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What does a neon pumping system really need?
For many years at the ISA Sign Expo, I’ve heard salespeople doom neon to the graveyard. Nonetheless, after nearly 100 years, neon signs are still shining and will definitely continue.
At major industry shows, companies sell the newest neon-equipment developments as “must haves.” Don’t trust any salespeople. Decide for yourself after having carefully weighed what you really need. Do your homework well, especially if you’re considering a new neon-pumping system.
Vacuum
A complete (read: as good as possible) vacuum must exist inside a neon tube, and that vacuum must be achieved before the tube cools down after bombardment. If this doesn’t happen, the impurities loosened by the “cooking” resettle on the tube’s walls and won’t be removed. I‘ve discussed the pumping characteristics of different pump principles awhile ago (see ST, November 2006, page 26).
A two-stage mechanical pump won’t produce standard-quality neon; you need a secondary, or “high vacuum,” pump. The mechanical, or “roughing,” pump should have at least a 12- to 16-cfm capacity. The secondary pump can be a diffusion/aspirator pump or a turbomolecular/molecular drag pump (quite an expensive investment, but it consumes less power and has a quick startup).
Because of different characteristics, an approximate 20- to 40-liter/second diffusion pump or molecular drag pump is sufficient for processing two, 8-ft, 25mm-diameter, cold-cathode tubes at a time, but a pure turbomolecular pump should have at least a 100-liter/second capacity.
The manifold should be short and wide; the bore of the vacuum stopcocks, at least 12 to 15mm; the manifold tubing, not smaller than 20 to 25mm up to the tube connection. The only obstruction between the neon tube and pump should be the electrode tubulation; all other tubing should be large diameter.
For precise filling, a good needle valve is a must. Cheap valves/transfer systems tend to leak or stick before they can pay for themselves. The manifold must at least provide connectors for two (one flushing, one filling) or three (flushing, red and blue) laboratory, standard, 12-liter (1 liter at 12 bar), aluminum, gas canisters.