Editing Getter (section)

==Flashed getters==
[[File:Opachki dead vacuum luminescent display bednyaga da.JPG|thumb|right|Dead [[vacuum fluorescent display]] (air has leaked in and getter spot became white)]]
Flashed getters are prepared by arranging a reservoir of volatile and reactive material inside the vacuum system. After the system has been evacuated and sealed under rough vacuum, the material is heated (usually by radio frequency [[induction heating]]). After evaporating, it deposits as a coating on the interior surfaces of the system. Flashed getters (typically made with [[barium]]) are commonly used in [[vacuum tube]]s. Most getters can be seen as a silvery metallic spot on the inside of the tube's glass envelope. Large transmission tubes and specialty systems often use more exotic getters, including [[aluminium]], [[magnesium]], [[calcium]], [[sodium]], [[strontium]], [[caesium]], and [[phosphorus]].

If the getter is exposed to atmospheric air (for example, if the tube breaks or develops a leak), it turns white and becomes useless. For this reason, flashed getters are only used in [[hermetic seal|sealed systems]]. A functioning phosphorus getter looks very much like an oxidised metal getter, although it has an [[iridescent]] pink or orange appearance which oxidised metal getters lack. Phosphorus was frequently used before metallic getters were developed.

In systems which need to be opened to air for maintenance, a [[titanium sublimation pump]] provides similar functionality to flashed getters, but can be flashed repeatedly. Alternatively, nonevaporable getters may be used.

Those unfamiliar with sealed vacuum devices, such as [[vacuum tubes]]/thermionic valves, [[high-pressure sodium lamp]]s or some types of [[metal-halide lamp]]s, often notice the shiny flash getter deposit and mistakenly think it is a sign of failure or degradation of the device. Contemporary [[high-intensity discharge lamp]]s tend to use non-evaporable getters rather than flash getters.

Those familiar with such devices can often make qualitative assessments as to the hardness or quality of the vacuum within by the appearance of the flash getter deposit, with a shiny deposit indicating a good vacuum. As the getter is used up, the deposit often becomes thin and translucent, particularly at the edges. It can take on a brownish-red semi-translucent appearance, which indicates poor seals or extensive use of the device at elevated temperatures. A white deposit, usually [[barium oxide]], indicates total failure of the seal on the vacuum system, as shown in the fluorescent display module depicted above.

===Activation===
The typical flashed getter used in small vacuum tubes ''(seen in 12AX7 tube, top)'' consists of a ring-shaped structure made from a long strip of nickel, which is folded into a long, narrow trough, filled with a mixture of [[barium azide]] and powdered glass, and then folded into the closed ring shape. The getter is attached with its trough opening facing upward toward the glass, in the specific case depicted above.

During activation, while the bulb is still connected to the pump, an RF [[induction heating]] coil connected to a powerful [[electronic oscillator|RF oscillator]] operating in the 27&nbsp;MHz or 40.68&nbsp;MHz [[ISM band]] is positioned around the bulb in the plane of the ring. The coil acts as the primary of a transformer and the ring as a single shorted turn. Large RF currents flow in the ring, heating it. The coil is moved along the axis of the bulb so as not to overheat and melt the ring. As the ring is heated, the barium azide decomposes into barium vapor and nitrogen. The nitrogen is pumped out and the barium condenses on the bulb above the plane of the ring forming a mirror-like deposit with a large surface area. The powdered glass in the ring melts and entraps any particles which could otherwise escape loose inside the bulb causing later problems. The barium combines with any free gas when activated and continues to act after the bulb is sealed off from the pump. During use, the internal electrodes and other parts of the tube get hot. This can cause adsorbed gases to be released from metallic parts, such as anodes (plates), grids, or non-metallic porous parts, such as sintered ceramic parts. The gas is trapped on the large area of reactive barium on the bulb wall and removed from the tube.