The Roswell Report: Fact vs. Fiction in the New Mexico Desert

When the atmospheric pressure outside the diaphragm increases to 5 mb. above the internal pressure, compass fluid will be discharged at the rate of 160 grams per minute under a 1-foot head. When the automatic 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 descends 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. Thereafter, the capsule contains a volume of air which has been trapped at the pressure 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 a trapped volume of air that is allowed to escape through a mercury pool as long as the outside pressure is decreasing. As soon as the exterior pressure increases, mercury 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-thousandth of the original volume. The distance between the two electrodes (one under mercury, the other within the capillary tubing) was influenced by considerations of safety and sensitivity. If the distance is less than 6 mm., shaking during launching is likely to move the mercury