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Re: Thorated vs Pure Tungsten

At 04:08 AM 6/15/99 +0200, you wrote:
>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
>

Hi Reinhard,

	The sparks are thin purple blue that occasionally shoot from the edges.
Today I picked up some pure tungsten and cut it but I haven't tried it yet.

Apparently TIG rods come in all kinds of "flavors".  These are from NASCO
and they offer the following types:

ZIRTUNG
2% Thoriated Tungsten
2% Ceria Tungsten
PURETUNG (pure tungsten)
1% Thoriated Tungsten
TUNGSTEN-LANTHANA ONE POINT FIVE

They are available in the following stock diameters:
0.040"
1/16 "
3/32 "
1/8 "
5/32 "
3/16 "
1/4 "
3/8 "

They also come in the following lengths:
3 "
6 "
7 "
12 "
18 "

The guy at the shop said he could order any of these, although, I hate to
think what 18 inches of 3/8" would cost!  However, 3/8 inch tungsten
electrodes would run a lot of current!!

I'll see tomorrow if the pure tungsten cures the arcing.  If not, I'll try
to round the ends. If all else fails, I have brass rods that I could solder
1/4 inch steel balls to ;-)

Cheers,

	Terry
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