by: John M. Hill
After a day-long hold at 500 °C to make sure all systems were working
satisfactorily, we continued the increase in furnace temperature this
morning. The furnace has now reached a temperature of 615 °C and is
heating at 11.7 degrees/hour. At the moment we are consuming about 430
kW of power.
The heating will continue at this rate through the night until we reach
750 °C at 08:10 tomorrow morning. At that point the glass blocks
have begun to soften and we begin rotation at high speed. By 09:00 the
furnace will have reached the full rotation speed of 6.81 rpm. In the
morning we will also start our two (rented from Empire Power Systems)
750 kW diesel generators so that 4/9 of the heaters and 1/2 of the
drive system will be on generator power. Every experiment should have
at least one big diesel engine.
The oven is running quite nicely and the temperatures are all nicely
controlled. Because of the 21 tons of E6 glass laying on it, the mold
is lagging about 50 °C behind the ambient temperature of the
furnace. This is the expected thermal lag --- we heat about 20% slower
than the 6.5 meter mirrors to compensate for the increased thermal
inertia of the larger 8.4 meter mirror.
There are many minor problems and happenings to keep life interesting.
The late night oven pilot reported some sightings of small furry
mammals. No, not red squirrels, but bats inside the lab. Arizona
football stadium is home to one of southern Arizona's largest colonies
of bats, and once in a while they find their way into the Mirror Lab.
At least one bat was escorted back outside this afternoon. If all else
fails, we have the phone number of the "bat man" --- no, not the Caped
Crusader, but a researcher who specializes in reviving trapped bats
with sweetened condensed milk and releasing them.
Late this afternoon one of the video cameras started overheating.
Barry McClendon and I pulled the camera off the oven and disassembled
it to see what was wrong with the thermo-electric coolers (TEC) than
cool the camera body. It turned out to be a short in the cable that
supplies power to the TECs. We hadn't noticed it earlier because the
weather had been cool enough that the cameras didn't need cooling until
the skin of the furnace started getting hot. The video images of the
melting glass are flash images in the blue captured with a PC-base
framegrabber system from CCD video cameras that look into the furnace
through 3 mm pinholes and security lenses. Even with only a 3 mm hole,
the big engineering challenge is to keep the camera from melting under
the radiation load at high temperature. We cool the camera bodies with
TECs and ducted fans. We use blue filters and flash illumination to
overwhelm the uniform blackbody glow of the hot oven.
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