When the atmospheric pressure outside the diaphragm increases to
5 mb. above the internal pressure, canpass fluid will be discharged at
the rate of 160 grams per minute under a 1-foot head. When the a utanatic
ballast valve is completely open {at 6.5 mb. pressure differential}, 300
grams per minute will flow.
The automatically operated needle valve is held closed by a loaded
diaphragm until the balloon reaches altitude. This diaphragm is open to
the atmosphere until the balloon d ascends from the minimum atmospheric
pressure attained. At that time, an electrical contact is made, firing
a squib which seals the diaphragm mechanically from any .further access to
the external air. ntereafter, the capsule contains a volume of air which
has been trapped at the pres·sure and temperature existing at the time of
operation of the sealing switch. When the ambient pressure increases to
the point where the entrapped air is compressed below this original volume,
the diaphragm will withdraw the ballast control needle valve allowing
ballast discharge to occur •
Figure 14 shows the minimum pressure switch which makes the electrical
contact at the time of seal-off. It consists of at r!<pped volume of air that
is allowed to escape througn a mercury pool as long as the outside pressure
is decreasing. As soon s.s the exterior pressure increases, merc\lry is
drawn into the tube making the seal off contact between two electrodes.
The dimensions of the air chamber and capillary tubing are chosen so
that during operation the change in the volume of the air would be less than
one one-thousendth of the origi~l volume. The distance between the two
electrodes (one under mercury, the other within the capillary tubing) was
influenced by considerations of safety an4 sensitivity~ If the distance is
less than 6 mm., shaking during launching is likely to move the mercury
- 8 -