When thirds are not enough....

[DA Aquamaster]

Most gas rules state, "Always reserve at least 2/3 of your gas supply for exit.

I have to admit, I dive thirds - unless the conditions suggest sixths. It keeps everything simple. Every cave diver understands (or should) calculating thirds with same and dissimilar tanks. Thirds becomes the line in the sand that you absolutely cannot cross. No exceptions. No excuses. Thumbs.

If decompression is a factor, then without deco gas it's a great idea to take away rock bottom gas and calculate thirds based upon usable gas. If you drop deco gas, then you may just calculate thirds for penetration. A diver can decide how much caution he or she wants to build into any dive.

Once we've established that line in the sand, we are now free to draw upon our experiences, our emotions, the formation of the cave, the conditions, how the dive is proceeding, etc., to decide when to turn the dive. I've turned dives before thirds for all manner of reasons: time, depth, deco, multiple directional changes, silt, gut instinct, name it.

There are so many factors that can go into turning the dive.

I agree with this and with being thoughtful in how you determine your turn point in general.

I calculate thirds on most dives, but in most low flow cases will turn 100-200 psi early as a standard practice and will usually pad that based on a variety of factors including but not limited to:

1. comfort level/familairity with the dive/cave
2. confidence in team mates
3. flow and visibility
4. severity, length and/or number of restrictions
5. Anticpated deco obligation and lost deco gas contingencies
6. presence of old line
7. complexity of the navigation
8. depth

I am not sure many cave divers conceptualize the benefit of turning just 100 psi early. For example, if you are at 90 feet, diving double 130s or cave filled 95s and are using approx 2 cubic feet per minute (a SAC a bit under .6) you are only giving up about 3 minutes and perhaps 150 ft of penetration by turning 100 psi early. In exchange for the early turn you are saving 7.2 cu ft of penetration gas and are in effect adding 14.4 cu ft to the 86 cu ft of reserve gas - a 16% increase. Assuming use rates and depths are comparable on exit, that extra gas gives you 7 minutes and perhaps 350 additional feet worth of gas in an emergency.

If you turn 200 psi early, you are adding 28.8 cu ft to the 86 cubic foot reserve, a 33% increase, worth perhaps an additional 14 minutes and 700 feet.

 

[ucfdiver]

I don't depend on flow to get me out. Besides, the types of dives I do at JB always take me off the mainline and there's hardly any flow there, even when the flow is up. That being said, if I were just doing a main line dive, I still wouldn't. I would cut it back by 100 psi. Not much, but enough to keep me happy.

Because you think it's actually needed, or just to say it's conservative? I'm curious, as an instructor I'm sure you're used to explaining how to determine when/how much to back off-- what situation do you think 100psi would help, when the 1200+psi already in reserve wasn't enough?

 

[LiteHedded]

Because you think it's actually needed, or just to say it's conservative? I'm curious, as an instructor I'm sure you're used to explaining how to determine when/how much to back off-- what situation do you think 100psi would help, when the 1200+psi already in reserve wasn't enough?

pull out the thumb -> cave gremlins steal all the gas

I see it time after time

 

[rjack321]

I am not sure many cave divers conceptualize the benefit of turning just 100 psi early. For example, if you are at 90 feet, diving double 130s or cave filled 95s and are using approx 2 cubic feet per minute (a SAC a bit under .6) you are only giving up about 3 minutes and perhaps 150 ft of penetration by turning 100 psi early. In exchange for the early turn you are saving 7.2 cu ft of penetration gas and are in effect adding 14.4 cu ft to the 86 cu ft of reserve gas - a 16% increase. Assuming use rates and depths are comparable on exit, that extra gas gives you 7 minutes and perhaps 350 additional feet worth of gas in an emergency.

If you turn 200 psi early, you are adding 28.8 cu ft to the 86 cubic foot reserve, a 33% increase, worth perhaps an additional 14 minutes and 700 feet.

+1
Although you can quibble on the exact distances, cutting back by 100psi on the penetration adds 300psi to the available exit gas.

 

[Trace Malinowski]

I agree with this and with being thoughtful in how you determine your turn point in general...

I think most of us dive this way. Turning a dive at 100 - 200 psi from thirds is common. In a sense that is still "thirds" compared to planning a dive on sixths or another such plan from the beginning. We'll look at the factors you mentioned and think, "Better safe than sorry," and turn.

I haven't been on any dives where a diver keeps checking the gauge to make sure we turn exactly on thirds and no one wants to go beyond thirds, so more often dives are called close to thirds, which to me, is still diving "thirds".

 

[MXGratefulDiver]

I think I follow what you're trying to say now. You're advocating that the RB diver needs to retain their own bailout at all times so with an OC/CCR team the OC diver needs to have their own bailout/buddy bottle.

That's exactly the bailout strategy me and my buddies use for any tech dives involving mixed OC/CC teams ...

... Bob (Grateful Diver)

 

[Dive-aholic]

I think I follow what you're trying to say now. You're advocating that the RB diver needs to retain their own bailout at all times so with an OC/CCR team the OC diver needs to have their own bailout/buddy bottle.

Exactly!

Because you think it's actually needed, or just to say it's conservative? I'm curious, as an instructor I'm sure you're used to explaining how to determine when/how much to back off-- what situation do you think 100psi would help, when the 1200+psi already in reserve wasn't enough?

Because it could be needed. An extra 100 psi isn't going to get me much farther in the cave. If I really want to go farther then it's time to stage or dive bigger cylinders. In a true emergent situation most divers will experience an increase in RMV.

Let's say you start with a common cave fill of 3600 psi and you have an issue 1000 psi into the dive. That leaves 2600 psi to exit. 1000 psi is allocated for each diver, leaving an extra 600 psi, or 300 psi per diver.

Now let's turn this into volume. Both divers are diving LP95s. Total gas is 259 cf at 3600 psi. The issue occurs at 72 cf. No change in RMV leaves 144 cf to exit. That's a total of 216 cf, leaving 43 cf in reserve. However, now instead of an RMV of .6, the divers are both breathing with an RMV of .9. That means 108 cf per diver (216 cf for both) is required to exit the cave. That's not enough. And that's not even turning at thirds. Will flow help the exit? It might. But, hey, it didn't help those 2 guys in JB a few months ago. If someone else hadn't been scootering in at the time he was, we'd have 2 dead divers about 300 feet penetration in JB. I don't know what their exact gas plan was, but I do know that when things go bad they usually go really bad. 100 psi might not be enough to get me out of the cave alive, or it might. I feel better knowing I have it. Besides, it's already been mentioned that SPGs are very accurate. In fact, I've even compared SPGs and found as much as 500 psi difference in a couple (not mine).

 

[ucfdiver]

Will flow help the exit? It might. But, hey, it didn't help those 2 guys in JB a few months ago. If someone else hadn't been scootering in at the time he was, we'd have 2 dead divers about 300 feet penetration in JB. I don't know what their exact gas plan was, but I do know that when things go bad they usually go really bad. 100 psi might not be enough to get me out of the cave alive, or it might. I feel better knowing I have it. Besides, it's already been mentioned that SPGs are very accurate. In fact, I've even compared SPGs and found as much as 500 psi difference in a couple (not mine).

From my understanding, the issue wasn't gas with those guys, it was failure to act as trained when there was a buddy separation. From what I heard, they were separated around kings canyon and wasted way too much gas looking for each other in a silt out. None of these gas plans work if you don't follow standard procedures. As a matter of fact, since these guys stuck around looking for each other until gas was critically low, I wonder if they had double the gas, would they have still been in the same situation? To my knowledge, no gear failure was involved in the situation.

I will say that I'm not entirely sure if my story is correct there, everyone seems to like to hint at this situation from a few months back and not put all the facts on the table.

Edit- I understand the logic behind -100 psi for spg error. That's what I was trying to get out of you-- the why.

 

[Dive-aholic]

From my understanding, at least one of them was lost and didn't find the main line right away. They were both in zero visibility during at least one part of the dive. The first one back to the main line looked and was about to exit when he saw his buddy's light coming down the main passage so he stayed to exit with him. If he hadn't waited for his buddy, he should have made it to his deco cylinder without running out of gas. I can't say I fault him for staying an extra minute when he saw his buddy's light. Had they both had an extra 300 psi in reserve, they would both have made it the last 300 feet to their deco cylinders. As it was, they both almost died.

 

[Cave Diver]

That's exactly the bailout strategy me and my buddies use for any tech dives involving mixed OC/CC teams ...

... Bob (Grateful Diver)

Exactly!

Depending on the dive, we'll approach this a little differently. Bailout is reserve, the same as 1/3's in doubles.

If CCR has an issue, they switch to bailout, dive is over and both exit.
If OC has an issue, CCR can hand off bailout to them, dive is over, both exit.

If you've planned your bailout properly (and this is the important part) you have enough to get either diver out in the event of a single failure. That's really no different than two OC divers - if they have to share gas, there is only enough to cover a single gas failure.

In both scenarios, you're rolling the dice that you won't have multiple gas failures on the same dive.

On a more complex dive we will carry multiple bailouts and the OC diver may carry a bottle as well.