In repair shops, Where acetylene will be needed continually for the work at hand, it is best to install an acetylene generator. One of the types approved by the underwriters should be selected. It should be placed in a separate shed outside of the shop. This will* safeguard the workman and the building in case of accident. Nowadays, however, acetylene can be used with perfect safety, provided there is ordinary good sense employed in the installation and use of it.

There are two general types of generators: one in which powdered or granular calcium carbid is fed into water, the other in which water is dropped upon the carbid. The reaction generates considerable heat. This is the element of danger. For this reason, perhaps the safer generator is the style first named, in which the carbid would be quenched in water while giving off the gas. One part of carbid will boil six parts, by weight, of water. Furthermore, water-feed generators give off gas long after the water is stopped, and carbid-feed only for a short time.

Fig. 44. - Diagram of hopper and feed mechanism. Davis carbid-feed acetylene generator.

Some carbid contains phosphates which are decomposed with the formation of hydrogen phosphid. This gas, which comes over with the acetylene in small quantities, is said to have a bad effect on the metals to be welded. While tanked acetylene has been cleansed of both hydrogen phosphid and sulphid, the generator acetylene is not. It is important to use in the generator a carbid that is quite pure chemically. The presence of small quantities of phosphorous can be easily told by the white smoke it adds to the acetylene flame. Sulphur cannot be so easily detected.

One pound of lump carbid gives 4 ½ feet of gas. One pound of ground carbid only 4 feet,1 due to previous decomposition. At this writing the cost of carbid per candlepower-hour is about 4/10 cents for a 24-candlepower burner consuming ½ foot of gas hourly.2

There are a number of good generators on the market, and the purchaser can make his choice from them. The Davis generator is at present recommended by the Davis-Bournonville Company. This is a lump-carbid feeder made in sizes ranging from the portable size, charged with 20 pounds of carbid, up to the largest size of 300 pounds' carbid capacity (Fig. 44). Lump carbid, 1¼ by ⅜ inches, is charged into the hopper at the top, whence it slides down an inclined plane onto a circular pan. This pan hangs on an axle, which rotates it according to the working of the overhead motor, and the carbid is brushed off the edge of the pan when it rotates. A pressure diaphragm controls the feeding apparatus. The operator sets the diaphragm for a given pressure by moving a weight along the lever controlling the diaphragm. The pressure of the gas can be raised to 15 pounds, is uniform, and is safe-guarded by a blow-off. The water levels are also maintained by overflow pipes. The act of opening the hopper to charge the machine locks the motor, while the position of the motor weight shows at once how much carbid is left in the hopper.

The advantages claimed for this generator are:

1 Rules of National Board of Fire Underwriters.

2 Davis Acetylene Company, special information.

1. That being carbid-fed, the resultant gas is always of a safe temperature, because of the great excess of water it is quenched in.

2. The use of lump carbid ensures slower generation, which takes place after the lumps have gone to the bottom. And lump carbid gives at least 10 per cent, more gas than the pulverized stone.

3. The feed motor is effective: the carbid cannot be overfed and is fed as the gas is needed.

4. The water levels are automatically maintained.

5. All parts are accessible.

6. The charging hopper is perfectly sealed from leakage. This generator should be installed in a shed outside of the building where the welding is carried on. The shed should be of sufficient size to allow easy access to all parts of the generator, and should be steam-heated with pipes coming from without, so that the water will not freeze up in the winter. No light or fire must be allowed in or near the shed, because of danger from explosions. The shed should be kept locked and closed to all but the regular attendant, who is an experienced hand.

Acetylene gas is much more dangerous than the other illuminating gases, as it will readily explode in mixtures of more or less air than the other gases. Accidents are not so common now as formerly, but happen often enough to show that there is much carelessness in the installation and management of generators, The underwriters' associations of this country and abroad are very rigid in their specifications concerning acetylene generators.