In this type of balloon, the internal and external pressure is always the same.
Zero-pressure stratospheric balloons are filled with helium, giving them the ability to carry heavy payloads at stratospheric altitudes. They have valves to discharge excess gas.
Zero-pressure balloons only stay aloft for a short time. As the surrounding air cools at night, the gas in the balloon contracts and the volume of the envelope decreases. As a result, the balloon inevitably starts to sink after its first night aloft.