As automotive technology has changed, so has the technology of diagnostic tools and processes. When I started in this business in 1966, a good volt/ohm meter and test light pretty much rounded out the arsenal for electrical test equipment. Several years later came the digital volt ohmmeter, then labscopes. Finding the temperature of something required a mechanical thermometer; years later, along came the non-contact infrared thermometers. Today we have the use of infrared thermal imaging.
In the March 2016 issue of Motor Age, I wrote an article on this same subject — “A picture is worth 1,000 words.” This month I would like to carry on this thought with a little more in-depth study of how thermal imaging works and a case study on an electrical system problem. Let’s take a few minutes and explore how this works.
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The right way to take a temperature
I think most of us are familiar with the handheld non-contact pyrometer, or thermometer. Those tools can save a lot of time and money when it comes to testing the temperature of automotive parts. Have you ever considered the knowledge needed to use the power of this tool? There is more to its use than just pointing and shooting. To make this sort of tool take accurate temperature readings, the technician must also have some knowledge of the system they are working on to understand the story the temperature readings are telling.
When using the non-contact thermometer, the tool is looking at the infrared radiation that is radiated from the object. That sounds simple, but what is infrared radiation? The dictionary defines this as light waves in the 8 to 15 micron wavelength. These are light waves that are invisible to the human eye, but can be seen by sensors in a non-contact thermometer or an infrared camera.
All objects emit infrared radiation as a function of their temperature. This means all objects emit infrared radiation. Infrared energy is generated by the vibration and rotation of atoms and molecules. The higher the temperature of an object, the more the motion and hence the more infrared energy is emitted. This is the energy detected by infrared cameras. The cameras do not see temperatures. They detect thermal radiation.
Why do you need to know this? Different surface colors radiate the light waves differently and the sensors in the infrared device will not report the proper temperature. As an example, a shiny surface, such as bright aluminum, will radiate the heat differently than a flat black surface. Why does this matter? In some cases, it does not matter, especially if the technician is only using the temperature readings to compare the temperature of two different things that are made from the same material and have the same surface color and texture. Now if the technician is trying to get an accurate temperature reading, then the surfaces must be a dark color, preferably a flat black. An easy way to accomplish this is to keep an aerosol can of flat black paint in your tool box and just give the shiny surfaces a quick shot of the black paint.