I Hope this isn't cheating!

[Phoenix]

I am currently going through the knowledge reviews in my PADI DM manual, and one question has me stumped, I think I have it down right, but I'm very unsure, so any help as to my mis-interpretations would be appreciated

a simple you got it right, or you got it wrong, heres how it works, will be more useful to me than giving me the correct answer to the question, if thats possible

thanks for any help

Question 4, Knowledge Review 5, Decompression Theory and the RDP

In metres/feet of pressure, how much nitrogen would the 5, 10, 20 and 60 minute halftime compartments each have after 60 minutes at 18 metres/60 feet?

5 =
10 =
20 = 57.5 fsw
60 = 30 fsw

that number for 20 just looked a little high
basically i worked it out as three half times (20x3 = 60)
so thats 30fsw after 20 mins, then a further 15 after another 20, then another 7.5 after another 20 totaling 57.5m, am i on the right track? that would make for some seriously small numbers at the far end of the 10 and 5 calculations

someone tell me if i've got it all wrong

thanks all

 

[Dr Deco]

Hi Phoenix :

The system you are using is correct, that is, each halftime moves you half the remaining distance to the completion point. After about six halftimes, you are within about 99% of completion, and the process is generally considered finished.

For gas tensions, you are using the gauge depth for pressure. For gases, one will usually employ the absolute pressure. Thus sea level (= 0 fsw) is really 1 atmosphere pressure (= 33 fsw) and 33 fsw is really 33 + 33 = 66 fsw or 2 ata of pressure.

Depending on how your problems are written, this definition can make a considerable difference.:mean:

Dr Deco

 

[Snowbear]

Phoenix once bubbled...
In metres/feet of pressure, how much nitrogen would the 5, 10, 20 and 60 minute halftime compartments each have after 60 minutes at 18 metres/60 feet?

5 =
10 =
20 = 57.5 fsw
60 = 30 fsw

that number for 20 just looked a little high
basically i worked it out as three half times (20x3 = 60)
so thats 30fsw after 20 mins, then a further 15 after another 20, then another 7.5 after another 20 totaling 57.5m, am i on the right track? that would make for some seriously small numbers at the far end of the 10 and 5 calculations

someone tell me if i've got it all wrong

thanks all

As Dr D said - you're using the right system, but the numbers/answers for the 5 and 10 min compartments are NOT small (hint - "fast" compartments...)

Edit - reading your question again, I think I misunderstood your "small #" comment. Yeah, they get small, but after (I think?) six 1/2 times, that compartment is essentially considered fully saturated.