Super Pressure Balloon

Super Pressure balloon shortly before release. Note the yellow machine on the ground.

While reading about some NASA balloon projects, I came across this picture of the launch of a balloon from an airport in New Zealand. I thought it gave a pretty good idea of the size of the thing.

Super Pressure balloon being inflated. Now you can see how big that machine is.

Especially when you find out that the little bitty yellow device on the ground in the top picture is actually a giant frigging 80 ton crane.

Super Pressure Balloon's route around the bottom of the world.

This was a Super Pressure balloon, designed to maintain a constant altitude for days on end. This one managed to stay aloft for a month before a leak prompted them to bring it down in Australia.
     Most high altitude balloons are zero pressure - the pressure of the helium inside is the same as the air outside. During the day the balloon warms up, the helium expands and the balloon goes up. During the night, it cools off, contracts and the balloon loses altitude. With a Super Pressure balloon, the volume is fixed. It stays the same day and night. Since the volume doesn't change, the altitude doesn't change, at least not as much as a conventional balloon. These things are flying around at an altitude of about 25 miles, three times as high as your typical jetliner.
    I suspect the term 'Super Pressure' is a bit of a misnomer. I suspect you would be hard pressed to be able to measure the pressure in the balloon when it is at altitude. Since there is almost no air at that height, it wouldn't take much to keep the balloon inflated. Just how much pressure? At 33.5 kilometers, which was this balloon's nominal altitude, the atmospheric air pressure is about 19 millibars, so one millibar of pressure is probably enough to keep this thing happy. (Ground level air pressure is around 1,000 millibars.)