diving 1/6's

[H2Andy]

ok... on doubles, easy to dive 1/6's on the first dive. you just halve thirds. i do
this because i'm intro cave and must dive 1/6's on doubles.

but... how about on the second dive?

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)

so on the second and subsequent dives, if any, where i used 1/6's on the first dive,
isn't it more accurate to just use thirds?

to clarify:

my question is whether it's ok to use 1/3's on the second dive on doubles and
still be within intro. cave limits?

 

[Otter]

I am guessing the intent is to get two dives off of your doubles, right? If so, then the second dive would be 1/3s. If you kept diving 1/6s, you would NEVER (theoretically) run out of gas and could dive forever. I am sure you know this, so I guess I am assuming the answer is 1/3s on the second dive....but if not, I'll pay attention to the answer.

Edit: I guess another way of looking at it is during your second dive, you dive the same PSI amount as the first dive.....so Dive 1: 1/6 out, 1/6 return, 1/6 reserve. Dive 2, 1/6 out, 1/6 return 1/6 reserve (leaving 2/6s in reserve).

 

[H2Andy]

that's my "common sense" answer too, but i wouldn't put it past myself to miss
the obvious...

glad to hear you're reading it the same way

although (post-edit), i would not use the same 1/6 from the first dive, since
actual dive gas usage will almost inevitably vary (i almost always come back with
more gas than i planned for, usually because of flow on the way out helping)

 

[Charlie99]

I think you need to look at the purpose of diving with the 1/6 rule before you use a different rule for the second dive.

Going in using 1/6 of starting gas, then having a total gas loss at the turnpoint, leaves you with 5/6 of starting gas to share between two divers on the way out. That means you can increase consumption rate by 2.5x each and still get out. (Rule of 1/3 does NOT allow any increase in gas consumption if total gas loss is at turnpoint).

 

[H2Andy]

sorry, charlie, you lost me (not your fault)

as i understood it, the 1/6 rule for Intro divers is to limit their penetration.

in other words, i "pretend" i have a single cylinder on my back when in fact i
am diving doubles.

that's why i was thinking it made sense to treat the "doubles" as a single on the
second dive, since i have already used up one dive's worth of air.

make sense?

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 B]

sorry, charlie, you lost me (not your fault)

as i understood it, the 1/6 rule for Intro divers is to limit their penetration.

in other words, i "pretend" i have a single cylinder on my back when in fact i
am diving doubles.

that's why i was thinking it made sense to treat the "doubles" as a single on the
second dive, since i have already used up one dive's worth of air.

make sense?

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)

Andy I'm pretty sure that at the intro level, doubles are always looked at as doubles, meaning your second dive should be turn at 1700.

Jason

 

[Rick Murchison]

Of course if you say "a sixth is a sixth is a sixth" then you can only penetrate a sixth.
But the sixth rule only applies if you're wearing doubles, so it isn't a sixth rule for the purpose of reserving five sixths of your gas, but rather a rule to limit penetration. After all, if you're wearing a single you're allowed to penetrate to a third...
Common sense says that if you can start with a single 104 and dive it to thirds, then you can start with the same 104 CF distributed in two tanks (as in a second dive with double 104's) and still dive thirds...
But then there are the rules.
I vote for common sense, and would say that the sixth rule is better read that penetration is limited to one sixth the capacity of doubles or one third of the gas you're carrying, whichever is less... but then I ain't in charge.
Rick

 

[Charlie99]

sorry, charlie, you lost me (not your fault)

as i understood it, the 1/6 rule for Intro divers is to limit their penetration.

I was looking at it strictly from gas management point of view.

The rule of thirds is NOT all that conservative, since if you are right at the turnaround pressure when the other guy loses all his gas, you will just barely make it out, provided that you each use the same or less gas than what it took to get in. 1/3 to get in. 2/3 left to share between to get out. Any increase in SAC could lead to OOG.

The rule of 1/6 leaves you with 5/6 gas left. If sharing, each guy has available 2.5 times what it took to get you in, thereby giving you some margin for increase of SAC, time to handle other problems, etc.

If you assume that this is the purpose of the 1/6 rule (and I'm NOT sure that it is), then to maintain the same relative safety, you would apply the rule of 1/6 to any and all dives. Of course, you would base the 1/6 on whatever pressure you had when starting the overhead penetration.

 

[loosebits]

I am also intro certified and I have moved into doubles in preperation for full cave so the rule of 1/6ths applies to me.

Now, there are no scuba police. No one is going to arrest you if you violate sixths or thirds or halfs (hey this is a high flow cave, I'll make it out in less than half the time it took me to get in - halves should be fine). If you feel like you need to interpret the rule in a way more advantageous to you, I wonder if there are other loopholes in other rules we could all exploit?

I guess my point is dive within your knowledge but don't use loopholes in the rules to pretend your knowledge is any further along than it is (hey, if I read this rule like this, than it must be safe for me to do that).

My personal limits on doubles are to penetrate no further than I can on 1/6th of full tanks, do not start a dive with less than 2000 psi. I dive 104's, the rule says 72 cuft starting volume (about 900 psi) but I don't trust my analogue guage in the 500 psi range and I don't want to be guessing what my guage is reading if I'm searching for a lost buddy - in other words, in an emergency I want to be well above 500 psi even if I'm almost to the cavern. Doing 500 from 2000 psi (1/6th of a conservative fill), I'll be out by 1000 at the very least leaving 750 or so for emergencies (the "or so" fudge factor depends on how close a friend I am to my buddy or how much money he owes me).

 

[H2Andy]

I guess my point is dive within your knowledge but don't use loopholes in the rules to pretend your knowledge is any further along than it is (hey, if I read this rule like this, than it must be safe for me to do that).

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