# Pressure inside a balloon

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 ?

DaveC426913
Gold Member
Sort of depends on what you mean by 'capacity'. It's really a maximum volume before bursting.

The balloon's volume will contain a certain mass of air. The pressure will be a direct function of the balloon's volume at any altitude.
So, when you fill it from 1/3 to 2/3 capacity, you are doubling the mass, but not doubling the volume. Depending on how large the balloon gets, that will determine the pressure.

jack action