What purpose does getter flash serve?

[BmWr75]

I've read multiple times explanations on how a vacuum tube works. But don't recall anyone ever explaining the purpose of "getter flash". A Google search and search of titles in this forum turned up nothing. I know when is turns powder white, that is a definite sign of vacuum loss in a tube.

So, I ask the question, "What purpose does getter flash serve?"

 

[dcgillespie]

Flashing the getter (which creates the brownish-chrome deposits on the envelope of a tube) is one of the final processes in the manufacturing of a vacuum tube, and serves to help eliminate any residual gas left in the tube from the manufacturing process. This is necessary as otherwise, the cathode could become damaged from the presence of this gas. The getter also then serves to absorb any gas that is released from the internal elements during the subsequent use of the tube during its normal useful lifetime. As it does this, it slowly evaporates, causing the deposits to thin out to the point of almost becoming clear on well spent tubes.

Dave

 

[70salesguy]

That's the wrong question.

The "getter" is fired after the tube is sealed. It helps remove any last bit of oxygen in the envelope. It leaves a flash (the silvery patch) inside the envelope. If the seal goes bad, the getter flash turns white.

 

[cubby01]

Someone posted this just the other day: Journey to the Center of a Tube by the folks over at TubeDepot:
http://www.youtube.com/watch?v=WnZRJjmOZb8&feature=mfu_in_order&list=UL

There's a nice visual explanation of getter, getter flash, and why it's there at the 2:20 mark.

The 'how it works' question is related to barium used as the getter material, it's general properties and reactions with oxygen. Based on my limited understanding, it's oxidation state of 2, which means a high degree of reduction, is good for sucking up oxygen molecules.