here is something confusing me about air pressure inside balloons. I got three questions: Assume I am sitting at sea level, with a balloon. I fill it to 1/3 its capacity with air. Then I place an absolute pressure sensor at its opening. 1- What pressure value do I read ? Do I get the 1013 mbar which is the atmospheric pressure at sea level ? Because I read that the pressure inside the balloon must equal the outside pressure. Now, I fill it to 2/3 its capacity with air. 2- What do I read now ? Is it double the pressure I got at question 1 or is it still the same ? Remember, the outside atmospheric pressure did not change. It is still 1013 mbar. Also note the balloon has doubled its air mass. Now, with 2/3 its capacity still filled with air, I move the balloon up to an altitude where the atmospheric pressure is half that at sea level. 3- What pressure value do I read there ?