Examining- Table 6.4, it is seen that air 

 under a pressure of 4,498 psia (10,000 ft) and 

 70°F has a density of 20.59 pcf (pounds per 

 cubic foot), and at 2,246 psia (5,000 ft) and 

 70°F its density is almost half this or 11.43 

 pcf. Seawater has an average density of 64.4 

 pcf; air at 4,498 psia is approximately Vs as 

 dense as seawater, whereas air at 2,246 psia 

 is about Ve its density; hence, air's ability to 

 provide a buoyant force decreases with in- 

 creased depth or pressure. At much greater 

 depths the density of air can reach that of 

 seawater where it supplies no lift whatever. 



Boyle's law states, in part, that the volume 

 of €1 g€is will vfMry inversely as the absolute 

 pressure; in other words, the greater the 

 pressure the less the volume. This is another 

 major factor influencing the use of com- 

 pressed air for water deballasting. For exam- 

 ple. Fig. 6.1 shows the interior of the AU- 

 GUSTE PICCARD with the tanks holding 

 compressed air for blowing main ballast af- 

 fixed port and starboard to the top of the 



hull. There are 42 air tanks of 1.67-ft'' volume 

 each which are charged to a working pres- 

 sure of 3,570 psia. The main ballast tanks on 

 this vehicle, of which there are 12, have a 

 total capacity of 842.5 ft^. If the entire air 

 supply was used to blow ballast at 2,500 feet 

 (1,127.5 psia), 221.6 ft^ of water would be 

 displaced; in order to completely empty the 

 main ballast tanks, almost four times the 

 number of air tanks now carried would be 

 required. In short, the mere physical re- 

 quirements of the air tanks to blow water 

 ballast at great depths would be unaccepta- 

 ble on a weight and volume basis alone in 

 present submersibles. Additionally, the air 

 itself in these tanks would weigh in the 

 neighborhood of 1,206 pounds (70°F; 3,570 

 psia), which is in excess of the payload in 

 most currently operating vehicles. In spite of 

 the disadvantages noted, compressed air re- 

 mains the chief source of water deballasting 

 and buoyancy in the majority of shallow- 

 diving vehicles. 



TABLE 6.4 AIR DENSITY (PCF) AS A FUNCTION OF PRESSURE AND TEMPERATURE^ 



•Taken, in part and extrapolated, from the U.S. Navy Diving-Gas Manual, U.S.N. Supervisor of Diving, Research Rept. No. 3-69, 1 Oct., 1969, 

 by Bateele Mem. Inst. 



284 



