A flotation control system comprising: a rigid pressure containing vessel (V, 10, 40, 70, 100, 161, 163, 165); a first source of a flotation medium (M F ) capable of increasing a buoyancy of said vessel in a liquid (L, W) and a second source of an incompressible ballast medium (M B ) capable of decreasing said buoyancy of said vessel in the liquid; a pig (P, P B1 , P B2 67, 97, 127) held in confinement by the rigid inner walls of said vessel and dividing said vessel into reciprocal serial hydraulically discrete rigid compartments (C F , 33, 63, 93, 123, C B , 35, 65, 95, 125), a first said compartment (C F , 33, 63, 93, 123) in discrete communication with said first source for transmitting said flotation medium therebetween and a second said compartment (C B , 35, 65, 95, 125) in discrete communication with said second source for transmitting said incompressible ballast medium therebetween; a first valve (F, 17, 47, 77, 107) connected between said first source and said first compartment for controlling flow of said flotation medium into and out of said first compartment and a second valve (B, B 1 , B 2 , 19, 49, 79, 109) connected between said second source and said second compartment for controlling flow of said incompressible ballast medium into and out of said second compartment, said first valve being operable to fix a quantity of said flotation medium in said first compartment and said second valve being operable to vary a quantity of said incompressible ballast medium in said second compartment to control an elevation of said pressure-containing vessel in the body of liquid; and a flow meter (Y) measuring said quantity of incompressible ballast medium injected into and exhausted from said second compartment. a rigid pressure containing vessel (V, 10, 40, 70, 100, 161, 163, 165); a first source of a flotation medium (M F ) capable of increasing a buoyancy of said vessel in a liquid (L, W) and a second source of an incompressible ballast medium (M B ) capable of decreasing said buoyancy of said vessel in the liquid; a pig (P, P B1 , P B2 67, 97, 127) held in confinement by the rigid inner walls of said vessel and dividing said vessel into reciprocal serial hydraulically discrete rigid compartments (C F , 33, 63, 93, 123, C B , 35, 65, 95, 125), a first said compartment (C F , 33, 63, 93, 123) in discrete communication with said first source for transmitting said flotation medium therebetween and a second said compartment (C B , 35, 65, 95, 125) in discrete communication with said second source for transmitting said incompressible ballast medium therebetween; a first valve (F, 17, 47, 77, 107) connected between said first source and said first compartment for controlling flow of said flotation medium into and out of said first compartment and a second valve (B, B 1 , B 2 , 19, 49, 79, 109) connected between said second source and said second compartment for controlling flow of said incompressible ballast medium into and out of said second compartment, said first valve being operable to fix a quantity of said flotation medium in said first compartment and said second valve being operable to vary a quantity of said incompressible ballast medium in said second compartment to control an elevation of said pressure-containing vessel in the body of liquid; and a flow meter (Y) measuring said quantity of incompressible ballast medium injected into and exhausted from said second compartment. <EOS>