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Old 02-12-2007, 12:55 AM   #1
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spectrum shift on MHs? and now with cool incubator pics

I'm trying to puzzle out a problem at work, and just thought of something. We use metal halides to run primary productivity experiments, and we've been getting bad data lately. I'm wondering if the bulbs are getting old and the spectrum has shifted, like PC and NO fluorescent bulbs will do. I'm not that familiar with MHs, so do they shift spectrum and output as they get older, and what would be a typical life span for them?

I'm not sure what wattage and kelvin the bulbs are (just know that they are huge, hot and blindingly bright, though!) I will check tomorrow when I'm at work.
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Old 02-12-2007, 12:57 AM   #2
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I would have to say that they do, as they have a "burn-in" period that they go through before they are buring at the temp they are supposed to. I think they would have to degrade over time, hence the reason to change them after 6-8months of use.
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Old 02-12-2007, 01:05 AM   #3
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OK, that's good to know. My rough estimate is that the lights are on 6 hours at a time, about 25 times a year, and haven't been changed since I've been there, so 5 years, that's 750 hours. 6 months at 10 hours per day is 1800 hours, so unless they were changed not too long before I got there, they are definitely getting to the end of their life.....hmmm......don't know why I didn't think of this sooner since we've all been going nuts trying to figure out why the data is bad!
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Old 02-12-2007, 01:12 AM   #4
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If I may......What do you do?
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Old 02-12-2007, 01:30 AM   #5
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I'm a graduate student, and I work in a lab that does a lot of water quality testing, primarily seawater. We do nutrient analyses (NO3, NO2, NH3, PO4, SiO2), chlorophyll, dissolved inorganic carbon, whole-water respiration and primary productivity. It's the primary productivity testing that uses the MHs. We have a 6 tray incubator setup that has slots for vials with screens underneath to create a light gradient. we incubate 5 ml seawater samples that have been spiked with radiolabeled sodium bicarbonate in the incubators for 1 hr, gas off the excess radioactive material, then measure what's left, ie what was taken up by the phytoplankton. This then can be used to calculate how much photosynthesis has occurred. We've been getting inconsistent and low results for the past few months, and have been going crazy trying to figure out why. We always measure the intensity of the light coming out of the bulbs, but not the spectrum. The bulbs are old, and I bet the spectrum has shifted enough (and the intensity has dropped) that it's affecting the rate of photosynthesis.

Hope that made sense....that was the simplest way I could explain it. It's pretty cool, actually, maybe I'll take some pictures next time the lights are on (which might be tomorrow) and post them.
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Old 02-12-2007, 01:37 AM   #6
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WOW....that is very interesting...I'm a lover of learing! TLC NGC Discovery I love it!!! I'd love to see some pics of that!!
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Old 02-12-2007, 11:28 AM   #7
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Newfound, the saltwater lighting guru Sanjay Joshi, says that you can substitue for the spectrum shift by lowering the lights closer to the water surface, but be careful to monitor temps. Basically, when the lights become older they emit more red spectrum than blue, which can cause algae to grow, and I'm assuming, mess up some of your experiments. By lowering the light, you are essentially cutting off a lot of the red lighting, and subsituting for the loss in intensity.

Anyways, here is a cool article if you are interesting that shows some spectral plots.

http://www.personal.psu.edu/sbj4/aqu...lideLamps2.htm

And his reef lighting page in general.

http://www.reeflightinginfo.arvixe.com/index.htm

Or..you can just coax someone into buying new fixtures.

HTH
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Old 02-12-2007, 11:31 AM   #8
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You need to burn in the bulbs for a about a week before you take any tests IMO. The ballast could also be going bad so you may wish to check into this as well.
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Old 02-12-2007, 06:58 PM   #9
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Thanks for the info everyone. DT, the experiments are only run for 1 hour so algae isn't the problem. The lights are 8 cm below the bottom of the trays and heat is already a problem, so we can't really move them any closer. The water runs through two circulating chillers and they really have a tough time keeping the water cool, especially in the summer when the air temps are high, and now, when the water temp that we need to match is very low; we just ran an experiment where we had to get the incubators to 2.7 C, imagine that and 8 cm over 6 250 watt MHs!! Lots of ice and ice packs, and pretty heavy duty (ie very expensive) chillers.

Not sure what Kelvin the bulbs are as they don't say and neither do the boxes. They are 250 watt Philips single end screw in bulbs. I will check on the ballasts as well but they appear to be OK; the bulbs come on pretty quick.

DragonForce, what do you mean by a week? 24 hrs a day for a week? or 7 days of "fishtank hours", ie ~10 hrs a day for a week. I'd be very leary of leaving these things on while there is no one here at night and we don't have more than two weeks where we don't need the lights until next fall!

here are some pics and descriptions:
The lights in the lab are off because ironically despite how bright the lights in the incubators are the samples need to be in the dark at all times when they are not in the incubators.

First pic is a back view of the three incubators. Each has two trays, with a MH underneath. The two chillers are at either end. All the right hand trays connect to the right hand chiller, and the left trays to the left chiller. We need to be able to set the trays at different temps, hence the weird plumbing.


Here is a closeup of the MHs from the front. They are very close to the underside of the trays. The metal shields keep light from one MH from spilling over to the next tray.


Here is a close up of one of the trays. Each one has 60 slots for sample vials, and a lid when the vials are in there.


Another closeup of a tray, showing the screening to create a light gradient. We have a list of the expected light intensities for each slot (they are labeled A-F across the columns and 1-10 on the rows, putting the vials in is like a weird game of battleship!) so each vial gets a specific light intensity. Every sample gets 18 vials, 16 in a gradient and 2 dark vials.


A view of the incubators from the front. Some poor person has to sit in front of these things and measure the light in every one of those slots with a light meter.....and try not to get blinded every time they look into the trays....sunglasses are a necessity, which looks pretty weird when you're sitting there inside with sunglasses on! If you forget to not look in when you open the trays you see rows of green dots for about 5 minutes after!
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