Remember that neon tube that came off the pump table so well, burning in nicely; well now it's been up for a couple of months and it doesn't look so good. What happened? The electrodes were cherry red. The glass got plenty hot. The vacuum pump seemed to be working all right. The mercury was good. It must be bad electrodes or glass.
Maybe not. There are some things about neon that we are unable to control: certain phosphors in the glass, physical constraints that force us to use glass or electrodes in sizes that are unsuited for the job and so on. And there are things that we need to control that may be getting left out. One of these is the pressure in the tube while it is being processed.
Everyone, I am sure is familiar with the sputtering of the electrodes that occurs as the result of pumping at too low of a pressure and the resulting stains. Another less known source of staining occurs because of the moisture and trash present in the phosphors at the beginning of the pumping process. Initially bombarding the tube at too high an amperage (which can only occur at low tube pressures) can damage the phosphors.* This leads to darkening through out the length of the tube. Most of the glass / electrode manufacturers that I spoke to say that these phenomena begin to occur at pressures less than .5mm.
There are also problems associated with processing the tubes at too high of a pressure. As the electrode becomes cherry red it is releasing gasses, among them carbon dioxide (this is just a natural part of converting the phosphates in the emitters). If this carbon dioxide is not drawn off by the vacuum pump as it is produced, when it comes in contact with the relatively cold wall of the glass it can change and be deposited as carbon an inch or so up from the electrode shell. This is the beginning of a stain in a mercury tube.
This pressure build-up is not just because of gasses being released by the electrodes but is in addition to the gasses (from moisture, trash, etc.) being released by the phosphors. And different phosphors release different quantities of gasses. Most glass / electrode manufacturers say that this problem can occur at pumping pressures above 3 or 4mm. **
So, problems at too low of a pressure and problems at too high of a pressure -what are we going to do? Obviously we need to monitor the pressure and adjust it to keep it within some boundaries.
This is very easy and simple to do if you have a (mechanical) pressure gauge on your pumping system. They are simple and accurate and make life a breeze; I think mine is great. They also cost about 700 bucks. If you are not so lucky as to have one it is possible to use your U-gauge to do the same thing. This is the method that works best for most people:
1) attach the tube(s). Open the main stopcock and let the system pump down for about 30 seconds. Use this time to check for leaks.
2) close the main stopcock. Close the valve for the U-gauge. Pinch the blowhose near the blowhose valve and quickly open and close the valve; this should let in 2 to 5mm of air. (Make sure the blowhose is clean and dry.) Adjust the pressure to strike an arc and begin bombarding.
3) as the tube heats the pressure will rise inside the tube; you can now monitor this pressure on your U-gauge. When necessary, open the main stopcock to maintain the pressure between 1 and 3mm.
4) as the tube temperature reaches 150 to 175 degrees C, lower and maintain the pressure at 1mm. in order to bring the electrodes up to temperature.
(The bombarding pressures and temperatures used here are an average based on information provided by the major glass / electrode manufacturers. For best results contact the manufacturer of your electrodes for specific instructions.)
In the beginning these may seem like unnecessary steps in an already rushed process but it is crucial to the long term brilliance of the tube. This method also goes a long way in making bombarding a more consistent and repeatable process.
* Actually, it is the activators (primarily metals) in the phosphors that are damaged or poisoned when excessive energy converts this moisture and trash into stuff that combines with these metallic activators and prevents them from doing their job.
** Darkening due to too low pressure is difficult to prove immediately - it usually takes months for it to show up. However staining caused by pumping at too high a pressure is easily verified: using one stick of purple (it seems to release more gasses than anything else) pump it using the prescribed procedures for monitoring the pressure in the tube. Instead of limiting the pressure build-up to 3mm. let the pressure rise as high as possible; 8mm or greater. Burning in at 60 mA you should be able to see this staining within two hours.
THIS MONTH'S TIP: HAND TORCH STAND
After making that beautiful splice with the hand torch do you immediately mess it up while trying to put the torch back on its stand or on some clothes-ripping hook on the side of the table? With these simple legs the torch can be placed on the table with the flame out of harm's way. Use soft aluminum or even coat hanger wire and bend to shape with pliers.
Sorry photo not yet available - KG)
Morgan Crook is a mechanical engineer, tube bender, and president of Neon Design, a wholesale, retail and consulting firm in Columbia, S.C.