Quiz - Physics - Approximate Time A Tank of Air Lasts

[homerdoc]

dang I just took one atmosphere at the surface and ratio'd it to 4 at 99 ft

 

[loosenit2]

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?

i used the standard that pressure changes by 1ATM every 33 feet in salt water and every 34 feet in fresh water. So 99/34+1 = 3.9117 ATA vs 4 ATA in salt water, hence slightly less ATA and slightly more gas.

 

[Scraps]

dang I just took one atmosphere at the surface and ratio'd it to 4 at 99 ft

And you were correct to do so. We're noodling through the fresh water "what if."

 

[dorsal]

Remember PADI questions assume saltwater unless said otherwise

 

[Pedro Burrito]

Spoiler: Answer

a. 22.5 minutes

 

[Brett Hatch]

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

Oh yeah. *face palm*