The National Oceanic and Atmospheric Administration (NOAA) developed this release valve to enable a more controlled descent of a balloon rather than the typical burst and parachute method. Mike Manes describes it:
These photographs are of a valvedown device for latex balloons that was developed at NOAA to produce a controlled near-constant descent rate, sans parachute, for their vaporsondes. This prevents adiabatic cooling of the airstream around the balloon from screwing up the PPM-scale water vapor measurements in the stratosphere. NOAA uses a 4" diameter valve on a 3000 gm balloon with a 5 or 6 lb payload, and regularly gets a 1000 fpm constant and very smooth descent. There's still He in the balloon on landing, making visual tracking a piece of cake. They also use a GPS-equipped cutter driver mounted right at the valve to fire it at a predetermined MSL altitude.
We "cloned" a version for a 1200(?) gm bag, using 3" PVC pipe and an acrylic valve plate. The plate is held in place with 15# monofilament fishing line which is cut by a burner on ground command. During our test flight, the cord running up the lift line got twisted like a double-knotted model airplane rubber band, thus developing a bad case of air-gap-osis before we were able to fire the cutter.
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