The pen drops off the shelf at a safe distance below the expected pressure altitude and ballast then flows until the pressure pen reaches the insulating section of the commutator. In order to prevent the overshoot mentioned as one undesirable feature of the automatic ballast system, the high pressure end of the insulator may correspond to the expected maximum altitude of the balloon, any loss of lift due to impurities or escape of lifting gas will cause the balloon to level off at a ceiling within the ballast-dropping range. Continued ballast dropping will result in the rise of the balloon. Thus, an overestimation of the ceiling is not as critical as in the case of the previous system.
(3) Rate-of-Ascent Switch
With the displacement-switch control just described there remain the problems of ballast waste and balloon oscillation resulting from discharge of ballast during rises of the balloon after a descent has been checked. To eliminate this, a ballast-control switch acting on the rate of rise of the balloon is put in series with the displacement switch to close the ballast flow circuit only when the balloon is coming down or floating below pressure altitude. When it is rising, no ballast flow is permitted. This "rate-switch" is seen in Figure 12.
Figure 12. Rate-of-ascent switch.