Re: [T3] Dual 40 DCNF Carbs

[Jim Adney <jadney@vwtype3.org>]

On 23 Sep 2002, at 23:03, Jens Vagelpohl wrote:

> > That's about .003", if they are Tungsten, it would be a pretty tough 
> > wire.

> they're made of platinum according to my literature

Any idea why? Platinum is a LOT weaker than Tungsten in wire form. 
Perhaps it has something to do with the change in electrical conductivity as 
a function of temp, which is what this system uses. Most metals increase 
their resistivity as the temp goes up, and I know that Tungsten does this 
dramatically, but it may happen at a higher temp than Bosch wanted to 
operate. Maybe Platinum changes at a lower temp.

I finally found RTD (resistance-temperature dependence) data on both W 
(tungsten) and Pt (platinum.) At least within the 0-100C range W has the 
stronger response. Pt is much more widely used as a RTD, however, 
because it is more linear. In this application linearity is irrelevant because the 
temp is held constant. I think I would have been tempted to use W instead, 
for the reasons of cost, temp dependence, and strength, but maybe Pt is 
more inert in air and hence more stable over time.

-
Jim Adney
jadney@vwtype3.org
Madison, WI 53711-3054
USA

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