The S trap is, upon the whole, the best, but it should be provided with a vent-pipe to prevent siphonage.
Typical simple forms of vent-pipe suitable for acetylene apparatus are shown in Fig. 7.
It is indispensable that every acetylene apparatus shall be fitted with a safety-valve, or more correctly speaking a vent-pipe.
Nevertheless this form of vent-pipe produces a gurgling noise, and would be better for a trumpet-shaped mouth.
So tall a vent-pipe, however, seriously raises the centre of gravity of the bell and may make it top-heavy.
No water enters the vent-pipe from the holder, and therefore no gurgling or irregular pressure is produced.
Every apparatus must be fitted with a safety-valve or vent-pipe terminating in a safe place in the open, and of adequate size.
There are two minor points in connexion with this form of vent-pipe often overlooked.