Tuesday, August 9, 2016

Stretching 36" Qualatex balloons

Stretching 36" Qualatex balloons


In flight, the 36" Qualatex balloons will stretch over the first few days.  In order to maximize altitude, conventional wisdom is to stretch them prior to flight at a pressure a little higher than they will ever see.  That way, the balloon will climb to maximum altitude right away.  Also, pre-stretching may catch any defects in the balloon prior to flight.

The Numbers

In flight, the balloon will experience about 0.2 - 0.3psi.  A few other pico flyers are pre-stretching to 0.4psi.

Filling rig

My filling rig is shown here, along with one of the fill nozzles I put in the balloon.  Starting at the top, the first valve goes to my helium tank.  The second and fourth (bottom) valve lead to balloons fill lines.  The third one is a short line that goes to my manometer.

I'm holding one of my fill stems here.  The rigid plastic fill stem is actually stolen from a can of Great Stuff Insulating Foam.  It fits neatly into the 1/4" vinyl hose.  The red tape is Silicone Self-fusing adhesive tape.  It provides  a somewhat spongy surface to help seal around the balloon fill hole.

I'm using hose pinch-clamps as show below.

Here's a pinch clamp on the hose, prior to insertion in the balloon.

I inserted the fill line deeply into the valve, so only a little of the silicone tape was showing.

I roll the excess balloon film around, and slide the hose clamp into place.

The gold balloon was just for practice.  The two silver balloons are being stretched in preparation for the next flight.  With two balloons on the filling rig, I can alternate back and forth as I fill and stretch.  Once filled to 0.4 psi, the balloon will stretch and the pressure will gradually go down.  While that's happening, I switch over to the other balloon, by simply turning a few valves, and top up the other balloon.  I'll alternate back and forth for a day or two until they seem stable.

Here's my manometer.  If you look carefully at the valves, you'll see that nothing is open except the manometer at the moment, so the fill rig is pressurized to 0.435 psi.


The rig works pretty well.  I think I have a few tiny leaks, but I can easily get a balloon up to 0.4 psi and I can see it stretching nicely.  While I would like it to be completely leak-free, as my friend Kevin / WB2EMS says, "Good enough is the enemy of better."

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