I couldn't resist until Tuesday. Due to the lack of wind I only was able to fly up to 5 m without any load.
Hydrogen balloons have the advantage of not needing any wind so I made a few calculations. Since I want to make the project as cheap as possible I'm going to focus on party balloons. I measured the pressure inside an inflated balloon to see if there was any significant overpressure that I would need to take into consideration. The overpressure is very small (about 30 hPa) so I'm going to neglect it.
Using Archimedes' principle I can get a rough idea of the maximal mass that a spherical balloon can lift up :
with a radius of 15 cm that makes a mass of about 15 g. Not that much so I'll have to use multiple balloons or find a cheap alternative for a bigger balloon.
Hopefully there will be more wind tomorrow.