how often to replace T5s
 

Hi,
I'm now using T5s as supplemental lighting in my greenhouse only on overcast days. Can someone please tell me after how many hours they should be replaced?
Vicki

Usually, they should be changed after a year. However, if you're only turning them on on certain days, perhaps they will last longer. The best way to know when bulbs need replacement is by measuring the light output with a reliable light meter.

I can't say this enough. My light meter was one of the best investments I've made pertaining to my orchid hobby.

Tracy is correct. Trust a good light meter. When I install the new tubes, I tape a paper to the fixture with the foot-candle reading at a certain distance. About a year later I take a reading and if the output has dropped, I change the tubes. Usually it has, so I'm pretty-much changing all the tubes out at 1 year. Mine run 7 days a week all year (various hours, though.)

Thanks Tracy and Ross!
Vicki

I change mine when the ends of the tubes start getting black (darker).

I'm with Tracy and Ross: once a year. One thing you might want to bear in mind is to be careful that you don't burn your plants after you change the bulbs. The light will suddenly be quite a bit stronger. It might be a good idea to move the more light-sensitive 'chids a few inches lower for a week or two and give them time to adjust. Just watch the leaves for any signs of red or purple streaks. ;)

Once a year is fine if you're leaving them on a long time, like 10-12 hours a day. That's what I do with the aquarium lights. My orchid lights are only on a few hours a day (only when I'm home) so once a year is probably a waste of my money. A light meter is probably the only way to know, but I can't afford one. The terrarium shop where I get mine recommended every 1,5-2 years for about 4 hours daily use, which is what I'm going to do. They aren't T5, but power compact florescent tubes.

Great advice - can anyone recommend a good light meter?

The one I use is Buy Light Meters Lux & Foot Candle l 5 Year Warranty l Sper Scientific It's a bit pricey, but it's self calibrating (calibrates the zero point every time you turn it on, so it eliminates drift due to battery strength, etc.) so I can depend on accurate readings (both lux and foot-candles).

Thanks again, Tracy and Ross. Thanks also to you...BioWheel, Stephen, Camille, Dave.
Vicki