IR Thermometer Accuracy

Thermo Gun

From our Facebook Community – Shawn Staggs

Anchor Bay Inspections LogoIn your opinion, how accurate is an IR Thermometer? Assuming the closer you are to the object being measured would give you a better reading. But still being really close are we talking about a 1 to 2 degree temperature difference? – Shawn

An infrared thermometer or temperature gun as many people refer to them, is actually a long wave “spot radiometer”.

This means it is subject to the same science as an infrared camera. The accuracy is determined by many factors.

First, are you close enough?

With an 8:1 device I need to be within 8 inches to measure a 1 inch diameter spot, 16 inches to measure a 2 inch spot and so on. If I’m 24 inches away and my object is only 1 inch in diameter, then I am only filling 1/3 of the circle and it will average all the temperatures together.

It will not give you a reading that says “the maximum temperature within this area is xxxxxx. If the 1/3 areas’ temperature is say 110*F and the other 2/3 of the area is 75*F, you might get a reading of approximately 85*F. The minimum size it will usually measure is 1/2 inch, and in this units case I would have to be at 4 inches away.

Second, what is the emissivity of the surface you are measuring?

Most temperature guns do not have an adjustable E setting. They are usually locked in at .95. If I’m measuring a flat painted surface (e = ~.92-.94) and I’m close enough, then I should be within 1 or 2 degrees F. If I’m attempting to measure a shiny metal surface, forget it! You won’t be close at all.

Infrared cameras have a much tighter distance to spot size ratio. It is a function of the spatial resolution. (The detectors characteristics and the lens). I have measured the d:s ratio on a lot of cameras. The E6-E8 has about 60:1 d:s ratio, the E40bx is about 135:1 and so on.

Hope this helps.

If you have more questions, please don’t hesitate. – Bill Faban – Monroe Infrared


Professional Home Inspector John Buckley in New Jersey shared this image with us in our open discussion group. He uses the FLIR TG 165 to perform spot thermal measurements. Find John Here. |  Join our INSPECTOR ONLY group here.

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