The most inner tube contains the lasing medium (the discharge). Immediately surrounding this tube is a larger tube through which the cooling fluid flows, and finally there is the outer tube (the one you actually touch) which acts as a reservoir holding extra gas to replenish the discharge.
The three tubes, or at least any two of them, come together at some point inside (this is obvious since they can not be held within each other and never touch). Since the 'glass type' of the three tubes are sometimes different (depending on what country it was made in) grades of different glass will be used to seal the ends together. In addition, at one end or both ends there is generally a glass-to-metal seal, which again calls for various grades-of-glass to make a smooth transition from glass-to-metal (smooth here refers to the coefficient of expansion).
If a crack has developed along a point where two or more parts (grades) have come together, it may at times not be possible to remove the crack, mainly because, as the tube is heated for sealing operation, the crack 'runs away'. This becomes a losing game because as you follow the crack with the torch, it gets further and further away from you.
Therefore, depending on where the crack is and what the tube material is and how it was constructed, the glass shop may declare that: 'this tube is not salvageable'. And when that happens we have a tube that is 'Beyond Economic Repair'.
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