Sputtering ?

[gregebert]

I've noticed black residue collecting on the inside of at least one of my nixie tubes over the past 1-2 years. What's interesting is that it is collecting right where the glass of the tube is touching the glass cover of my case (yes, I goofed on the case; nothing should touch the tubes...). It's a small dot, perhaps 1 mm in diameter so it doesn't bother me much.

While I'm not at all surprised that sputtering is likely to be what's happening, I'm rather amazed that it would concentrate on a single point especially because glass is a very good insulator. Furthermore, the tubes are run at DC so there isn't any high-frequency/capacitance issue as far as I can reason.

Have any of you folks out there who work with neon signage seen deposits on the inside of tubing near supports, etc ?

I wonder if this could be exploited on nixies to keep sputtering from soiling the entire inner surface of the tube and draw it somewhere away from the viewing-area.

[John Rehwinkel]

> I've noticed black residue collecting on the inside of at least one of my nixie tubes over the past 1-2 years. What's interesting is that it is collecting right where the glass of the tube is touching the glass cover of my case (yes, I goofed on the case; nothing should touch the tubes...). It's a small dot, perhaps 1 mm in diameter so it doesn't bother me much.

That may not be sputtering, but mercury condensation. The mercury will tend to condense on the coolest part of the tube, so if it's warm in your enclosure, the contact with the case might provide that cool spot.

Close examination with a magnifier might reveal telltale tiny beads of mercury there.

- John

[Tristan]

I'd say that firstly that it could be also condensed mercury vapour as opposed to the cathode material. The reason it had condensed there is because the temperature of the glass in that spot has been lowered relative to the rest of the glass envelope.

[Tristan]

Beat me to it. I have observed this affect on ZM1040 tubes.

[gregebert]

OK...sounds like time for another experiment: Swap with a non-contacted tube and see if the dot goes away in a few months....

BTW, the inside of the case is warmer than room-temp so the point where the tube contacts the glass cover will be cooler than the rest of the tube.

The clock draws 14 watts, of which ~10 watts is just for the 6091 nixies and their drivers.

[John Rehwinkel]

> OK...sounds like time for another experiment: Swap with a non-contacted tube and see if the dot goes away in a few months....

Or gently warm that part of the case (ideally with the clock off, but it's not required) and see if the spot dissipates. Shouldn't take more than a couple of hours.

- John