A zero pressure balloon is made from very thin (7.5 micron) plastic sheet, like a laundry bag. It is filled with helium through a tube connected to the upper portion, and it has a vent to the atmosphere at the very bottom.
The balloon is filled only partially on the ground. As the balloon rises, the gas bubble expands to fill out the balloon from the top downward. When the gas bubble expands to completely fill the balloon, excess gas exhausts through the vent at the bottom; at this point, the ascent stops, and the balloon will hover at a constant altitude.
At night, after the sun goes down, the gas bubble cools and shrinks. This causes the balloon to begin to descend. If the balloon mission must go for several days, some ballast material, such as sand, can be dropped to maintain altitude. But during the next day, the gas will heat and expand again, but since the system is now lighter, it will ascend higher and dump some more gas. And so on until the supply of ballast is used up. Eventually, it will hit the ground, but this may take several days.
A zero pressure balloon MUST be provided with a means of terminating the flight to keep it from floating in civil airspace at night. This is usually vent at the top of the balloon which can be opened via a radio command. Another method is to put some weight at the top and to cut away the payload; then the weight at the top will cause the balloon to turn upside down so that the gas can exhaust through the vent.
Because of the difficulty and possible failure of the termination means, EOSS prefers to use rubber (latex) balloons, which will self-destruct (almost always) at altitude. We have used a 3000 gram latex to lift over 10 kg of load to nearly 30 km altitude; in fact, we will do that tomorrow on EOSS-73!