Dive theory question

[jedi_diver]

Hi

I am wondering if some out there can help me understand how to work out this question. If you have the PADI OWSI candidate book for 2008 it is practice exam question number 12 on the physics exam.

At 40 m/132 ft of salt water a flexible container is filled to only 1/5 of it's capacity. If this container is taken to 20 m / 66 ft what will happen to the volume of air inside?

A) the volume will be the same at both depths
B) the air volume will double
C) the air volume will increase by 5/3
D) the air volume will descrease by 3/5

The correct answer is c but I can't understand why B is not the right answer...
Could some one help me to get my head around this? Maybe it will be of some help to some others too.

Thanks
JD

 

[maged_mmh]

coz you have to account for atm pressure

 

[E.C.Hansen]

Maybe dont think from the bottom up, as in 40m is twice as deep as 20m. Think top down, at the top your at 1 atmosphere, at 10m 2atm, at 20m 3atm, etc. So 40m is actually 5 atmospheres, and there's 3 atmospheres at 20m... So 5 atmospheres vs 3 atmospheres is a 5/3 ratio. Am I saying this right? Not a dive pro but I remember this from somewhere...

Eric

 

[lobsterlover]

I'm currently finishing my post-work beer but I think this is right.

40 m is 5 atm
20 m is 3 atm

(1/5)*X=(1/3)
solve for X
X=5/3

C would only be correct if you were transitioning from 2 atm (10 m) to 1 atm (surface).

 

[caver]

Think about this,,,and then reverse it.

The pressure change from 1 atm to 2 ata doubles or 100%
The pressure change from 2 ata to 3 ata increases by 33.33%
The pressure change from 3 ata to 4 ata increses by 25%
The pressure change from 4 ata to 5 ata increases by 20%

So, the deeper you go, the rate of change decreases. The inverse is true while ascendng.

 

[maged_mmh]

Maybe dont think from the bottom up, as in 40m is twice as deep as 20m. Think top down, at the top your at 1 atmosphere, at 10m 2atm, at 20m 3atm, etc. So 40m is actually 5 atmospheres, 2 more than the 3 atmospheres at 20m. So 5 atmospheres vs 3 atmospheres is a 5/3 ratio. Am I saying this right? Not a dive pro but I remember this from somewhere...

Eric

either way... same result.
you got it right though.

 

[maged_mmh]

in other words: think in terms of absolute pressure not depth.

 

[mikeyjoe]

The easiest way to do this is bring the container to the surface and then submerge it to the new depth. If the container is @ 132', it is at 5 ATA - 4x33'= 4 ATA, plus the one from surface atmosherice pressure. So the air in the container will expand 5 times it's orig volume when brought to the surface. 5x 1/5 = one full container. Submerge it to 66', which is 3 ATA (2x 66' plus one from the surface) and the volume of air will reduce to 1/3. So, expanding by a factor of 5, and then reducing by a factor of 3 is 5/3.

Again, the easiest way to work through these type of problems is to bring it to the suface and then re-submerge it to the new depth, and always add one atmoshpere for surface air pressure.

Let me know if I have made it clear as mud.

Michael

 

[Web Monkey]

132' = 132/33 ATM = 4 ATM + 1 ATM (surface) = 5 ATA
66' = 66/33 ATM = 2 ATM + 1 ATM (surface) = 3 ATA

According to a dead guy named Boyle, P1(V1) = P2(V2), so 5(V1) = 3(V2), which makes V2 = (5/3)V1.

This translates into "Don't hold your breath"

Terry

Hi

I am wondering if some out there can help me understand how to work out this question. If you have the PADI OWSI candidate book for 2008 it is practice exam question number 12 on the physics exam.

At 40 m/132 ft of salt water a flexible container is filled to only 1/5 of it's capacity. If this container is taken to 20 m / 66 ft what will happen to the volume of air inside?

A) the volume will be the same at both depths
B) the air volume will double
C) the air volume will increase by 5/3
D) the air volume will descrease by 3/5

The correct answer is c but I can't understand why B is not the right answer...
Could some one help me to get my head around this? Maybe it will be of some help to some others too.

Thanks
JD

 

[jedi_diver]

Ahhhhh.... That's it! I got it now. Thanks to all that took the time to reply.