[figure]
Figure 10. Automatic ballast valve.
The second undesirable feature of the automatic ballast valve system is the lag induced by the use of a minimum pressure switch to seal off the aneroid capsule. This is in addition to the lag of the aneroid itself. If a mercury switch is used, the differential between minimum and seal-off pressure is about 8 millibars; with a less dense liquid, the operation will still require about a 2-millibar difference. If the sealing is done by a fixed pressure switch, it is then necessary to predict the altitude to which the balloon will rise. Failure to reach this height would leave the aneroid open and useless. Deliberate under-estimation of the ceiling causes a relatively long period of uncontrolled slow descent before control begins.
The third unwanted feature is the waste of ballast which flows during both descent and ascent of a balloon whenever it is below the seal-off elevation. Since the balloon is no longer "heavy" when its downward motion has been arrested, flow during the return to the datum plane is needless and indeed
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