Subject: Re: [amsat-bb] Re: Satellite Thermal Lesson
Fascinating report, Bob - thanks for sharing... I recently bought a cheap IR thermometer (gave into a long-standing state of curiosity) and had great fun with it; can't wait to do some more testing, but lots of fun walking around pointing at things in the environment.
But if you read the docs, the display from such an IR thermometer is completely meaningless unless you know the EMISSIVITY of what you are pointing at...
Now the good news is that MOST things have a reasonablly good emissivity, such as BLACMK and WHITE and Fingerprints etc... Remember, even my fingerprints changed the emissivity of my aluminum from about 4% all the way up to near 90% like black and everything else.
But point that thermometer at CLEAN aluminum and the reading will be meaningless for TWO reasons:
1) If it is clean, it will see hardly any IR from the aluminum and so "it should read a real low temperature"...
2) But it WONT! The reason is, that ALUMINUM is also a great REFLECTOR of IR! So therefore your IR thermometer will not even see the alluminum plate anyway, because all it will see is what it sees reflected in the aluminum.
You can prove this to your self by holding a sheet of aluminum as if it was a mirror. Point is so y our IR gun see's your body in its reflection and you will see your body temperature. Point the reflection to somethig cold, and it will read the cold temperature.
So again, those IR thermometers are only as good as the knowledge of the person using them. That's why those "window" snake-oil salesmen can come out and show you all the "leaks" and lost heat in your home, and sell you thousands of $$$ of new windows, because they either 1) don't have a clue how their instruments work, or 2) They do know, but they use the missleading display to fool the gullible.
Thanks for the reminder about those IR thermometers. Someone in our department got some and was running around doing all kinds of measurements... Then we showed him how his measurements didn't mean a thing unless he knew the emissivity of what he was shooting at... Sure pulled the rug out from under him. But again. Almost everything around you has a high emissivity... But if he doesn't know what does and what doesn't, then he is just messing around... And should not use those numbers for anything valuable...
Bob, WB4APR