On Saturday, March 22, 2014 2:55:34 AM UTC, Johny B Good wrote:
On Fri, 21 Mar 2014 19:29:09 GMT, Harry Bloomfield
tiscali.co.uk wrote:
After serious thinking Johny B Good wrote :
I'm afraid to say this, but that last paragraph is basically a load
of bull****. Regardless of 'poor quality of early filaments' the
design criteria is still affected by the same physical laws that
determine the life versus luminous efficacy trade offs made with
today's high quality filaments.

Is exactly right!

There has been little if any improvement or development in lamps and
filaments, since the 1950's maybe even back to the 1920's.


The improved quality of filament just allows for an improved luminous
efficacy for a given lamp life (1000 hours being the standard lamp
life arrived at all those years ago as offering the best TCO between
the electricity and lamp replacement costs).

They were quoting 1.00 hours back in the 1950's and coiled coils.


Once you've decided what the lamp life should be, the voltage is
pretty well determined within 1 or 2 percent for a given wattage and
lamp life.

Tungsten filament GLS lamps are very sensitive to voltage variations
that can readily be dismissed by most other domestic appliances.

For voltage variations within +/-25% of nominal, each 5% increase
will approximate to a halving of lamp life (and associated improvement
of efficacy). Conversely, a 5% reduction will double the lamp life and
reduce efficacy.

My experience suggests life is reduced by much more than 1/2 the life.
My problems with the oven lamps is not the first time I have been sold
220 -240v lamps and have them need to be replaced within a very short
time.

I would expect oven lamps to typically last for several years in a
domestic oven, rather than just a week or so.
That's certainly the case with our Bosch double oven which we
acquired (2nd hand but hardly used) just two and half years ago.
Only one of the E14 40W Philips 300 deg C rated lamps was bought new
at the time, the other being an original lamp. Neither have needed to
be replaced so far but it's only the top smaller grille/oven that's
seen regular use and I can't undo the jamjar bottom pyrex glass cover
to take a closer look at the lamp itself without using some sort of
'Jar Lid Opening Tool' to get the extra purchase and leverage to
examine that lamp to see whether it was it was the new or the old
lamp.
Even when trying to undo the more accessible lamp glass in the lower
oven, I found it a bit of a struggle but I was able to verify that the
lamp rating was indeed a full 40W, as well as testing that the totally
ubiquitous 15W E14 300 deg C oven lamp (2 for a quid) would fit and at
least relieve the darkness of the oven's interior recesses 'in a
pinch'.
At somewhere in the region of a fiver a pop for the Bosch 40W oven
lamp, I'll happily make do with the standard 15W Pygmy oven lamp thank
you very much. I think the only reason I 'splashed the cash' in this
case was on account we needed a replacement jamjar bottom pyrex glass
lamp cover anyway so I added the 'special lamp' to the order.
The thing is with oven lamps, they have to be rated for for an
ambient temperature of 300 deg C as well as for the mains voltage in
use. Even if the standard glass envelope is still ok at that
temperature the filament could well land up running a good 200 deg C
higher than designed for if an ordinary lamp 'dressed up as mutton, so
to speak' were to be used.
Even a 15W pygmy oven lamp needs to specified precisely for the mains
supply voltage in use by the oven. In the UK, it should specify 240v,
definitely not 220-240v which makes me suspicious straight away as to
whether it would actually be rated for 300 deg C ambient.
Where are you buying your oven lamps from? Does your oven use the
readily (and only) available 15W E14 Pygmy oven lamp, or is it
something a little more inconveniently exotic like the ones used in
our Bosch oven?

The 300C rating is the standard, but unnecessary. You have a lamp enclosure with upto 220C one side, 20-40C the other, so the rise in operating temp is 100-110C. This does push filament temp up, but that's of minor consequence given how little ovens are used. The big difference is the cement in the bulb base, which is definitely not rated for the temp rise. You thus get a higher failure rate there, with glass parting from base sometimes. Some appliance lamps also have hardened glass to resist shattering into food, I forget whether that includes oven rated lamps. In practice though theyre behind a glass cover.


NT