Maximum depth at which you can breathe using a simple air pipe

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Came across this question in the Flying Circus of Physics,

My initial assumption was that this would be where the water pressure becomes greater than the maximum pressure difference that can be created between the lungs and atmospheric pressure.

a search revealed the normal pressure difference while breathing to be just around 500Pa, which would make the depth very small. Given that the maximum volume of air that the lungs can hold is around 6 times the normal capacity (this is dubious), then the maximum pressure difference would be around 3kPa, however this still gives an answer which is around 3 times less than that given.

I wonder where my mistake is?
 

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  • #2
CWatters
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Work out the water pressure at say 0.5-1 meter and multiply by the area of your chest to get the force needed to expand it...

At 1 meter the water pressure is around 1.4 psi.

If the area of your chest is say 0.5 square foot then...

force = pressure * area
=1.4 psi * 72 sq inches
= approx 100lbs

1.4psi is about 9600 Pa
 
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Thank you for your reply, 1m is indeed the depth given.

How would you estimate the maximum force that the lungs can exert? 100lbs comes to around the weight of a light person - so would you go from the fact that you wouldn't be able to breathe with someone sitting on your chest?
 
  • #4
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Thank you for your reply, 1m is indeed the depth given.
How would you estimate the maximum force that the lungs can exert? 100lbs comes to around the weight of a light person - so would you go from the fact that you wouldn't be able to breathe with someone sitting on your chest?
You could do a test to determine the pressure your lungs can produce. If you have access to a pressure gauge, you can blow/suck through the gauge and it'll display the pressure. If it's an analog pressure gauge, it needs to be calibrated for low pressure (a tire pressure gauge won't be very accurate).

If you don't have access to a pressure gauge, you could use a vertical tube with some liquid in it, suck the liquid up the tube and record the height you manage to get it to. You can then calculate the pressure at the bottom of that liquid column to determine the pressure you created.
 
  • #5
K^2
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If you don't have access to a pressure gauge, you could use a vertical tube with some liquid in it, suck the liquid up the tube and record the height you manage to get it to. You can then calculate the pressure at the bottom of that liquid column to determine the pressure you created.
I don't know about now, but this is how it used to be done. I believe, in Soviet Union it was actually part of standard tests of physical fitness for military, etc. A U-shaped tube of liquid is usually used, with other end exposed to air. So difference in column heights gives you a pressure differential with respect to atmospheric directly, and that's what you are looking for.
 
  • #6
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You could do a test to determine the pressure your lungs can produce. If you have access to a pressure gauge, you can blow/suck through the gauge and it'll display the pressure. If it's an analog pressure gauge, it needs to be calibrated for low pressure (a tire pressure gauge won't be very accurate).

If you don't have access to a pressure gauge, you could use a vertical tube with some liquid in it, suck the liquid up the tube and record the height you manage to get it to. You can then calculate the pressure at the bottom of that liquid column to determine the pressure you created.
Thank you, I may well try that with a foot pump.
 
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I've done this experiment under water with a rubber hose. Breathing becomes very suffix difficult only a few inches under water. At three feet it is completely impossible.
 
  • #8
jbriggs444
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You could do a test to determine the pressure your lungs can produce. If you have access to a pressure gauge, you can blow/suck through the gauge and it'll display the pressure. If it's an analog pressure gauge, it needs to be calibrated for low pressure (a tire pressure gauge won't be very accurate).

If you don't have access to a pressure gauge, you could use a vertical tube with some liquid in it, suck the liquid up the tube and record the height you manage to get it to. You can then calculate the pressure at the bottom of that liquid column to determine the pressure you created.
If all you are going to do with the calculated pressure is to compute the depth at which you can inflate your lungs underwater through a breathing tube, save yourself some effort and just write down the original measurement.

The depth underwater at which you can inflate your lungs through a breathing straw is equal to the height to which you can suck water up a drinking straw.
 

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