A glass flask is open to atmospheric pressure through a fine capillary tube. With various rates of change of pressure, various differential pressures exist between the air in the flask and the outside air. This pressure difference controls the level of liquid in a manometer switch, filled with 24% hydrochloric acid. When the internal pressure is 0.2 mb more than the ambient pressure, the switch opens and ballast flow is stopped even though the balloon may be below the floating level. (The switch is set so that a rate of change of .1 mb/minute acting for three or more minutes will open the switch.) By thus restricting flow when the balloon is rising, balloon oscillations are minimized and ballast is conserved. A sketch of this operation is shown as Figure 13.
Figure 13. Height-time curve, showing ballast control action.
Since the rate switch is much more delicate than the displacement switch, safety considerations have caused the combined control to be supplemented by a pure displacement switch control. In this, the conducting segment of the pressure modulator is divided, and only a limited pressure height range (set for desired floating level) is controlled by both switches in series. If the rate switch is damaged at launching (by spilling some of its electrolyte, for instance) or in flight (perhaps by evaporation of the electrolyte) and the balloon descends, simple displacement control becomes effect when the high pressure lower altitude) segment of the conductor is touched by the pres-