Hunting and repairing leaks

If a pinhole appears whilst the vacuum line is in use and cannot be opened, a temporary repair can be made with black wax (Apiezon or similar) as follows The wax is warmed and worked into a short rod about the diameter of a match-stick and about 1 cm long. The region of the pinhole is warmed with a very small flame, the black wax is applied to the pinhole as a spot not more than 2 mm in diameter, and then warmed with a very small flame just sufficiently to flow and wet the glass. Before a pinhole patched up in this way is repaired properly, all traces of the wax must be removed with a solvent, otherwise the charred remains and ash from the wax will spoil the glass and make a proper fusion repair that much more difficult. 

Leak hunting is a craft which can only be learnt with practice and it is bedevilled more than most aspects of vacuum work by Murphy s Law (see Chapter 1). Pinholes and cracks usually appear in the most inaccessible places, and for good reason. Just because a region is inaccessible, e.g. the back of a tube-to-tube joint, near a wall or metal support, the glass-blowing will have been difficult and its quality and finish less than perfect. 

It should not be thought that faults (pinholes or cracks) can appear only at fused joints. The author once spent three weeks hunting a leak which turned out to be a minute crack in the thick wall of a commercial U-tube manometer, and (of course) on the side towards the plank to which the manometer tube was fastened, invisible from the front. 

When it is necessary to dismantle a vacuum line, as much care is needed as during its construction, but the considerations are different. This is because the well-used line will probably have attached to it various reservoirs of solvents and liquid reagents and containers with drying agents (Na Alms, liquid Na + K alloy, molecular sieves, etc.), and there will be condensers 

No vessel containing a highly reactive compound must be opened by hot-spotting in the open laboratory, but any such must be made safe in some other way. The simplest method is to detach it from the line and then smash it, with appropriate precautions, in a safe destruction area. 

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