The tap A must never be used to regulate the supply of acetylene; in fact, after the hydraulic back-pressure valve has been charged with water, it is best to leave this tap always on.
The working pressure for oxygen previously given should not be too rigidly adhered to. Even in the same sizes of blowpipes the conditions must vary slightly, and a little practical experience with each blowpipe will soon indicate the best working conditions. If the flame is not properly regulated it may fire back and go out. If so, the taps C and E should be shut off at once, and a few seconds allowed to elapse before relighting. When work is carried on for a long time at a stretch and the burner becomes warm, it will be found necessary to slightly open the acetylene tap C from time to time. If work is being done which involves the nozzle of the blowpipe being held in a confined space, it is advantageous to cool this end of the blowpipe by immersing it from time to time in a bucket of water. While this is done the gases must be turned off at C and E.
Welding should be done at the apex or outer extremity of the small white cone.
If the hole in the nozzle of the blowpipe gets obstructed at any time through beads of iron being splashed into it, or from any other cause, it may be cleared with a piece of copper wire and cleaned with a wire brush. No steel reamer or other sharp instrument should be used in the hole, which otherwise will be altered in size.
On stopping work the acetylene tap C should be closed first and then the oxygen tap E. When work is completely stopped, the oxygen cylinder should be shut off. The oxygen cylinder valve should never be opened until taps F and E are open, and it should then be opened slowly. In this way sudden impact of oxygen in the regulator is obviated.
The following instructions on the methods of welding copper, cast-iron and aluminium are contributed by a foreman welder.
=Welding Copper.=--Copper to be welded should have its edges bevelled to enable the welding to penetrate the entire thickness of the metal. Bevelling is not generally practised below a thickness of 3/32 in. From 3/32 in. to 3/16 in., a slight open bevel is sufficient; 3/16 in. thick and over, the angle of the bevel should be about 90°. It is not necessary to go beyond this even with great thickness. The bevelling should be regular, especially at the bottom, so as not to produce holes or excess of thickness at the bottom of the bevel.