[amsat-bb] Re: Satellite Thermal Lesson

Robert Bruninga bruninga at usna.edu
Wed Nov 18 23:22:37 PST 2009

> 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

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...


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