diving thirds vs rock bottom

[smokey braden]

a diver on a different thread posted that he thought rock bottom was better than diving thirds.

what are the pros for diving rock bottom?

what are the cons for diving rock bottom?

what are the pros for diving thirds?

what are the cons for diving thirds?

when addressing rock bottom vs diving thirds, is there a winner?
or is this a case of apples and oranges?

thank you for your time and i'm sure i will receive lots and lots of good info.
smokey

 

[rhwestfall]

lets say you dive thirds -

statement: it takes 1/3 to get "there", then 1/3 to get back, and 1/3 in reserve

with that, you do not have enough if you are sharing, to allow for anything more than "getting back"....

now, establish a reserve, and divide the balance by thirds, and you are planning "smarter"...

 

[DevonDiver]

Rock bottom assumes a known ascent time. Contingency planning for diving in an overhead environment, assumes potential delays to a predicted ascent time. Rule of Thirds covers that contingency.

 

[DBailey]

Both systems are alloting for a certain amount of air to be held as a reserve. In diving thirds, you are holding 1/3 of the air as the reserve. In rock bottom, you are holding an amount that is derived from a more detailed look at the dive plan. In some cases, the rock bottom calculation will show that 1/3 is not enough. In other cases, the rock bottom calculation will show that 1/3 is too much.

For each diver, it is a risk assessment that needs to be discussed in the team.

 

[flots am]

Rock Bottom:
Pro: More accurate if you have accurate numbers for you and your buddy.
Con: A little harder to calculate. If your SAC rates are overly optimistic, you might not have enough gas.

Thirds:
Pro: Easier
Con: Less accurate, Might not actually give you enough gas in an emergency. Might make you reserve more than you need to, cutting your dive short.

My personal preference is Rock Bottom with known good stressed SAC Rates for me and my buddy.

If you're not sure about your buddy, diving thirds and turning the dive when the first diver hits 2/3 of a tank might be a better guess.

See? Clear as mud.

Edit:
I see you listed as a Solo Diver.

This changes the numbers considerably, since you don't need to reserve any gas for your buddy.

Generally, you would need to calculate how much gas it takes to get you safely to the surface in an emergency from the deepest part of your dive, while stressed, and also make sure that your emergency gas (pony) would contain at least this amount.

 

[decompression]

A discussion I've had with my students involves the "solo" diver or a diver from another team....the scenario involves what happens when an OOA diver (not your buddy) approaches you in a panic near the end of your dive? Will your third be enough for 2? If not, what would you do? Does a rock bottom calc cover this?........just fish food for thought.

 

[Moonglow]

You plan a dive, calculate how much gas that you'll need, then take twice as much. Pony up. Hang bottles. Stage bottles. This isn't tennis, you know.

 

[flots am]

A discussion I've had with my students involves the "solo" diver or a diver from another team....the scenario involves what happens when an OOA diver (not your buddy) approaches you in a panic near the end of your dive? Will your third be enough for 2? If not, what would you do? Does a rock bottom calc cover this?........just fish food for thought.

Rock Bottom should be enough, since it takes your "buddy's" gas usage into consideration. However some random panicked diver is not your buddy and might use a lot more than you expect.

A diver with a "really freaked out" SAC rate of 2CFM will use about 34 CuFt of gas to do a safe ascent from 100', assuming a minute to get his act together on the bottom, a 30'/m ascent rate and a 3 minute safety stop. If you ascend faster and skip the safety stop, it's less, but it's an interesting number in any case.

The calmer diver with a 0.5 SAC would only be a little more than 8 CuFt, so if you assume the OOA diver is hoovering and the donor is calm, it could take about 42 CuFt to get to the surface including a slow ascent and a safety stop.

If you assume a stressed SAC of a little more than 1, it becomes more manageable, with an ascent requiring about 28Cuft total for both divers, which is about 1/3 of an AL 80.

I guess if you're diving thirds on an 80, you should only share with moderately stressed divers.

flots.

 

[Adobo]

a diver on a different thread posted that he thought rock bottom was better than diving thirds.

what are the pros for diving rock bottom?

what are the cons for diving rock bottom?

what are the pros for diving thirds?

what are the cons for diving thirds?

when addressing rock bottom vs diving thirds, is there a winner?
or is this a case of apples and oranges?

thank you for your time and i'm sure i will receive lots and lots of good info.
smokey

The questions are actually too vague to answer definitively. It's like asking, which is better, a butter knife or butcher's knife?

Are we talking about open water recreational dives or tech dives? Or are we talking about cave dives?

I use minimum gas (what you refer to as rock bottom) in recreational and technical dives in open water. I use thirds (or actually, a more conservative variant on cave dives). I would use a combination of both on an open water technical dive wherein I would have to return to a specific spot to begin my ascent. (I haven't actually done this as I am no fan of doing deco on an anchor line.)

I haven't thought it through but thirds seems like it would work on recreational dives but it seems incredibly sub-optimal. In dives where no emergencies occur, you will wind up coming back with a ton of unused gas.

In dives where thirds (or even more conservative gas plan) is really required, a diver without some form of technical or cave training is inadequately trained and likely inadequately equipped to execute the dive.

 

[windapp]

In dives where thirds (or even more conservative gas plan) is really required, a diver without some form of technical or cave training is inadequately trained and likely inadequately equipped to execute the dive.

In high current, where the only way you are getting back to the boat is up the anchor line, thirds (or half way to rock bottom) is required to ensure that you don't end up on an unintended drift dive.