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In Reply to: Re: "Disappearing" Getter Question posted by Jim McShane on March 22, 2006 at 05:38:55:
Jim,
Follow Ups:
Okay, here's my understanding - which I would be GLAD for someone to correct if I'm wrong.Since the barium (that's the usual material in receiving tubes) deposited on the glass got there when it condensed, the barium has a specific amount of mass. The barium was heated to a gaseous state ("flashed") when the tube was made, and the gas condensed on the cooler glass outer surface of the tube. So while small, there is mass in the getter flash deposit.
When a gas molecule (other than the inert "noble" gases) comes in contact with the barium it is absorbed and held. Therefore the mass of the getter increases as it "disappears". But the material must still be within the tube glass envelope.
Which leaves only two possibilities as I see it:
1. The getter barium (now barium oxide or some other compound) is present in the tube in very small solid particles, or...
2. The getter barium (now barium oxide or some other compound) is present in the tube in a gaseous state. For this to occur it seems to me the gas formed by getter "evaporation" would have to be a "noble" gas (like helium, for instance - a gas that will not chemically react with any other element) that can circulate harmlessly in the tube.
...are definitely NOT gases at room temperature. But as a thin layer they could be transparent, hence you don't see them anymore. Also, as you say, these compounds may accumulate into small partcles instead of a continuous metal film, so the film becomes partly transparent even if the material in the particles is not.
The actual getter is a small ring or structure in the tube which is filled with a metal (usually Barium) and then heated to a very high temperature causing a flashing of the getter material. The silver coating you see inside a tube is the result of this operation. The flashing is consumed by the residual air and other gases in the tube left over from the original evacuation as well as the additional ones produced from operation.The amounts of flashing present does not necessarly determine whether a tube is good or bad. I have several Tung-Sol 6550's with almost no flashing left and they still test nearly new and sound fine. However, tube life is not exactly linear, so it can mean the tube is nearer the end of its usable life.
Hope this helps…Jim, I'm sure could add to this.
Regards,
"Kico"
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