metal halides over acrylic

[mattsfishes84]

Hey All -

I posted this over in the lighting forum, but haven't had any replies, but since it applies to a large tank, I thought you all may know..

I understand that Metal Halides directly over acrylic can cause damage over time and cause cracking of the acrylic top panel of the tank. However.. is this damage caused by:

A) Heat only - and the thermal expansion and compression of the acrylic

B) UV Radiation from the bulbs causing damage

or

C) Both.


If it is heat only, then wouldn't it be OK to have the MH fixtures hanging high enough to prevent them from directly heating the acrylic?

Any thoughts?

 

[Acrylics]

Should be little UVC coming from MH bulbs sufficient enough to cause breakdown too rapidly. That said, UV is generally regarded as an "aging accellerant" in many materials and acrylic being one of them. Under normal (outdoor) conditions, most acrylic mfrs now warranty their materials to (essentially) not be structurally affected by UV for ~10yrs.
Heat is the bigger issue, while one could raise the bulbs high enough to not cause much of a heat issue, it is generally recommended that you don't place MH or any other HID bulbs directly over the acrylic. The other bad side to doing this is that any spray or saltcreep on the acrylic will act as a filter so not as much light will get through.
Another thing you can do is to place a piece of 1/4" over the crossbraces with "feet" under them so that there is an air gap between this piece and the brace. Doing this allows the 1/4" piece to act as an insulator and absorb almost all of the IR & UV energy.

HTH,
James

 

[mattsfishes84]

Good idea.. i may have to try that. I'm not sure there is a way to place the lights such that they are not right over the acrylic without having a void in the tank..
so perhaps i will try the "shield"

thanks!

 

[wmilas]

I was told by my manufacturer that heat is the killer and its heat that causes the crazing that is the precursor to failure.

You have a few choices:

1) Don't put a MH over a brace. What does "over mean"? Don't rightly know because with something like a Lumenarc that throws a 2x2 or 3x3 square depending on its height, I'd imagine that being on the outer envelope of the square wont produce enough heat to cause damage providing you ventilate adequately.
2) Add a shield as stated above. I would also still ventilate with a fan.
3) Do what I'm doing.. either go full t5's or put the t5's over the braces and keep the MH's over the openings.

Good luck

 

[BADGUS]

i have a 125 acrylic and only use 2 MH 400w over the openings,i run 4 total cooling fans,so far so good.