Spark gap degradation

[Zak28]

If ordinary 304 or 316 stainless steel was immersed in mineral or some other oil wouldn't its service life be substantially improved as opposed to operating in open atmospheric gasses were oxygen and heat would destroy it?

Oil immersion also carries heat away from the conductors and the oil is unlikely to become conductive after a long period of operation. This is for 3-8kvAC tesla spark gaps, Im not sure what higher voltages would do to the oil, very high voltages might even heat it to boiling temperature. Would be nice to have some publications about this.

 

[wwfeldman]

the purpose of a spark gap is to arc over at some voltage
if you insert it into mineral oil, or (almost) any other fluid, it will (likely) require more voltage to arc over.
or you get to move the two metal plates closer together.

once you arc in mineral oil, there will be some chemical changes. as there are more arcs in the oil, the degradation and contamination will continue.
after a while, the spark gap will not operate as it did at the beginning of its useful life.

since you want it for a tesla coil, it will be exercised a lot.
I think you're better off with a spark gap in air, with a cooling fan as needed. its cleaner, easier to use, build and replace.

 

[Zak28]

the purpose of a spark gap is to arc over at some voltage
if you insert it into mineral oil, or (almost) any other fluid, it will (likely) require more voltage to arc over.
or you get to move the two metal plates closer together.

once you arc in mineral oil, there will be some chemical changes. as there are more arcs in the oil, the degradation and contamination will continue.
after a while, the spark gap will not operate as it did at the beginning of its useful life.

since you want it for a tesla coil, it will be exercised a lot.
I think you're better off with a spark gap in air, with a cooling fan as needed. its cleaner, easier to use, build and replace.

Did you mean by chemical changes there are metallic ions emitted from the electrodes into the oil?

 

[wwfeldman]

Did you mean by chemical changes there are metallic ions emitted from the electrodes into the oil?

i wasn't talking about metal ions (or even little bits of metal in the oil), but that is a possibility also.

i was thinking about any arcing in the oil changing the composition of the oil by breaking molecules and perhaps re-combining into new things. When there is a lot of energy flying around, one simply does not know.

 

[dick_freebird]

You could consider the spark gap as a tiny "FIB milling
experiment". Every electron or ion impact stands a
chance of kicking off material. In a non-vacuum there is
also the possibility of chemical action.

How about a spark plug, with its electrode already chosen
for durability in high temperature explosive operation?
Just be sure the polarity favors the tungsten / platinum /
iridium / whateverium tip taking the abuse for the most
part.

Bonus, cheap and replaceable.

 

[Zak28]

You could consider the spark gap as a tiny "FIB milling
experiment". Every electron or ion impact stands a
chance of kicking off material. In a non-vacuum there is
also the possibility of chemical action.

How about a spark plug, with its electrode already chosen
for durability in high temperature explosive operation?
Just be sure the polarity favors the tungsten / platinum /
iridium / whateverium tip taking the abuse for the most
part.

Bonus, cheap and replaceable.

I was considering thoriated tungsten rods since they are tiny in comparison to those spark plugs which cannot have their gaps adjusted.

This is the issue, there arent publications dealing with immersed spark gaps. Im sure immersion would accelerate spark frequency since liquid would quickly fill the void and wouldn't ionize as rapidly as air would.

 

[wwfeldman]

I was considering thoriated tungsten rods since they are tiny in comparison to those spark plugs which cannot have their gaps adjusted.

This is the issue, there arent publications dealing with immersed spark gaps. Im sure immersion would accelerate spark frequency since liquid would quickly fill the void and wouldn't ionize as rapidly as air would.

why would immersion lead to "accelerated spark frequency" ? that seems contrary to " ... wouldn't ionize as rapidly..."

 

[FvM]

There's much literature about oil and gas filled HV switches, some results can be probably applied to sparc gap behaviour. I agree with wwfeldman that your assumptions are rather speculative.

I expect that degradation of stainless steel electrodes is mostly due to electron and ion bombardment. It won't be lower for an immersed sparc gap.

 

[Zak28]

why would immersion lead to "accelerated spark frequency" ? that seems contrary to " ... wouldn't ionize as rapidly..."

If there is enough oil it would not saturate with impurities quickly. Agitation and filtering can also clear it of impurities and increase spark frequency.

 

[Nik_2213]

IIRC, you need very special oils, which may be toxic. Alternative would be SF6 gas, but that has powerful greenhouse effects and may not be available to the amateur experimenter...

IIRC, some big spark-gap transmitters used a pair of rotating metal disks as the arc gap electrodes both to allow them to cool and to distribute the damage evenly...

 

[wwfeldman]

SF6 is good as an insulator for short term use.
Sparking in SF6, repeatedly, as in a Tesla Coil, is dangerous
The SF6 reacts with water vapor i air and you get HF and SO2 (?)
HF is very dangerous

 

[dick_freebird]

If you want negative (differential) resistance then you need
an ionizable medium.

Here's a big tank of SF6, used to corral and abruptly release
a megavolt or so to produce a flash X-ray burst. I have one
of the burnt-out "capacitor pucks" from inside its little brother
(only a 750 keV) in the office as "wall art".

 

[Zak28]

If you want negative (differential) resistance then you need
an ionizable medium.

Here's a big tank of SF6, used to corral and abruptly release
a megavolt or so to produce a flash X-ray burst. I have one
of the burnt-out "capacitor pucks" from inside its little brother
(only a 750 keV) in the office as "wall art".

Thats quite a container. Whats it lined with and is it insulated from inside?

That wall art is missing from your posts or is it bound to certain grounds with prohibition to dissemination?

Im not sure what capacitor pucks are, maybe you referenced thyristors or large ceramic doorknob capacitors, in which case they might need a plurality of screws/nails to maintain attached to a wall since they can be quite sizable.

...It won't be lower for an immersed sparc gap.

It should last longer since oxygen is absent along with substantial thermal transfer the metals remain with less hot areas which includes conducting surfaces.

 

[Easy peasy]

for standard over-voltage protection, e.g. 3 - 6kV lightning protection, two brass acorn type nuts set 3mm apart is a good standard solution - as chrome ( as in stainless steel) tends to reinforce the oxide layer and the next spark over potential is higher - whereas brass is nice and constant ...

 

[c_mitra]

You could consider the spark gap as a tiny "FIB milling experiment"

In fact there are commercial machines that use electric sparks to machine very hard (but conducting materials)- commonly called spark erosion machines.

Work a bit slow but are pretty good in final finish. The spark produces ions that affect both the electrode and the medium (air or oil or what so ever).

Of course the effect depends on the total energy output per spark event...