[ARC5] Filaments - How Low Can You Go?

[Richard Knoppow]

----- Original Message ----- 
From: "J. Forster" <jfor at quikus.com>
To: <kgordon2006 at frontier.com>
Cc: "ARC-5 Mail List" <arc5 at mailman.qth.net>
Sent: Friday, October 19, 2012 9:26 AM
Subject: Re: [ARC5] Filaments - How Low Can You Go?


> The Diffusion Rate for Solids is explained here:
>
> http://en.wikipedia.org/wiki/Mass_diffusivity
>
> As you can see, the diffusion rate (rate at which surface 
> is repopulated
> from the bulk) is strongly dependant on the absolute 
> temperature. This is
> a result of Statistical Thermodynamics, verified by 
> experiment.
>
> Best,
>
> -John

     I think it depends on the type of filament.  There are 
three generally used:
Pure tungsten
Thoriated tungsten
indirect heaters using coated surfaces

    My understanding is that reducing voltage of pure 
tungsten filaments will greatly extend life, RCA published a 
chart during WW-2 showing the rate. I also understand that 
the emission of thoriated tungsten and coated indirect 
heaters does not bear a simple relationship to voltage 
(temperature) so that reducing voltage may cause problems. 
I've seen a detailed discussion of this somewhere but can't 
pull the source out of my memory.
     For pure tungsten filaments (used in some high-power 
tubes and in lightbulbs) a reduction of 5% voltage 
approximately doubles the life, a reduction of 10% gives 
about ten times the life.