Got the Right Gas?

Contrary to the title, I won’t be discussing "good" or "bad" gas in this column. Rather, I’ll address available filling gasses for neon tubes, and what is most suitable for which application.

Historically, nitrogen or carbon dioxide filled the first luminous tubes; in that era, noble gases were unknown or unavailable. Gases used with "Moore tubes" weren’t inert; thus, they underwent a chemical reaction with electrode material. Eventually, the tube consumed the gas supply. To remedy this, Moore invented tubes equipped with an automatic-refill valve that supplied additional gas when the tube emptied.

George Claude’s landmark innovation — the use of inert, noble gases to fill Moore tubes and achieve an "unlimited" lifetime — succeeded because noble gases don’t endure chemical reactions (except xenon under very exotic conditions). This chemical inactivity occurs because of the electron constellation in its atoms. All possible positions in the outer electron shell are occupied, so electrons of other atoms have nowhere to dock. Nor are there free surplus electrons the gas could easily share with others.

How are noble gases made?

Today, we can easily obtain all noble gases (except radioactive radon) in significant amounts. With the exception of helium, which is found in a few oil wells, all rare gases are extracted from air. Table 1 lists the average gaseous chemical composition of the air we breathe.

Gas extraction is completed through boiling (evaporation) of liquefied air. Each rare gas has a different boiling temperature; thus, they separate from other gases and remaining air components.

For commercial oxygen production, air is cooled to -320