General comments on safety

Vessels under vacuum do not explode but implode, and the consequences may be significantly worse than the pressure difference of 1 atm suggests. Implosions are, however, very rare and usually result from apparatus being incorrectly designed or handled. The author, with 40 years of laboratory experience, has never seen an implosion. 

Because flat glass surfaces are weaker than outwardly convex ones, and outwardly concave ones are even worse, no such surface of more than a few square centimetres should ever be evacuated on the convex side, and the wall-thickness must, of course, be adequate. 

Ring-seals of all kinds tend to be mechanically weak and sensitive to thermal shock. Tungsten-through-glass seals and pockets for thermocouples or conductivity probes are the most frequent instances of ring-seals on vacuum equipment, and these should never be subjected to extreme cold. 

A warning about glass blowing The flame of a glass blowing torch should never be directed into the neck of a flask because the mixture of air and unburnt gas is very likely to explode violently. 

Since one of the major reasons for working with a vacuum line is the sensitivity of the materials being handled to oxygen or water, it follows that many of these materials present severe hazards should an accident occur. Of course, these chemicals must also be introduced somehow into the vacuum 

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