will cause an overshoot, hence the unnecessary exhaust of some lifting gas.
The effects of temperature on the aneroid capsule of the automatic ballast valving system were eliminated by the use of a ballast switch which uses a vacuum-sealed aneroid, set to permit ballast flow through a valve whenever the balloon is below a given pressure altitude. In this system the minimum pressure switch and the lag caused by its use are eliminated. This displacement-switch control has the disadvantage that the flow which it permits is not proportional to the displacement of the balloon below a datum plane but is constant through the valve. Normally this flow is large to permit rapid restoration of equilibrium. A second disadvantage is the requirement of batteries to supply power to the electrically operated valve. However, the advantage of eliminating the temperature effects on the aneroid compensate for these two comparatively minor disadvantages.
In practice, the displacement switch has consisted of a modified radiosonde modulator in which the standard commutator is replaced by a special bar which is an insulator above a certain point and a conductor at lower levels (higher pressures). When the aneroid pen arm is on the conducting section of the commutator, a relay opens the ballast valve. To prevent excessive flow on ascent, the pen arm rides on an insulated shelf above most of the contact segment of the commutator (Figure 11).
Figure 11. Pressure displacement switch.