How do you know when to replace a T5 lamp

I've gotten two grows out of my existing T5 lamps. I've been reading that the lamp life isn't all that long, so I am wondering how do you know when it is time to replace the lamps?
Do you go by the number of hours the lamp has been on? All of my original lamps are working just fine - should I go for another grow on these lamps?

Is it that they put out less lumins as they age, and by using them for longer than intended would result in the plants receiving less light

Well you will be able to tell if its putting out less lumens cause it will appear to be dimmer, not as bright. IMO its better to be safe then sorry and invest in new t5's,, theyre pretty cheap. You can get them at lowe'/home depot

My t8s get grey at both ends when the bulb is toast. Probly the same with t5s. They last for a long time.

i thought they were good until they burn out?

doniawon said:

i thought they were good until they burn out?

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Ya, two grows aint shit for T5s and to the guy who said when they appear dimmer, pffffh, youll never look at your light and say..oh it appears dimmer today than yesterday lol T8 s start to fail at around three years usually and even after three years they grow some nice plants. Those bulbs are expensive when you multiply them x 16 so if you want, switchem but i think you should give them at least four more grows and then consider it....id do ten more but thats just me

Ninjabowler said:

Ya, two grows aint shit for T5s and to the guy who said when they appear dimmer, pffffh, youll never look at your light and say..oh it appears dimmer today than yesterday lol T8 s start to fail at around three years usually and even after three years they grow some nice plants. Those bulbs are expensive when you multiply them x 16 so if you want, switchem but i think you should give them at least four more grows and then consider it....id do ten more but thats just me

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I would agree with that you can't look at a lamp, let alone a bank of 8, and tell if they are dimmer. flourescents do get the grey end caps as they age but if they still burn then I am hoping to keep using them. I have been using the aquarium lamps that are not inexpensive and hope that I can get another couple grows out of them. Looks like that would be the case

Ok this is coming from my reef aquarium keeping background since the lighting needs and types are very similar HID, LED, T5. The general rule we follow in aquarium keeping is replace your bulbs every 12 months when running HID/HPS/MH or T5/Florescent. However this may not hold true with MJ growing or it could since light output is weight. I would say the very best way would be to have a photometer and check the output of the lighting every few months or at the start and end of each grow and keep track of the amount of output lost over time. Another way you could go about it is to replace the bulbs once you start to have a lose in yield. You will not be able to see the dimming of the bulbs over time since you see the lighted area each day but you can tell a new bulb next to an old bulb. The ends of the bulbs turning gray has nothing to do with the life of the bulb yes older lights will have more gray or even black on the ends but new bulbs can have grey on the ends as well. This is a result of the gasses and mercury burning and being excited but the flow the electrons threw them.
Knowing when to replace T5 bulbs is something I will need to dial in since I am growing with T5. I would expect you can get 3-5 grows out of T5s depending on how long you light during veg and how long you veg for.

get a brand new bulb fixture and a light meter, and compare the light output. always keep a fresh bulb so you can compare. I bought a combo moisture/light/ph/ppm for 20$ i think but i dont think that the ph+ppm are accurate..

Those combo meters suck and really dont work. You will need a good photometer and they are not cheap. A local fish store near me as one that they rent out for $20 a day for testing your home aquarium lighting. Ones like this are the only good and accurate ones http://www.hydroponics.net/i/138064. Funny I just did a google sreach for photometers and it gave a link to a hydroponics store LOL.

Replace bulbs at 12 month intervals or sooner for better growth and production . I know some people who replace them within 6 months at the same time they replace there HID bulbs .. I have went longer than a year but expect diminished returns and lower growth rates than before .