Sunday, April 7, 2013

This Is Really Hot (especially for geeks)

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I finally found a good chart for color temperatures.

The German chart is based on tool steel.  It shows working temperatures for blacksmithing on the left, and the tempering range on the right.
Left: Incandescence: These glowing heats include the red-hot working range for iron and steel, and the yellow where steel tends to burn and then melt.  Incandescent light bulbs, tungsten filaments, and distant stars also follow roughly this range of colors.  Eventually, the colors shade through into white and blue-white.

Right: Iridescence: The lower-temperature colors are not glowing, and you would see no effect on plain black iron.  But on polished steel or sheet metal, you can see a surface iridescence when it's heated... weird rainbows with red in the middle.  It's due to very thin films of oxidation, which interfere with light much the same as a soap bubble or wet-parking-lot oil films.

And then I found this very simplified color temperature scale:

This appears to be what photographers and interior designers are currently using as a color reference: a sort of re-calibration of what used to be called 'warm' and 'cool' light effects.
This scale is interesting to me.  It acknowledges that the 'warm' colors like candles and low-watt bulbes are actually cooler than the 'cool' white lights like tungsten-filament theatrical bulbs.  But it also includes the diffuse light from the blue sky as a 'high' color temperature - when the sky itself is not particularly hot.  So it's loosely based on current physics, but somewhat arbitrary when it comes to actual temperatures.

Then there are the charts of star temperatures.  Most list red as under 3000 K (2700 C, around 5000 F), with yellows and yellow-whites in the 4000-6000 K range.

This puzzles me, because both stars and iron are classic textbook examples of 'blackbody radiation.' Yet the stars appear to be much hotter in the same color range, compared with the numbers given for steel or iron.  Are the visible colors of the stars merely surface temperatures, and star charts estimate the (hotter) core temperatures for the same stars?
  Or is it just a case that actually measuring these extraordinarily hot temperatures is difficult if not impossible, and different theories give different estimates?

Why do I care?
Well, for starters, being a geek means I'm allowed to care about things even if it's not normal to care about them.
  I like colors; I like art and science; and knowing about color effects helps me do both.
 I can stare into the fire and estimate how hot it is.  I can watch a steel can heating up and estimate how hot it is - and how soft it would become if I cool it quickly.  I can make art by heating polished steel, or make realistic drawings and paintings of fire.

And I can guess how hot my copper kettle got last time I left it on the stove with no water in it.  (Classic absent-minded professor move, that last one.)

On another practical front, we are looking into testing equipment for measuring the interior temperatures of our rocket mass heaters.
   Refractory materials to handle 1000-2000 F are pretty widely available, and firebrick can handle up to something like 2700-3000 F under reasonable conditions.   But if we use better insulation it doesn't just improve combustion efficiency, it also pushes temperatures upward.  If we use insulation rated to 2600 F, for example, it might melt around 3200 F.
   Testing thermometers include $4000 thermocouple probes (accurate temperature at a specific point, sacrificial tips for higher temperatures); and $800-2000 infrared thermometers (they are more easily confused about whether you are measuring the brick surface or the flame itself).  And $4000 infrared cameras, which would give a portrait of the heat at many places at once.
   We have a grant application out for some of this equipment.  We may be doing some fundraising soon for the rest of it, and we're also very interested in finding collaborators who have already invested in similar tools and would be willing to rent them out.

All this research may shortly be leading to some EPA independent-lab testing, and to more technical builder's guide.
It's easy to get into abstractions and learning for its own sake, and get distracted from the main project.  If I write up these 'sidetracks', it frees up space in my head.
You might think of it as 'outtakes' from our new book.