, giving a credible, measured temperature of 95°C (Sp1). This is a useful method of measuring the temperature of low emissivity or reflective objects.

Painted metals have normal emissivities, providing the paint isn’t metallic. It’s the surface that determines emissivity, not the underlying material. Treat emissivity tables with caution, because the actual values vary depending whether a metal surface is tarnished, indented, rough or polished. Most cameras can be set to read the temperature of a low emissivity surface, but as well as the uncertainty of what emissivity to set, the readings are error-prone because of the reduced emitted radiation for the camera to work on.

If you need the temperature of a metallic, glass or other low-emissivity surface, it’s best to apply PVC tape or another high emissivity material, as described here. Let it equilibrate for maybe 30 seconds and then read its temperature with a TI camera, set to the normal 0.95 emissivity.

In Practice
Don’t trust temperatures of bare metal in thermal images. See above if you really need to know their temperature.

Also see page on Reflections, a related source of confusion.

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