Quiz - Physics - Approximate Time A Tank of Air Lasts

[Pedro Burrito]

From the Physics Section of the PADI Dive Theory Exam:

If it takes a diver 90 minutes to breathe all the air from a scuba tank at the surface, approximately how long will the air in that tank last at 30m/99ft?

a. 22.5 minutes

b. 20 minutes

c. 30 minutes

d. 37.5 minutes​

I will post a daily question from my exams to help newer divers and to encourage more experienced divers to interact gracefully and helpfully with the newer divers.

Reminder - this is a post in the Basic Forum and it is a green zone. Please be nice and on topic.

Thank you for your patience while we try to give people something to discuss other than Covid-19 and/or Politics. I will post the answer covered by the spoiler tag later today.

 

[Scraps]

Legal disclaimer: My choosing the correct answer should not be construed as an endorsement of breathing all the air from one's tank at 30m/99ft.

 

[loosenit2]

FSW or FFW? The answers all lend themselves to FSW but with Fresh water the answer is 23 min.

 

[Scared Silly]

Legal disclaimer: My choosing the correct answer should not be construed as an endorsement of breathing all the air from one's tank at 30m/99ft.

Quit ruining all our fun. A CSA from 30m/99ft is a fun experience which is how one obtains good judgement.

 

[Scraps]

FSW or FFW? The answers all lend themselves to FSW but with Fresh water the answer is 23 min.

I think Pedro is covered by the "approximately" in the question and by the absence of another answer that would be more accurate even if a test taker were to assume the question posited a scenario involving the 1% or so of the world's water that is both fresh and not frozen.

Quit ruining all our fun. A CSA from 30m/99ft is a fun experience which is how one obtains good judgement.

Sort of like CrossFit: The few customers whose bodies don't get busted up can achieve excellent fitness levels. If you survive that 100-foot CESA, you'll develop some awesome judgment.

 

[NAM001]

fresh water salt water it is a theory question that has so much relevance in future training.

 

[loosenit2]

I think Pedro is covered by the "approximately" in the question and by the absence of another answer that would be more accurate even if a test taker were to assume the question posited a scenario involving the 1% or so of the world's water that is both fresh and not frozen.

i'm not dinging Pedro, I love what he is doing. The real benefit of these quizzes is not the question but the conversations that happen as a result of them. My comment was meant to stir discussion.

 

[Brett Hatch]

FSW or FFW? The answers all lend themselves to FSW but with Fresh water the answer is 23 min.

How do you arrive at the 23 min? I can find a similar number -- slightly larger -- by multiplying the FSW answer by 1.03, since salt water is 3% denser than sweet water. However, intuitively that does not strike me as correct. I would expect the pressure difference to increase with depth, rather than remaining flat at 3%. Ie to answer this correctly we would need an expression that captures the diverging pressure as a function of the depth. Something like 1.03 ^ depth. Am I off base here, or onto something?

 

[Scraps]

How do you arrive at the 23 min? I can find a similar number -- slightly larger -- by multiplying the FSW answer by 1.03, since salt water is 3% denser than sweet water. However, intuitively that does not strike me as correct. I would expect the pressure difference to increase with depth, rather than remaining flat at 3%. Ie to answer this correctly we would need an expression that captures the diverging density as a function of the depth. Something like 1.03 ^ depth. Am I off base here, or onto something?

In fresh water, each 34 feet adds one atmosphere of pressure:

99 ft / 34 ft = 2.91 gauge or 3.91 atm

90 minutes @ 1 atm = 23.02 minutes @ 3.91 atm

 

[tursiops]

How do you arrive at the 23 min? I can find a similar number -- slightly larger -- by multiplying the FSW answer by 1.03, since salt water is 3% denser than sweet water. However, intuitively that does not strike me as correct. I would expect the pressure difference to increase with depth, rather than remaining flat at 3%. Ie to answer this correctly we would need an expression that captures the diverging pressure as a function of the depth. Something like 1.03 ^ depth. Am I off base here, or onto something?

The 3% takes care of that. 3% of a big depth is more than 3% of a small depth.