On 29/10/2020 18:38, Jeff Layman wrote:
On 29/10/2020 10:54, Max Demian wrote:
On 28/10/2020 22:29, Jeff Layman wrote:
On 28/10/2020 20:32, Theo wrote:
Jeff Layman wrote:
What interests me is /why/ those elements failed. They have no moving
parts and are effectively just wire resistances.Â* Any idea why yours
failed? It seems to me the elements must be designed that way; is
there
something they do to the (nichrome?) wire which almost guarantees it
will fail after a few years?

I assume it's the same reason lightbulbs fail.Â* They run at high
temperatures.Â* Eventually the material ages (in the case of lightbulbs
due
to filament evaporation - not sure of the mechanism of a ceramic
element but
possibly some kind of migration into the substrate) and a positive
feedback
loop causes thermal runaway and catastrophic failure.

But oven grills run at a lowish temperature - a dull red heat (around
700 - 800 deg C), e. which is about half the melting point of nichrome.
They are also supported along their length, unlike an incandescent
filament, which runs at yellow-white heat at around 1300 deg C. It also
seems to me thatÂ* toasters run at a higher temperature, but they seem to
burn out at a much lower rate than oven grills. Our toaster, for
example, is around 25 years old, and is used much more than the oven
grill.

What I don't understand is halogen radiant heaters and radiant rings (on
a vitroceramic hob).

Surely the point of the halogen gas is to enable white heat without the
filament burning out - heaters and rings are only red hot.

I would expect "darker" rings to be more efficient, in that they don't
waste energy as visible light. However, I think that halogen hobs were
developed as they could be turned on and off almost instantly in a
similar way to gas. The problem with ordinary rings is that they are
thermally very slow. Do halogen hobs burn out their "elements" at the
same rate as ordinary rings?

But why halogen? Why not argon/nitrogen like the old fashioned tungsten
lamps?

--
Max Demian