As there have been over 100 hits on this topic, I shall carry on mapping my progress, just in case some other idiot wishes to build one too. 
It is old vintage sparking , so near original radio transmission topic too.
I hunted around for some small valves for gas inlet/air outlet.
As the whole rig is only 200mm dia x 250mm high I didn't
wish for large ones, so I bought two model railway engine valves. (60mm high)
Both mounted on the top acrylic plate where the argon gas is not inclined to escape
anyway as it is heavier than air.
The rig will be tilted to one side to fill the chamber, argon pushing the air up
and out.....I hope. If a problem I shall have to pipe the gas to the bottom of the chamber.
Now how to know when the chamber is full?
I shall use a lighted match burning at the exit valve and when it goes out
I should be nearly there !
Is there a better way?
It is old vintage sparking , so near original radio transmission topic too.
I hunted around for some small valves for gas inlet/air outlet.
As the whole rig is only 200mm dia x 250mm high I didn't
wish for large ones, so I bought two model railway engine valves. (60mm high)
Both mounted on the top acrylic plate where the argon gas is not inclined to escape
anyway as it is heavier than air.
The rig will be tilted to one side to fill the chamber, argon pushing the air up
and out.....I hope. If a problem I shall have to pipe the gas to the bottom of the chamber.
Now how to know when the chamber is full?
I shall use a lighted match burning at the exit valve and when it goes out
I should be nearly there !
Is there a better way?
