The purification of acetylene which is to be used for welding is extremely important and necessary in order to obtain good results ; the degree of impurity present in the generated gas varies with the quality of carbide used, and unpurified acetylene contains traces of lime dust, ammonia, sulphuretted hydrogen and phosphorus, all of which are injurious to the weld.

Most generators allow the gas to bubble through water after generation. It is also usually filtered by passing it through felt or other suitable material before entering the purifier. By these means the bulk of the three first-mentioned impurities are removed, but this is not sufficient and subsequent chemical purification is necessary to eliminate the remaining traces as well as the phosphorus.

Small portable generators for metal cutting are not usually supplied with purifiers ; but all large portable and fixed generators should be provided with purifiers which should be charged with chemical material as recommended by the generator makers. This material should, as a rule, be changed after 100 cubic feet of acetylene per lb. of the chemical has passed through.

When there is any doubt as to the efficiency of the purifier the gas can readily be tested as it issues from the blowpipe nozzle by means of a piece of white blotting paper moistened with a 10 per cent, solution of silver nitrate. If the paper turns black in a few seconds when the gas is allowed to impinge upon it, considerable impurity is present and the purifying material should be changed.

If on the other hand the colour of the blotting paper remains unchanged or turns only a light brown colour, the gas may be regarded as sufficiently pure for welding purposes.

Dissolved Acetylene From A Cylinder

If desired, acetylene may be obtained in cylinders, in which the gas is dissolved in a porous material soaked in acetone. Acetone is a liquid hydro-carbon which has the property of absorbing 25 times its own volume of acetylene at atmospheric pressure, and it continues to do this for every atmosphere of pressure that is applied to the gas.

All injector blowpipes work as well with dissolved acetylene (D.A.) from a cylinder as with generated acetylene, although its cost is of necessity higher than that of gas obtained by a generating plant direct. D.A. has the advantage of purity and simplicity, and its extreme portability affords an excellent and safe method of transporting acetylene, rendering it frequently useful for welding work and for use on board ship, in buildings, or confined spaces, where generators are not available or generated acetylene cannot be conveniently piped to the job.

Gas must never be drawn from a D.A. (dissolved acetylene) cylinder without an acetylene regulator being interposed. See full instruction on page 87 and carry out carefully.

The Oxygen Cylinder

See full details and instructions on page 88 and carry out carefully.

Instructions and Precautions for Operating Welding Blowpipes.-We have dealt separately with the various components of a complete oxy-acetylene welding unit. Here we shall indicate the instructions and precautions which should be observed in starting up and operating the welding process.

Oxygen Cylinder

Read carefully through the notes under the heading of " Oxygen Cylinders" page 88.

Regulator

Couple up the oxygen regulator to the cylinder after making sure that it conforms with the conditions specified under the heading, " Endurance Pressure Regulator," page 82.

Tubing For Oxygen And Fuel Gas Supplies

See the notes re this subject on page 89.

Hydraulic Valve

Before the generator is put into operation the hydraulic back pressure valve must be filled with water in accordance with directions on page 83. This is a precaution of the utmost importance, and subsequently, at least once a day, and always after any displacement of water by backfire, it should be replenished in the manner indicated. The valve should be dismantled at intervals and cleaned ; when this is done make sure that the vent pipe and all passages are clear.