Compressed Air Work And Diving 1909

CH APTER II. Stage Decompression (conlinued). Table IV.—'This table is for use in caisson or tunnel work where long shifts may be employed by reason of the comparatively low pressure usually necessary for this class of work. With such long shifts and low pressures the calculated theoretical rate of decompression is nearly uniform atter the first rapid drop in pressure, and the rules for clecom- pression are greatly simplified by adopting uniform de- compression. The table gives the number of minutes for each pouncl of decompression after a first rapid drop in the proportion of two to one in absolute pressure. Suppose, for instance, that men working at a pressure of 30 Ibs. came out for a meal after three hours’ exposure, they would first decompress rapidly to J° +1 = 22^- Ibs. absolute pressure, or 7^ Ibs. by gauge. They would then lower the pressure 1 Ib. every six minutes, taking 6x7! = 45 minutes to pass through the air-lock. After a seconcl or thircl exposure the time required would be 7 x 7^- = 52^- minutes. After six hours or more continuous exposure 8x7^ = 60 minutes would be required. It is clear that in order to economise time spent in the air-lock it will be better for pressures over 2 5 Ibs. to keep the men continuously under pressure during each shift, and