TK-31's peculiar spot!

[N2IXK]

Originally Posted by BrianSummers

I am told that IO tubes, and for that matter other high vacuum devices, have a failure mode were natural helium in the air "seeing the vacuum in the tube" goes though the glass as the helium is so small. The helium then accumulates in the tube.

If anyone can confirm or deny this I would be interested to hear what is said.

Certainly possible, assuming you ran the tube in a helium-rich atmosphere.

This is the principle behind the helium leak detectors used on high vacuum systems. Helium atoms being able to diffuse through tiny cracks is certainly true. It often happened with early He/Ne laser tubes, but in the other direction, where the He slowly diffused out of the tube (through the epoxy seals), depleting the mixture of Helium until the laser stopped lasing.

A coworker once was able to rejuvenate one of those tubes by placing it in a chamber of pressurized helium for a while.

Some high speed counter (dekatron) tubes that included H2 or He in the gas mix were prone to issues as well.