Recalculating Thirds

[cavedivingwoman]

A question: I was also taught to recalculate thirds but cannot remember the last time I did this. Is this no longer current practice? 'Agree that it is a dicey way to fly. 'No such thing as too much gas. Thanks for clarifications.
Barbara

[lv2dive]

I'd suggest a separate topic on this discussion. 😊

A question: I was also taught to recalculate thirds but cannot remember the last time I did this. Is this no longer current practice? 'Agree that it is a dicey way to fly. 'No such thing as too much gas. Thanks for clarifications.
Barbara

[FW]

.. Is this no longer current practice?...

It is still used for things like the Grand Traverse from Orange Grove to Peacock. There is no real good reason to exit a cave, recalculate, and go back in after a surface interval anymore. There are plenty of places to top off your tanks, during your surface interval.

[victorzamora]

I think what they're referring to is mid-dive. I've done this before: Dive JB up to thirds (noting my gas used at the bottom of the chimney), turn and "recalculate" thirds and then go through Horseshoe, and then "recalculate" and go up Young's Siphon, then go up the chimney and head to the surface. JB is a pretty poor example because I'm still out with over a third left (or I was when the flow was high). I don't know if I could do that without the super-heavy flow, and my rule was to always exit with 1200psi left. SOMETIMES I'd get a little risky and go to 1000psi. There were dives where I'd piddle around the rock garden in JB until I'd exit with around 1000psi, but those were few and far between.

Having said all of that, I've only been Full Cave for 18 months and was trying to squeeze as much practice out of every dive as I could.

[jj1987]

A question: I was also taught to recalculate thirds but cannot remember the last time I did this. Is this no longer current practice? 'Agree that it is a dicey way to fly. 'No such thing as too much gas. Thanks for clarifications.
Barbara

Yes, it's still being taught. It's still very common.

One clever way I've heard to do it is have line markers with how much gas you've used (100, 200, 300, etc), and drop them on the way in when you pass a passage you'd like to see going out. For instance, if I'm heading in and have used 300psi, I'll drop the 300psi marker by the side passage I'd like to see on the way out. For example, if you had 1500psi left when you hit that point on the way out you would do 1500-600 = 900psi/3 = 300psi left over to penetrate into the side passage. Clearly this has no safety margin in my example, but you can/would add that.

The real risk here seems to be that you're making calculations underwater, on the fly, where mistakes are easily made.

[kensuf]

Yes, it's still being taught. It's still very common.

One clever way I've heard to do it is have line markers with how much gas you've used (100, 200, 300, etc), and drop them on the way in when you pass a passage you'd like to see going out. For instance, if I'm heading in and have used 300psi, I'll drop the 300psi marker by the side passage I'd like to see on the way out. For example, if you had 1500psi left when you hit that point on the way out you would do 1500-600 = 900psi/3 = 300psi left over to penetrate into the side passage. Clearly this has no safety margin in my example, but you can/would add that.

The real risk here seems to be that you're making calculations underwater, on the fly, where mistakes are easily made.

That's 1..

Another is gas consumption tends to go up when the fit hits the shan, and by re-calculating you're reducing your margin for error.

[SlowDiver]

Yes, it's still being taught. It's still very common.

One clever way I've heard to do it is have line markers with how much gas you've used (100, 200, 300, etc), and drop them on the way in when you pass a passage you'd like to see going out. For instance, if I'm heading in and have used 300psi, I'll drop the 300psi marker by the side passage I'd like to see on the way out. For example, if you had 1500psi left when you hit that point on the way out you would do 1500-600 = 900psi/3 = 300psi left over to penetrate into the side passage. Clearly this has no safety margin in my example, but you can/would add that.

The real risk here seems to be that you're making calculations underwater, on the fly, where mistakes are easily made.

To me the real risk is when something bad does happen you can forget about your so call SAC rate. I was amazed at how much gas I used until I got the problem sorted out and settled my breathing rate down.


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[nakatomi]

I can see where such a method of recalculating would be useful, for example for intro divers in high flow caves like JB or Ginnie where almost half of the penetration gas is used just to get down the chimney or through the lips.
However recalculating sixths while you're still carrying two tanks with anything above 3000psi is a different story from recalculating spent thirds. Why would anyone want to recalculate based on 1500psi? If the tanks are that emtpy, it's time to get a refill.

[twaldo]

I think I totally mis-understood... your talking about re-calculating on the fly WHILE you are still in the water?? that seems suicidal.... I thought it meant doing a full dive and re-calculating during surface interval to then be able to do another dive....

Tim

[Squirrel Girl]

I think it's very hard to make a hard and fast rule. If you're in Mexico and most of the caves are <10 meters (33 ft) deep, you can get a lot of diving in with less than a full cylinder.

I have found that more than the stress causing my breathing rate to go up when something goes wrong, I have had my breathing rate go up from the physical stress of trying to solve a physical problem in a high current.

I see no problem at all in going a shortish way into a cave to practice skills and blow off a tank of gas after a "bigger" dive. OF COURSE that means being prudent. Crazy/scary to come out with 100 psi. 500--not so good. But it still depends on where you are (cavern zone, etc) and the rest of the circumstance.