My 1st Diving Problem Ever!

[PvilleStang]

So I'm going to have to jump in here:

When you had your regs freeflow, were they both on the same first stage, or are you running separate first stages on an H valve? If they were on the same first stage, I would think it feasible that either you had a valve fail in it, or possibly you may need to adjust your IP (well, let the techs at the shop take care of that).

And do any of you follow the rule of halves? That's what I was taught: calculate your air requirements, and plan to turn your dive at half your usuable air (the final 500psi or so is thrown out). That in essence leaves you with ample gas for your buddy and you in multiple situations (of course it's aimed towards doubles usage).

 

[HybridDiver]

So I'm going to have to jump in here:

When you had your regs freeflow, were they both on the same first stage, or are you running separate first stages on an H valve? If they were on the same first stage, I would think it feasible that either you had a valve fail in it, or possibly you may need to adjust your IP (well, let the techs at the shop take care of that).

And do any of you follow the rule of halves? That's what I was taught: calculate your air requirements, and plan to turn your dive at half your usuable air (the final 500psi or so is thrown out). That in essence leaves you with ample gas for your buddy and you in multiple situations (of course it's aimed towards doubles usage).

I was reading about the Halves rule, with NWgratefuldiver Gas planner thing. I think im going to use that rule mostly, It seems good. I know cave divers use it.
Chris

 

[TSandM]

Actually, if you carefully read Bob's material, you'll see that gas management can be done four different ways.

The first way is "all available gas". That's like a drift dive off a live boat; you can dive until you reach your "rock bottom", or minimum gas, and then ascend. The boat will pick you up wherever you are.

The second way is "halves". That's where it would be NICE to get back to where you started, but it isn't critical. Our shore dives are like this. If worst comes to worst, we can surface swim back, but we don't want to. So we take out rock bottom, and use half our remaining gas before we turn around and head for shore.

The third way is "thirds". This is where you HAVE to get back to your starting point. An example would be diving off an anchored boat on a wreck. You have to get back to the anchor line to return to the boat. You can use a third of your gas (after you have taken out rock bottom), because, if your buddy goes OOA at the point where you are at maximum distance from the upline, you will have to air-share back, and that will take twice as much gas as you have used getting to where you are.

Note that all of the above strategies take out "rock bottom" -- This is because, at the end of your dive, you must do an ascent, no matter what has happened beforehand.

The fourth strategy is pure "thirds", and it's only applicable to situations where you must get back to your starting point, and your starting point IS the surface. In other words, the pure rule of thirds is really only used in caves.

 

[JessH]

Halves is good for when you can safely return directly to the surface. In situations where it is safer to turn around instead of going straight back thirds or even more conservative rules are often better. When diving in a lake with boat traffic I often prefer thirds. If you go very far out going straight up is often not the safest path since you may come up right in front of a boat. I carry an smb in case this is necessary, but plan to not have to do it. Obviously in hard overhead environments this will be even more necessary.

Of course you always need to subtract enough of a reserve to get you and a buddy to safety before using either of these rules. Thirds already has your buddy in mind to some extent. One third for me to get there, one third for me to get back and another third for my buddy to get back. The truth is that in an emergency you are going to use much more air and this may not be enough air. 500psi may or may not be enough of a contingency depending on the dive planned.

When diving in lakes I often do multi level dives in order to balance the need for adaquate reserves and making the dive as long as possible. As you move shallower less of a reserve air supply is required so you can spend time there even when you couldn't have deeper. This may also serves to slow the ascent and allow extra offgassing.

~Jess

 

[JessH]

The fourth strategy is pure "thirds", and it's only applicable to situations where you must get back to your starting point, and your starting point IS the surface. In other words, the pure rule of thirds is really only used in caves.

I actually find most of my lake diving to be similar in this respect. I go in where it is shallow, and follow the contour of the lake bed down to where it is deep before turning around and coming back up.

In a hard overhead environment such as a cave the rule of thirds isn't always going to give you enough gas for two divers to exit when the fit hits the shan. This is why fourths and sixths are sometimes used. The bottom line is that no rule is going to be perfect all of the time and that you need to think through your gas requirements before every dive.

~Jess

 

[PvilleStang]

Well, with the 500 psi, I've been taught to throw it out the window. For example:
3000 PSI tank fill, now take out 500 psi for the bottom reserve:
2500 PSI to work with. Using the rule of halves, you'd turn when you go through half of that gas, or 1250 PSI. Here, you'd take the 1250 and add back on your 500, and you'd get 1750, which would be a typical turning point on an aluminum tank. I try never to touch that 500 PSI: I see it like draining your gas tank in the car all the way, just causes too much trouble.

 

[TSandM]

The bottom line is that no rule is going to be perfect all of the time and that you need to think through your gas requirements before every dive.

Right on, JessH!

Pville, why is 500 psi the automatic reserve? If you're diving deeper, or have smaller tanks, you may have to reserve much more than 500 psi to be safe. "Rock bottom", or minimum gas to get two divers to the surface from the deepest point in the dive, may be almost half the volume in the tank, if you go deep on an Al80, for example.

 

[PvilleStang]

I actually find most of my lake diving to be similar in this respect. I go in where it is shallow, and follow the contour of the lake bed down to where it is deep before turning around and coming back up.

In a hard overhead environment such as a cave the rule of thirds isn't always going to give you enough gas for two divers to exit when the fit hits the shan. This is why fourths and sixths are sometimes used. The bottom line is that no rule is going to be perfect all of the time and that you need to think through your gas requirements before every dive.

~Jess

Where ARE the cave divers at 12:30 in the morning...

Guess I'll have to wake up to this one.

 

[JessH]

Are you refering to me comparing lake diving to cave diving or saying that the rule of thirds might not be enough of a reserve?

If it is the former then let me head things off by saying that lake diving is nothing like cave diving. My comparison was simply due to the fact that the way I do many lake dives the ascent becomes part of the dive not something I do at the end of it to get to the surface. If diving Lake Travis was anything like cave diving I wouldn't be itching to take a cavern(and eventually cave) class. So far I have 6 Cenote dives under my belt and I want more!

If it is the later, well there is a reason that sixths is mandated for divers diving doubles at certain levels of cave training.

~Jess

 

[JessH]

Pville- If you are deco diving that 500psi reserve may be no where close to what you need if things go bad. What if you loose all of your gas and have to share your buddies? Of course deco bottles can change things a bit for the better.

~Jess