Signs of the Times - July 2016 - (Page 28)

Lig hti n g T e c hn i ques By Marcus Thielen Marcus Thielen is a physicist and lighting-industry consultant from Duisburg, Germany. Grace Under Pressure Demystifying proper tube-filling techniques N eon signs are known for their lengthy lifespans - some have been reported to operate 24/7 for more than 60 years. But, when a neon sign reaches its end, when the tubing has gone bad and a signshop is called to make a repair, a sign's smallest neon tube is usually the main culprit in its failure. Looking more closely at why the tube failed (often prematurely), the neon shop typically claims that we've "done it the same way as ever," pointing to a material defect. But, after digging deeper, the tube-filling pressure - often set to follow a chart supplied by a transformer manufacturer - turned out to be incorrect. The person pumping the tube is always blamed first, although the chart he followed has often neglected important facts. I'll outline many of these facts. However, we can summarize that not all parameters affecting a neon tube's lifetime are known, including which tube-filling formula was followed. The analysis will remain Fig. 1 - The typical atomic structure of lead-free Sodalime glass. Gas can be occluded within voids inside the meshes. 28 SIGNS OF THE TIMES July 2016 Photo 1 - The neon tube was damaged by internal sputtering. The metal deposited inside the shell and on the glass in front has a spongy structure which has taken up (occluded) most of the filling gas. incomplete because the physics of gas discharges and plasma are still subject to ongoing research. Experience has shown that neontube filling pressure influences the tube's lifetime by its fourth or fifth power. Gas consumption Historically, the first gas-discharge lamps (Geissler tubes, Moore lamps) were filled with gases that could undergo chemical reactions, and most of the reaction products were solids. Given the limited amount of gas inside the tube, its functional lifetime was exhausted when most of the gas was lost to chemical reactions. Once Georges Claude pioneered the use of neon (and other noble gases) to create luminous tubing, chemical reactions were eliminated because, with very few exceptions, neon doesn't undergo chemical reactions. This greatly extends the lighting's lifetime. If chemical reactions are eliminated, how else may gas dissipate within tubing? First, gas may diffuse through the glass. However, except for helium gas combined with quartz glass - a rare luminous- tubing combination - this very rarely happens; the common gases used in neon signs (neon, argon, krypton) are practically impermeable with all glass types. So, ignore this as a possible reason. The next process to consider is occlusion. Occlusion refers to gas being absorbed in interstitial pores, like water in a sponge. The structure of typical soda-lime glass (Fig. 1) bears tiny voids between silicon dioxide tetrahedron. Neon gas can migrate there. The amount of gas occlusion increases with time as ionized neon atoms are shot with high kinetic energy into the voids. This process absorbs some of the neon, which you can see if you repair an old, clear red-neon tube. When the glass gets hot during bombardment, the gas is released from it and the tubes often turn a reddish-orange color, as if they're already filled with neon gas. Metal methods The most important occlusion process involves metals. Unlike glass, metals are polycrystalline materials with intergranular membranes or voids. Here, the inert gas can and will go in. The gas can be

Table of Contents for the Digital Edition of Signs of the Times - July 2016

Signs of the Times - July 2016
Columns & Departments
ST Update
Technology Update
Integrating High-End Effects in Your Design
Vinyl Apps
Strictly Commercial
Lighting Techniques
Moving Message
Technology Review - Computerized Cutters’ Accu-Form
Technology Review - Cyrious software
Design Matters
New Products
Signs that Invite
Horseshoe-Shaped Window Graphics
Driving West
State of the Industry
Returns to Glory
Enter ST’s Vehicle Graphics Contest
Advertising Index
Editorially Speaking

Signs of the Times - July 2016