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Neon-Processing FAQs

What actually happens inside a neon tube during bombarding?

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As an expert witness, I often analyze failed sign installations and report them in the courthouse. Because most judges possess a logical, but not necessarily technical, understanding, it‘s not always easy to explain the actual source of trouble.

But judges aren’t the only ones who lack a technical background about neon-tube production. The staff that manufactures neon lamps often lacks this knowledge, and, because workers need to know what they‘re doing, I’ll address tube-processing “FAQs.”

Background
To be honest, conditions vary so widely during the neon-tube bombarding process that we can’t definitively research or understand everything that occurs inside the tube during bombarding. This is also why neon-tube processing can’t be completely automated.

However, we can identify the primary physical and chemical interactions in the bombarding process. Bombarding serves several functions. Heating the glass vaporizes contaminants on the inside of the glass tube so they can be pumped away during evacuation. The electrodes’ metal shells are also heated to burn off impurities and activate the chemical emission coating on the shell’s interior.

Vacuum processing should completely remove “vacuum debris” (materials that outgas or decompose in the operating lamp’s plasma environment, inner surface and volume), which can impair lamp operation. The tubulation is the tube’s only exit point, but the vacuum pump can remove only gases, not liquids or solids. Thus, the bombarding process must release absorbed gases, evaporate liquids into gases and convert solid “vacuum contaminants” by chemical/thermal reaction into gaseous compounds the vacuum pump can expel from the tube. The bombarding process’ second goal is to properly turn the “emission coatings” into their active form so the electrodes can operate as designed.

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The initial strike
Why is it so difficult to strike the discharge in a neon tube the first time? Moisture. When the lamp is bent, and the glass tube is still open after having been worked in an open fire, water vapor enters the tube and condenses on the powder layer in the glass’s cold parts. Also, by blowing into the tube, the bender introduces additional moisture (spit).

Water at room temperature has a vapor pressure of approximately 15 mbar. When the main stopcock initially opens to remove most of the air, the tube’s total pressure drops lower than the vapor pressure, which causes the water to evaporate quickly. Evaporation absorbs thermal energy from the surrounding atmosphere and cools the water, which can form heavy, visible, water condensation. (You can feel the glass growing cold in these spots.) This slows the evaporation, and the water can freeze – and ice evaporates very slowly.

The tubulation is the main bottle-neck between the vapor-release point and the point at which the gases are removed, which results in a tube atmosphere of several millibars of nearly pure water vapor – the gauge reads lower than 1 mbar pressure.

Water vapor’s high ionization potential requires more voltage from the bombarding transformer to start the initial discharge. Many neon workers quickly decrease the pressure until the gauge reads 0.5 or 1 mbar, but then, insufficient oxygen remains in the lamp to complete the conversion process of the electrodes and won’t permit the glass to properly heat.

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Once the discharge has been ignited for a short time, it will restart much more easily after turning off for a moment. Why? First, in the glow discharge, water vapor partially breaks down into hydrogen and oxygen, and hydrogen has a much lower ionization potential (and starting voltage) than water vapor. Second, some oxygen is converted into ozone and remains ionized in the tube for several seconds or minutes. Ions in the lamp greatly facilitate the restart process.

If a neon shop experiences frequent problems when starting the lamps at 3 to 6 mbar, change to a bombarder with a higher output voltage (but not lower amperage, which would mean a higher kVA rating). In an urgent situation, when a tube won’t start at all, here‘s a trick: Evacuate down as far as possible for two to three minutes. Run the spark coil along the tube; close the main stopcock, and introduce approximately 3 mbar of air. Hit the tube with the spark coil again; quickly set the spark coil aside, and start the bombarder.

Heating glass
In the bombarding process’s initial stage, heat the lamp’s glass as hot as possible, without overheating or partially processing the electrode too early. When you heat the glass at pressures higher than 4 to 5 mbar and at low currents (approximately two to three times the electrode current rating), make sure the electrode shell doesn’t turn even a dull red. If the pressure rises too high (from released impurities), so that the discharge becomes unstable, switch off the bombarder, open the stopcock to reduce pressure to not less than 4 mbar, and continue until the glass is hotter than approximately 450° F.

Electrode processing
The bombarding process must convert the electrode-emission coatings, which, in their operating form, aren’t stable in atmospheric conditions. The emission coatings are introduced into the electrode shell as a compound, which must be broken down under a partial vacuum. For standard, triple-carbonate emission coatings, this chemical reaction starts at approximately 1,750° F and finishes at roughly 1,925° F.

Every (coated) part of the inner shell surface should reach this temperature level and remain at it for roughly 15 to 30 seconds to completely break down convert the compounds. The chemical-crackdown reaction releases gases, which are primarily carbon dioxide. Thus, the pressure gauge might roughly indicate when the conversion has been completed.

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During this conversion process, the lamp’s gas pressure is critical. Each electrode manufacturer’s instructions differ (follow them strictly), but, generally, during the electrode conversion, if you hold pressure lower than 2 mbar, as well as raise the temperature too high, for too long, you can damage the emission coatings. The glow, especially for small-diameter, long-shell electrodes, doesn’t always proceed smoothly through the whole shell.

It’s best to switch off the bombarder; close the main stopcock; wait for three to four seconds until the shells have slightly cooled, and continue to bombard at the same current level and maintain the same pressure.

Final pumpdown
When the tube is at a high temperature during the bombarding process, and the electrodes are glowing evenly, and, even with a closed stopcock at approximately 1 mbar, the pressure isn’t rising, you normally stop the bombarder and simultaneously open the main stopcock. At this point, the vacuum pump removes all impurities and reaction products. At the end of the bombarding cycle, the pressure in the lamp is more than a thousand times higher than the total pressure in the lamp before you can fill the rare gas.

Because of the physical laws of a vacuum (and independent of the vacuum-pump suction power), it will take roughly eight to 25 minutes (depending on tubulation and lamp volume) to reach roughly 1 micron of pressure (0.001 mbar) in the lamp. When pressure is reduced, further impurities begin to evaporate, or they’re released, when the total lamp pressure is lower than the impurities‘ vapor pressure. Shortening the pumpdown and filling lamps while they’re still hot guarantees bad quality and short lamp life. Sorry, you can’t bend physical laws, even if you can bend glass well.

Bombarding-Process Summary
• Read the electrode manufacturer‘s bombarding instructions ,and follow them strictly.
• Keep the tubulation short and wide. Don’t make any restrictions in it.
• Evacuate to as low as possible pressure for two to three minutes.
• Backfill 4 to 6 mbar of dry air (not only nitrogen!).
• Heat the glass at a low current at 3 to 6 mbar of pressure until the glass is hot.
• Reduce the pressure to a minimum 2 mbar, and increase the current.
• Continue until the entire metal shell has reached roughly 1,900° F for 10 seconds.
• Stop, open the main stopcock, and let the lamp evacuate for at least 8 to 25 minutes.

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