On Sat, 21 Apr 2018 11:35:12 +0000, Andrew Gabriel wrote:

In article ,
harry writes:

Both tube and starter have sub-atmospheric gas filled glass bulbs.
(Plus mercury in the tube) The common fault is for air to leak in.
The other fault is for a filiment in the tube to go open circuit.

The three most common failure modes for tubes at end of life a

1) Loss of the electron emission coating on the filament.

This was pretty much the only failure mode in all tubes until about
about 10 years ago. With switch-start control gear, it shows as a
blackened tube end which only glows orange and not white as the
starter repeatedly tries and fails to start the tube. No white is
because the filament is not emitting electrons when heated red-hot
(no thermionic emission), so there's no conduction into the gas-fill,
and the discharge can't start.
Electronic control gear detects this by seeing the tube start to act
as a rectifier and shuts down the tube to prevent it changing to
operate as a cold-cathode tube which has a number of dangers.

This was an issue aggravated by the use of cheap bi-pin switch started
lamps in domestic service (kitchens, basements, sheds and the circular
tubes used in hallways and landings). Each switch start attempt would
sputter some of the thoriated coating off the cathodes/cum anodes,
shortening the service life when used in such frequently switched
applications.

The older (half century older!) Quickstart transformer technology not
only gave an almost instant flicker free startup (200 to 300ms) but
significantly reduced this sputtering effect which could double or even
triple the service life of a tube used in a domestic kitchen over that
using the cheap bi-pin switch start circuit, especially useful since it
encouraged the occupiers to treat it as an instant start incandescent
(but with a service life around an order of magnitude longer).


2) Run out of mercury in the gas.

Environmental regulations now require minimum mercury dosing of tubes
for the expected life (typically 1/10th of what older T12 tubes
used).
Mercury is slowly absorbed into the electrodes and glass and lost
from the gas fill. This causes tubes to dim with age, and eventually
to run a dim pink when all the mercury has gone.

I lament the parsimonious mercury dosing of modern T8 lamps since it
lends an unwanted "run up" characteristic, especially noticeable in the
winter months, more associated with the mercury amalgam variants used in
CFLs. The earlier fully dosed T12 tubes only showed the slightest hint of
this with really low temperatures not usually seen in a domestic
environment even in the winter other than for outhouse lighting.

In regard of both startup and run up time, modern electronically
ballasted fluorescent luminaires(sp?) have taken a backwards step over
the ancient Quickstart fittings. I guess that's the price you pay to save
some 16 watt's of consumption on a 4 foot fluorescent fitting.


3) Phosphor worn out.

Tubes last much longer than they did and the phosphor efficiency drop
causes them to dim and become unviable.

Air doesn't leak into tubes, even ones which are very many decades old.


True enough, the phosphors have to deal with mercury poisoning as well
as the degradation from UV radiation - the phosphor coating on LEDs
doesn't have to contend with this so they last a lot longer (but
nevertheless still slowly degrade over time).

In office and factory environments, fluorescent lamps were replaced en
masse after clocking up the rated hours for the 80% of design lumens
point, thousands of hours before the more gross and obvious failures
would start showing other than for defectively manufactured lamps.

This was simply because changing out lamps by the gross was far cheaper
both in electricity consumption to meet the minimum lighting standard
required by regulations and the labour costs involved in relamping on an
ad hoc basis as each individual lamp failed to produce its design lumens
output one way or another.

A fluorescent tube with electronic "choke" is as efficient as an LED
bulb.

Not any more.

For the general public, that's a fairly recent development, I first saw
125LPW samples just over a year ago and I think prior to that, most of
the retail outlets were (and still are) offering 81 or 90 LPW lamps. You
can still see plenty of 60LPW lamps in the smaller wattages on sale even
today (about the same efficiency as the best CFLs of recent years).

The best efficiency tubes maxed out around the 90 to 100LPW mark.

If you take into account losses in the fluorescent luminare (getting the
light from the wrong side of the tube to where you want it, or losing
it),
they never were.

For situations where a nice diffuse, shadowless lighting effect is
desired, such as a kitchen or a shed come DIY workshop or a low ceilinged
basement, that's not quite the deficiency it would seem. However, these
days when such a lighting characteristic is deemed desirable, a better
solution would be the use of ceiling mounted LED flat panels with a
suitable diffusing cover to mute the horrendous glare typical of naked
LEDs.

--
Johnny B Good