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Old Aug 29th 2017, 07:08 AM   #1
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What is the pressure in closed container dependent on?

Hello!

I have a problem : I have an elastic container (I call it "balloon"). I fill it 100% with water (no air bubbles) and close it. There is a pressure sensor connected to the balloon to measure the pressure inside it.

I would like to ask You : what factors have influence on the pressure inside the balloon, when it is lying on the table and no one is moving this?

I noticed that the pressure sensor says, that the pressure increases, when I heat the balloon a little (just a few C degrees) with a hair dryer. I was expecting that, but there is on thing I don't understand and I would like to ask about: When I left the balloon with water (and of course with sensor) for the night, the next day in the morning the sensor says that the pressure increased during night. But the temperature at night didn't increased - it decreased. I don't fully understand this, that is I would like to ask: what else could the pressure inside could be dependent on?

Thanks for help!

Darek
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Old Aug 29th 2017, 07:57 AM   #2
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Perhaps the atmospheric pressure increased overnight.
Did you check the barometer?
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Old Aug 29th 2017, 11:19 AM   #3
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The pressure increase shown by sensor in the balloon was 50 hPa. I didn't check the barometer, but I am sure that the atmospheric pressure can't change 50 hPa just in one night.

Another ideas?

Thanks!
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Old Aug 29th 2017, 11:57 AM   #4
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How exactly is the sensor "connected" to the balloon? Is it inside the balloon? Where is it located exactly? Pressure will vary with depth of the sensor submerged in the water (i.e. the pressure at the bottom is higher than the pressure at the top due to the weight of water above). Another factor that could vary is the elasticity of the balloon material - if the material stretches over time it could expand and reduce the pressure inside.
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Old Aug 29th 2017, 08:59 PM   #5
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50 hPa is equivalent to the pressure of 50cm of water!

that's a big change , by moving ,squashing the balloon you should see a change of about 10 hPa , do this to see if the meter is working ....

Check to see if the barometer mirrors your pressure sensor over daily changes in pressure
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Old Aug 30th 2017, 12:04 AM   #6
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There is an elastic, rubber hose connected to the balloon and then this hose goes down about 20 cm ant there it is connected to the sensor, which is lying on the table (but it has no possibility to move).

The balloon is made of exactly the same material as the "air mattress" that we use to put on the lake/sea and lie on it. So If I put there 100% water then it strains itself and there is very little water I could add to it using a little pump.

OK, as You said Chip, when the material will stretch it will lower the pressure inside. I have the opposite situation: my pressure increase with time.

Oz: the meter is working. Now I have two sensors connected to my balloon: one electronic and one mechanical. Both gives me very similar results when I press the balloon or move it like you said.

Today I will try to check the pressure inside with the barometer, thanks!
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