diving 1/6's

[loosebits]

i completely agree.

but how do you explain that on the second dive, using thirds, my penetration is
only 100 psi more than on the first dive, using sixths?

if the second is dangerous, then so is the first. 100 psi is nothing

I didn't state it very well... it comes out to using sixths or 500 psi, which ever is greater, get a fill at 2000 psi. So lets say I get a fill to 3300... diving sixths, lets say I end my dive with 2500 psi, instead of doing sixths again and turning at 2100 (2500 - 2400 / 6), I'll dive to my max penetration on 1/6th of a *normal* fill or 500 psi so I'll turn at 2000 and exit with around 1750... That will be my last dive on that fill and won't be able to penetrate any further than I did on my 1st dive. I realize I only get an extra 100 psi on my second dive but my goal here is not to push it as far as I can but to leave the cave under my own power instead of being dragged by some recovery diver. On the other hand, I also would like 2 good dives out of a fill. If getting gas means an hour drive, I may do another dive using only 350 psi being out of the cave with 1050 to spare (and that a good 850 or so in reserve - 200 psi is not a reserve, that's a hmm, I wonder if the guage is right).

 

[Jason B]

Andy, I think loosebits hit on something. Correct me if I'm wrong but I think you dive double 85's. According to the NSS/CDS manual, minimum starting pressure for a set of double 85's is 2175 psi. That means that on your original post where you wanted to go back in with 1600 psi would be against guidelines (not just for intro).

Jason

 

[H2Andy]

That means that on your original post where you wanted to go back in with 1600 psi would be against guidelines (not just for intro).

yup...

i wanted to go back with 2400 psi, though, not 1600 ( i *end* the dive with 1600
if i use 1/6's)

say, for example, that i start the second dive with 2400 psi

thirds is 800psi, but 1/6's is only 400 psi, which means i come back with 1600 psi
(two thirds of 2400)

 

[StSomewhere]

What does your training agency say? What does your instructor say?

If I understand your question -- and you are obviously farther along than I am, although I'm working on it -- you are wanting to do apprentice dives (thirds, with doubles) but are still intro (thirds, with singles) or discretionary apprentice (sixths, with doubles). No offense, but it sounds to me like for whatever reason you are justifying are pushing past the limits you agreed to. Why not just move onto apprentice training first?

 

[H2Andy]

hehehe... i guess you didn't read my question

the first dive, with 1/6's, is almost identical to the second dive, with 1/3.
i have 500 or 600 psi worth of penetration.

my question is, how can one dive be safe and the other not?

i still start both dives above the minimum required to enter overhead

 

[Jason B]

Hey Andy, my bad. It was your third post where you said...

put another way:

dive one: 3000 psi. 1/3 is 1000, 1/6 is 500. i turn at 2500, get back with 2000

dive two: i can now use 1/3's. 600 is thirds, so i turn back at 1400, make it back
with 800

(if i had dived 1/6's on the second dive, 1/6 is 300, turn is 1700, make it back
with 1400)

Jason

 

[jonnythan]

Because on the second dive you have only have enough gas to get you and your buddy back to the surface if your exit is identical to the entrance, but on the first dive you have much more than necessary? But I'm not cave trained or anything..

 

[H2Andy]

It was your third post where you said...

oops... yup... that wouldn't be kosher (though i believe my minimum is 1200 psi
in LP 85 doubles (i.e. 70 cf per NSS-CDS)

let's say i started the first dive with 3200, made it back with 2200

 

[H2Andy]

Because on the second dive you have only have enough gas to get you and your buddy back to the surface if your exit is identical to the entrance, but on the first dive you have much more than necessary? But I'm not cave trained or anything..

like i said (dude!?!) i enter overhead with gas above the minimum required to enter
overhead on both dives

remember, if i were diving a single tank, all that extra gas during the first
dive would not be there

 

[Jason B]

hehehe... i guess you didn't read my question

the first dive, with 1/6's, is almost identical to the second dive, with 1/3.
i have 500 or 600 psi worth of penetration.

my question is, how can one dive be safe and the other not?

i still start both dives above the minimum required to enter overhead

Yes, but your starting pressure is too low to even enter the overhead on the second dive. The are more guidelines to cave diving than rule of thirds.

Jason