Tuesday, 24 July 2018

Exploring the IR thermometer

An IR thermometer (or indeed a thermal imaging camera if you have the funds) is a useful tool for anyone wanting to understand heating, insulation and heat losses around their home. You can learn a lot from them, but you need to understand some of the important facts, like how they work and they are not magic.

There is an illusion that the eBay cheapies aren't worth having, but depending on your needs they are most certainly worth having.

The primary limitation of the cheaper ones (like $10 ~ 20 range) is that they only provide readings of temperatures within a certain range, in the case of the two I own that range is -50C to +380C

Not enough for a blast frunace operator or someone checking temperatures of exhaust systems of diesel tractors (near the manifold), but totally fine for my uses (checking the floor and measuring food temperatures when cooking).

So today I decided to do a comparison using a thin walled plastic cup with hot water in it that would enable me to use a known high quality thermometer to measure its temperature and compare that to my IR thermometer. Interestingly I found them to be within a degree C (over repeated measurements) of the actual thermometer (which is a high grade one intended for laboratory use with the usable range restricted to that of liquid water).

The plastic cup is essentially a source of heat, and heat is actually part of the same spectra of stuff that is light. (see this link: https://en.wikipedia.org/wiki/Infrared). It is that "light" that the IR thermometer is reading and measuring.

However while the aluminium foil is a good reflector and conductor of heat, its not a good radiator. This is because of a property called emissivity.
(read this link: https://en.wikipedia.org/wiki/Emissivity).

The above video makes it clear why reflective and shinny surfaces are problematic for reading what can be thought of as "light" from the surface. The issue is are you seeing the source or a reflection from a surface.

As it turns out glass blocks thermal IR but the glass (being a good conductor itself) will show its own temperature. You just need to be aware of hot things reflected in it on your side of the glass, like a furnace).

So armed with that information I hope you can feel more confident in how to use an IR thermometer some of the pitfalls, and what to avoid you can go grab one and start gathering useful data.

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