Wes wrote:
Earlier this week I had to get a cam follower out of a machine. The follower was sticking
at times causing the machine that rotates a part and inserts a stud to fail on the last
hole.
-- snip --

So I set the handwheel over a range burner and gave it the heat. My IR temp gun got wildly
differing temperatures from the 4130 draw tube and the alumininum hand wheel. Like a 200+
F difference.
-- snip some more --

Totally aside from your success...

Look up "emissivity", and maybe "radiometry". Basically, a material is
as good at emitting photons as it is at absorbing them. Aluminum is
shiny because it's highly reflective, it's reflective because it's
really bad at absorbing photons, which means it's really bad at emitting
them. So when you get it hot the photons don't come out -- in a way,
they 'bounce' off of the surface back into the material instead of
getting radiated.

So when you point the IR temp gun at some aluminum, you're getting a
little signal from the aluminum, and a lot of signal that's just a
reflection of the room around it. Mirror-polish the aluminum, and the
situation gets worse (there's a reason that aluminum is one of the
materials of choice for Really Good Mirrors).

Steel is gray because it absorbs some photons, which means that it's
much better at emitting photons than aluminum, which means that it'll
read better in IR.

Put a dot of black paint on the aluminum and watch your accuracy go up
(unless your IR 'gun' has a built in, unchangeable, and wrong for black
paint emissivity correction factor).

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com