Karel.
Blowing the ballasttanks is done under different circumstances and you are using instruments and experience. F.inst if you want to ascend by ballasttanks from larger depths say 50 meters you use more precious HP air as the counter pressure would be 5 kg/cm2. It takes an experienced controlroom engineer to admit the right amount of air into the ballast tanks which are never blown empty at this depth, the controlroom engineer uses the blowing pressure, the counterpressure ( depth), the time and experience admitting the right amount of air. The aim is just to blow so much air into the the tanks so the submarine start to ascend, then shut the blowingvalve and the air in the ballast tanks expands, forcing out the ballastwater. As the counterpressure reduces it increases the acceleration of the ascend. If you blow too much air into the ballasttanks the tanks are empty before your uncontrolled ascend which is waste of energi, a lot of bubbles and dangerous as the sub break the surface without control. The correct ascend is: you take the sub up to 14 meter by hydroplanes, make a surface sweep with the periscope and then blow your ballasttanks controlled by checking the depth gauge, stop blowing before completely surfaced, then start the diesels and blow the residue air by the diesel exhaust. The exhaustblowing is done in a special sequence starting with the saddle tanks having the smallest resistance being almost at the surface. You are able to check the counterpressure (exhaust) manometer when the tank is empty but normally you checked when the exhaust escapes out of the Kingstons.
Tore