[trophyhead]

There are several approaches that can be taken with balloons. By design, they do not expand like a weather balloon does. One wants to put in enough lifting gas to make it fly high (mostly to avoid clouds that will bring down a balloon with such little free lift), but not enough that it will burst.

In the referenced article, an SBS-13 balloon is used. It's made from a clear Mylar-like material and will fly at 40000ft+ when filled with helium (higher with hydrogen). This is the 'gold standard' of balloons and can fly for months at a time as it's over most weather. The downside is that they cost close to $200.

Another approach is to buy a cheaper 36" diameter balloon made of similar material from AliExpress that cost about $1. These are often pressurized to a certain differential pressure to 'stretch' the balloon before launch and gain some extra volume, as well as to QA the quality of the balloon (the QA on a $1 balloon isn't extensive). If filled with hydrogen, these can fly at 35000 ft+, a bit lower for helium. Usually two of these cheap balloons are used for a launch for more lift, though a single one can be used and a lower altitude reached.

A light payload is desirable as it will increase altitude. I've seen as light as 5 grams and the heavier ones are closer to 20g. One can calculate the free lift where the differential pressure of the balloon at altitude will pop it, but it's usually around 7g (so the balloon + payload + 7g will neither rise or fall before launching).