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Using an Infrared Thermometer

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Occasionally there is reference in discussions about using an infrared thermometer (spot radiometer) to check the temperature of something; tires, wheels, transmission housings, etc. An IR thermometer is a great tool and can help diagnose a problem or prevent a failure, but there are several things that affect infrared readings.

Two things are very important and fairly easy to control: distance and surface.

Distance is how far the thermometer is from the target. Any decent IR thermometer will have instructions and in those instructions it will give spot size or distance/spot ratio, which is the size of the area the instrument reads at a given distance (or distances). My spot radiometer has a ratio of 8:1 which means at a distance of 8” it reads a spot of 1” diameter, 16”:2”, 32”:4”. At 8 feet it reads a 1 foot diameter spot. The important note here is that it is not reading the tiny dot made by the laser pointer. Too far away and the spot being read may be very large and not yielding a reliable temperature of the target.

More important is the surface of what is being measured. The instructions might mention emissivity, but can get fairly murky in this area. The main point here for RV folk is that bare metal or metal-like surfaces do not emit radiated energy like other surfaces. They reflect the energy from the surrounding stuff, so a reading on a chrome hub cap will be reflected temperature from the road, the sky or maybe even yourself, but not a reading of the true temp of the hub cap (chrome or aluminum wheel). Take a reading on a smooth metal surface like a clean (unpainted) oil pan, transmission pan, and it will likely read cooler or hotter than it actually is, depending on the reflected temperatures of the surrounding equipment. Tires, painted or textured surfaces generally reflect less and give better readings. Smooth, bare metal or metal coated parts (chrome) do not yield accurate readings of the target. If you want to test this for yourself, take readings on your chrome hubcap from several angles; one straight on, one from a high angle and one form a low angle. Each will likely read differently even though you have the laser dot on the same point.

I uploaded an image of a chrome hub cap and wheel taken with an infrared camera. The three temperature points are all going to be relatively close in actual temperature, but each reading is influenced by the reflected energy: SP1 is off my hand, SP2 off the ground and SP3 from the sky off the top of a lug nut cover.

IR 94007

Stay close to keep the spot size small and avoid bare shiny metal, chrome, etc. If in doubt, use a contact thermometer if safe to do so.

Hope this is useful.

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Couple of extra thoughts that may be useful.

A reading with any type of radiometer (infrared thermometer) cannot be considered reliable without a calibrated instrument and proper adjustments to compensate for emmisivity, atmosphere, etc. Thats not to say a reading with your handheld point and shoot unit will not be good enough. Many measurements are for comparison or trending and even without adjustments, they can tell you one area is warmer or cooler than another, one tire hotter than another, etc.

If there is a surface you are unsure of you can place a piece of electrical tape on it ( I use Scotch 33). If it reads the same on the tape as off, the surface is readable with the IR. Another option to verify the readibilty of a surface if you have a contact thermmometer would be to compare the contact reading with the IR reading. If the same you can read the surface with the IR. There are changes in readability as surface temperture goes up, but for most of what we use point radiometers for on an RV the difference would not be great, so you can check your surfaces when they are cold if it is safer to do so.

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