[APD] Re: DIY Metal Halide Fixture

[Bill Wichers <billw at waveform_net>]

> Two big problems!  First, the metal strip mounting the
> reflector gets TOO hot!  I am worried that it might
> catch the wood on fire by some misfortune.  Second,
> the acrylic sheets I used as a watershield have
> started to melt.  I only had the light running for 30
> minutes.

For the metal strip, use a material that is a poor(er) conductor of heat 
than aluminum. Also, use a thin piece and bend it such that there is a long 
amount of bracket material between the reflector and the mounting surface. 
This will minimize the amount of heat transmitted *through the bracket*, 
but radiated heat from the bulb will still heat the wood if there is little 
separation between the two. For the bracket, I'd use a piece of mild steel 
sheet, maybe 1/16" thick or maybe a little less. You can get this from 
Lowes in their hardware section.

For the water shield, use glass -- tempered would be best in this 
application. Glass won't deform or melt from the heat, and the tempered 
glass will have better resistance to shattering if you do get water on it 
while it's hot.

Also, make sure you have at least an inch or two all the way around the 
reflector to allow for good airflow. If you already have the maximum 
spacing around the assembly in your hood and the temperature is still 
higher than you'd like, try mounting a small fan in the hood to exhaust the 
hot air. Make sure there is good ventilation both in AND out of your hood 
to allow air to circulate and help keep the internal temperature down. If 
you use a fan, position it so that it will generate maximum air flow around 
the area of the reflector, possibly positioning one fan near each reflector 
assembly. You don't need much fan power for this, a little air movement 
will go a long way since you only need to remove a relatively small volume 
of high temperature air to make a big difference.

     -Bill


*****************************
Waveform Technology
UNIX Systems Administrator

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