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Re: Thorated vs Pure Tungsten
Hi Terry, all.
My comments below.
> Original Poster: Terry Fritz <twftesla-at-uswest-dot-net>
> I use 1/8 inch diameter thorated tungsten rods in my new neon
>protection circuit as safety gaps. However, they tend to
>sporadically fire at low voltages. The ends are cut pretty square
>instead of being rounded. I can replace the rods with the pure
>tungsten ones and grind them round on the ends. However, I
>was just wondering if the thorium was mostly to blame for
>the sporadic firing or the square ends? This happens with nothing
>else connected except the neon running at full normal voltage.
>
> I really think, now, that the pure tungsten is by far the best for
>Tesla coil applications. The slight radioactivity of the thorated
>versions is unsettling and this sporadic firing thing is a pest. The
>tungsten rods seem to be far far better than anything else I
>have used and are definitely the way to go for gap electrodes,
>as so many have found...
The reason why thorium is used at all, is to decrease wear in their
normal (tig welding) usage. For a safety gap (which shouldnīt fire
at all ;o]), this is of no interest, so you can use the non-thorated
ones here anyway. For welding aluminum, magnesium (i.e: easily
oxidized materials) you weld with AC (to continiously rip up the
oxide coating). During welding minute quantities of thorium can be
embedded within the weld bead. For this reason you canīt use
thorated electrodes, when welding Al, Mg, etc. This will contaminate
and weaken the weld bead (it gets brittle). For welding steel, the
thorated ones allow a higher current (for equal wear). Interestingly
enough, the thorium also prevents "sticking" of the tig electrode (you
can start the arc with HF power = no contact of the electrode with
the base material or you can "strike it like a match" to start the arc).
This is of great importance to the weldor, because any tungsten
in a weld bead (due to sticking and breaking off) will absolutely RUIN
the weld bead. It must be completely ground off and rewelded. The
metal tungsten is almost impossible to melt. Even the 50,000°F of a
plasma cutter or plasma welder will only very, very slowly erode this
material. This superb resistance to wear and high temperature stability
is what makes it such a great RSG electrode. The newer (but non
thorated) replacements for the thorated electrodes are a composition
of different materials (no idea what exact materials they are made of,
except that it isnīt pure tungsten), that have similar wear and start-up
characteristics of the old thorated electrodes. My guess is these are
probably the best for RSG usage (i.e: no radioactive material, but
better wear characteristics than pure tungsten). If you use a side
pick-up arrangement on the RSG, you probably can use the pure
tungsten ones too, because the arcing surface is much, much
larger than on an end-to-end setup.
You sporatic firing might be due to small quantaties of thorium being
burned in the arc. Where does the spark jump (from the middle or
more at the edges of the thorated electrodes)? What color is the
arc?
Someone asked (a few days ago) about using titanium as electrode
material. While I am not sure about the RF characteristics of Ti, I do
know that Ti can burn (very similar to Mg), so it might not be a wise
idea to use it. Esp. if they are small in diameter and run at high
power (multi kVA) levels. Due to this factor, the electrodes might
not quench (if they start to burn, you canīt "quench" with anything
but sand..... Sorry for the pun, but I couldnīt resist ;o]) very well.
Coiler greets from germany,
Reinhard