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Re: Compact fluorescent lamp life

Dave Gomberg said:

> I bought a pair of Panasonic 96w cf tubes from AHSupply in Dec. 
> 1999.   This month one of them failed.   They have been running about
> 10-12 
> hours per day split over two light periods per day (afternoon 
> thunderstorm).  So that is about 8000 hours when the first tube
> failed.
> But the surprising thing was that when I replaced the failed tube I
> would 
> estimate the replacement was 2 stops (4x) brighter than the remaining
> original.   I thought someone had written that cfs didn't dim much,
> run 
> them till they die.   Well I can tell you that with 5 hour on
> periods, they 
> dim a lot in the first 8000 hours.
> If you have older pc tubes, you should maybe get a replacement set
> and 
> replace one.  See if it makes a big difference in the brightness.  
> If it 
> does, you can keep the old faded one as a spare.
> And many thanks to AHSupply for the great replacement service, 2 days
> from 
> order to receipt!   And the best prices!

Dave is right on all counts.

All flourescent bulbs dim over their life span.  All! 

But note this, too:  If you replace a PC bulb you will notice that the
new bulb is much brighter than any older bulbs.  If you replace all of
them and then replace one again in a couple of months, the new one will
be much brighter than the older ones.  Why?  There is an initial steep
drop off in brightness for flourescents that makes brand new bulbs
outshine their barely older comrades by about 10% to 20%.  I don't know
the reason for the steep initial drop off, but its reported on more
than a few bulb manufacturers' sites.

After the steep initial drop off, a slower and steady decline continues
until one of the bulb filaments fails or the tube is cracked.  There
are no hard rules so far as I know for what Rated life span must mean. 
Some life spans are average time until failure based on 3 hour on-off
cycles.  Some are based on 40% decline in brightness but not that's 40%
from the initial brightness, it's 40% from the brightness of the bulb
after about 1,000 hours of use. 

Scott H.

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