I googled around and found some interesting facts, theories, stories on the subject of ceramic color for exhaust headers.
__________________________________________________ ___________________
One post:
99% of IR guns used are calibrated for use on a flat black surface. Colors have a different emissivity value. The example I use during training is the heating of a bearing ring which is shiny ground metal. The automatic shutdown thermocouple limits the temperature rise to a user set 225*F. Shoot the ring with an IR gun and the temperature reading will be 1/2 or less than actual. Spray the ring with some flat black paint and all is well.
So, what does this mean for the individual and for the group? For the individual it means that you can compare cylinder-to-cylinder temperatures if the gun has the temperature range. However, the temperature may not be accurate unless the color surface matches the IR gun calibration or the gun has an emissivity adjustment. For the group, it means that trying to compare different header coatings using different guns will yield results that will likely not be usable.
Another post:
Without getting too technical at first, Carl is on the right track here. Color isn't the only driving force behind emissivity though. You can take a flat paint of varying shades and their emissivities may be identical. IR is oblivious to color.
The basic equation for determining emittance is:
E + R + T = 1
Where:
E is Emissivity (IR emitted)
R = reflectivity (IR reflected)
T = Transmissivity (IR Energy passing through the medium)
If the part being measured is opaque, then transmissivity is zero. For most metals we can therefore rule this out.
The flat black surface Carl refers that these guns are calibrated to is actually referred to as a blackbody. There is no such thing as a natural blackbody. A blackbody is a perfect emitter. ZERO reflectivity. A good surface/contact thermocouple is great way to assist in determining how accurate your readings are. There are various techniques for calibrating these devices, but since we can actually see and access the surface to be measrued, a good surface TC can help you in determining how accurate you are. If you have an IR gun that allows you to adjust the emissivity, you can then back your way into an accurate reading with your IR gun based on the surface TC reading. And now you can take readings off other locations with the gun....
If this isn't coherent, well, thats because it is 3 AM... I carry a Level II certification through ASNT for Infrared inspection. Feel free to ask me anything you would like to know as far as IR is concerned.
__________________________________________________ _____________________
So - I guess from the above it may be hard to compare IR temp reading on headers of different color ceramic coating - and my flat black ceramic coating is probably going to naturally read higher just due to it being closest to a "perfect emitter". Interesting.
