# How do you measure the air pressure inside a balloon?

1. Feb 20, 2013

### thecooltodd

Unfortunately when I google the answer, the results are just how to use a balloon to measure air pressure. I want to be able to monitor the internal pressure of a balloon using a digital barometer.

1) How would I go about setting up such a system?
2) Would the barometer have to be on the inside of the balloon or are there ways of measuring the air pressure inside by letting out some air?
3) What type of digital barometer should I use?
4) What is the typical air pressure of a balloon?

2. Feb 20, 2013

### Staff: Mentor

You would use an "Absolute" pressure sensor, as described at this wikipedia page:

http://en.wikipedia.org/wiki/Pressure_sensor

You could use a relative/gauge pressure sensor if you could pass atmospheric pressure air through a tube that goes through where the knot normally goes on the balloon (like seal it with a rubber band around the tube).

3. Feb 23, 2013

### Staff: Mentor

If you know the deformational mechanical properties of the rubber comprising the balloon, the uninflated surface area of the balloon, the undeformed thickness of the rubber, and, if the balloon is close to a perfect sphere, then you can calculate the gas pressure inside the balloon as a function of the balloon diameter. Then, for any measured diameter, you will know the pressure inside the balloon.

The pressure inside a balloon will typically be less than 2 psi above atmospheric.

4. Feb 23, 2013

### DrZoidberg

You would need a digital pressure gauge, not a barometer. But those can cost several hundred dollars.
Why not instead simply use some aquarium tubing plus water to measure the pressure. Take a bottle with a plastic screw on cap (coke bottle), make two holes in the cap, put two tubes through and glue them in with epoxy or super glue. Then you fill the bottle halfway with water. One of the two tubes should reach all the way to the bottom, the other should not touch the water. Then you connect a filled balloon to the shorter tube and see how far the water rises in the other. If it rises by 1 meter that means the pressure is 1N/cm^2 = 10000 N/m^2 = 10 kPa
Most toy balloons have around 5kPa. Modelling balloons have 10 - 20 kPa.