ible is seen in Table 6.2, taken from reference 

 (3), which shows the numerous subcontrac- 

 tors the prime contractor dealt with at var- 

 ious stages in development and fabrication of 

 ALUMINAUT. 



COMPRESSED AIR AND 

 DEBALLASTING 



The most universally applied power source 

 on submersibles to empty the main or varia- 

 ble ballast tanks of water is compressed air. 

 However, compressed air is useful only to 

 certain depths where the volume and pres- 

 sure required to store it is practical, and 

 where its density under pressure provides 

 effective buoyancy. In many vehicles varia- 

 ble ballast tanks are pumped dry; in this 

 case, the tanks must be able to withstand 

 ambient pressures. The following discusses 

 the application of compressed air for water 

 deballasting. 



A discussion of compressed air is virtually 

 impossible without defining certain terms. 

 The following are taken from the U.S. Navy 

 Diving Manual. 



Gage Pressure (psig): The difference between 

 the pressure being measured and the sur- 

 rounding atmospheric pressure. The zero on 

 ordinary gages indicates atmospheric pres- 

 sure and, except where otherwise specified, 

 almost all pressure readings are gage pres- 

 sure. When the pressure in a tank is given as 

 1,000 psi, this means it is 1,000 psi above 

 atmospheric pressure. When it is desirable to 

 indicate that a pressure is gage, it is custom- 

 ary to express it as pounds per square inch, 

 gage (psig). Gage pressure is a commonly 

 used expression in the submersible field, al- 

 though many manufacturers do not state 

 psig. 



Absolute Pressure (psia): The true or total 

 pressure being exerted, consisting of the 

 gage pressure plus 1 atmosphere of pressure 

 (14.7). Absolute pressure is commonly 

 expressed as pounds per square inch, abso- 

 lute (psia) and this value is always used in 

 equations describing gas behavor. 

 Standard Temperature and Pressure (STP): 

 The volume a gas occupies at 14.7 psia and 

 32°F (760 mm Hg absolute; °C). Under these 

 conditions is derived a Standartl Cubic Foot 

 (SCF). 



Normal Temperature and Pressure (INTP): 



The volume occupied by a gas at 14.7 psia 

 and 68°F. "Normal" is a relative term and 

 can also be taken at 70° or 72°F. 



In addition to the above terms are two gas 

 laws which describe the behavior of air un- 

 der varying conditions: 



Boyle's Law states that if the temperature 

 is kept constant, the volume of gas will vary- 

 inversely as the absolute pressure ivhile the 

 density will vary directly fts the pressure. 



Charles's Law states that if the pressure is 

 kept constant, the volume of a gas ivill vary 

 directly as the absolute temperature. 



These two laws are combined to relate 

 pressure, volume and temperature in a gen- 

 eral gas law expressed as: 



Pi V, 



P,V, 



where: T, ~ Tj 



Pi = initial pressure (absolute) 



V, = initial volume 



Ti = initial temperature (absolute) 

 and 



Pj = final pressure (absolute) 



Vj = final volume 



Tj = final temperature (absolute) 



The air supply for blowing water ballast is 

 carried aboard submersibles in cylindrical 

 containers referred to as either tanks, flasks 

 or bottles. These tanks may be made of an- 

 aluminum alloy, steel or other special mate- 

 rials. The capacity (generally expressed in 

 cubic feet) of such tanks is the amount of air 

 or gas the tank holds when charged to its 

 rated pressure. The rated pressure (also 

 called "service pressure" or "working pres- 

 sure") is the internal pressure to which the 

 tank can be repeatedly filled without causing 

 abnormal metal fatigue. In the U.S., the 

 Department of Transportation maintains 

 regulations for design and manufacture of 

 high pressure cylinders, and, if made of steel, 

 their pressure rating is stamped on the cylin- 

 der. Cylinders with pressure ratings of 1,800 

 to 5,000 psig are available, but 2,400 psig and 

 3,000 psig are most common. A 70-ft^ tank, 

 for example, filled to its rated 2,250 psig 

 would contain sufficient air to fill an enclo- 

 sure of 70-ft3 volume at a pressure of 14.7 psi; 

 the actual physical volume of the tank would 

 be about 2 ft^. 



282 



