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Author Topic: 811A Tube lifespan  (Read 16825 times)
W6RMK
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Posts: 680




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« Reply #45 on: September 13, 2011, 10:30:53 PM »

Reply number 31
What Jim  N2EY said in reply #26 is exactly right according to my view. The black would be hotter. Although the black metal would emit heat the shiny metal would reflect the solar insolation rather than absorb it as well as black. That's why flat plate air or water heating solar collectors are usually black. If what was said were fact, then solar collectors would be shiny metal. They do employ mirrors to concentrate the solar insolation on a black absorber.

Frank

The difference is that in the "things laying in the sun" and "solar collector" is that the former has nothing taking the heat out except radiation.   Yes, the absorption is lower for the shiny metal, but the emissivity is much lower, so the heat comes in slower, but never leaves, so the equilibrium temperature is higher, than for the black paint which radiates about as well as it absorbs. So for solar collectors, black (or even better, selective coatings with high alpha and high alpha/epsilon ratio) gets used.
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W6RMK
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« Reply #46 on: September 15, 2011, 09:26:01 PM »

interesting experiment.
Did you say "non-contact" thermometer? Is that an IR probe type?  You have to adjust those for the emissivity of what you're looking at.

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W8JI
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« Reply #47 on: September 16, 2011, 08:12:35 AM »

I did not use the no contact as my harbor freight 12.99  no contact thermometer was not working. I used a multimeter with a contact  temp probe.
73s
Frank

Mine quit too, but my $300 IR that is 15 years old still works. :-)

I mostly use the probe type.
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