diving thirds vs rock bottom

[GrumpyOldGuy]

My opinion is a little different.

I view 1/3rds as a rule of thumb. Its easy to understand, takes no brains to follow and will keep you out of trouble 99% of the time.

Rock Bottom is the proper way to calculate and do a gas plan. Its much more accurate and will likely give you more bottom time.

A gas plan though is not the same as a dive plan, its only one component (although a big one). A WindApp pointed out the logistics of the dive and ascent options may change your turn pressure and dive plan. Currents and other issues (getting lost in my case) may play havoc with even 1/3rds. Doing a blue water ascent on an unplanned drift dive is not indicative of a good plan, but better than drowning.

 

[Adobo]

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.

In high current, where the only way you are getting back to the boat is up the anchor line, thirds alone is very much the opposite of conservative in terms of gas plan for a person who is not solo diving on a recreational dive. If your buddy has an out of gas emergency as you hit your thirds, even if you get back to the anchor line, which btw is not guaranteed, you'll likely not have enough gas to do anything but a straight ascent to the surface. In fact, in that scenario, I imagine the dive team would run out of gas on the way to the surface.

Also I don't know if you meant to imply this but halfway to rock bottom does not equal thirds. Either way, using a half way to rock bottom approach in a non solo dive also leaves the buddy team with potentially insufficient amounts of gas to do anything but a straight ascent to the surface once back to the anchor line if there is a gas emergency.

If one absolutely had to get back to the anchor line to get back to the boat, the way I would plan the dive would be to first subtract out rock bottom (which is calculated assuming an ascent starting at the anchor line). Once rock bottom is subtracted out, I could do something like thirds on the remaining gas.

And as I said previously, if a diver is planning a dive like this and has not had some form of tech or cave training, they likely do not have the training or the equipment to execute this dive properly. There are other concerns other than gas for a dive like this.

 

[boulderjohn]

First of all, this is way too complex for the Basic Scuba section.

The answer, of course, is "it depends," and what it depends upon is pretty advanced. I remember well the arguments I had about this with my instructor in my Advanced Trimix class. To put it as simply as possible, in dive planning, you make a plan that is the most appropriate to your diving situation. No one plan is superior to all others in all situations.

 

[decompression]

I'm not sure if it's too advanced for a basic scuba discussion.....rock bottom is included in gas planning/ dive planning in basic open courses. I'm glad that it's being discussed as its an important part of diving safely.

 

[Kevrumbo]

A Quick Contingency "Rock Bottom" Calculation and Gas Plan Estimate for Open Water. . .

For a single 11 litre tank (AL80), a total of 11 litres/bar metric tank rating and a volume Surface Consumption Rate (SCR) of 22 litres/min -same as a pressure SCR of 2 bar/min*ATA (divide 22litres/min by 11 litres/bar)- using an example NDL air dive to 30m (4ATA) depth in Open Water.

Emergency Reserve/Rock Bottom pressure calculation, from 30 meters with one minute stops every 3 meters to the surface,
-->Just "tally the ATA's":
4.0
3.7
3.4
3.1
2.8
2.5
2.2
1.9
1.6
1.3

Sum Total: 26.5

Multiplied by 2 bar/min*ATA equals 53 bar Rock Bottom absolute reading remaining on your SPG. --this also happens to be the pressure in bar needed for one person in an emergency contingency to reach the surface with the above minimum decompression ascent profile.

So ideally for a two person buddy team, multiply 53 by 2 which is 106 bar for both to reach the surface (sharing in a buddy Out-ot-Gas contingency).

But realistically, for two experienced divers stressed: 106 bar plus 30% of 106 bar equals 138 bar Rock Bottom SPG reading.

For two novice divers stressed: 106 bar plus 100% of 106 bar equals 212 bar (!!!)
--->obviously then, two novice divers on single 11L tanks should not be diving to 30m for any significant length of time. . .


(Note for my metrically challenged countrymen here in the US: A full AL80 tank is 3000psi which is approximately 200 bar). . .

Lets use the Rock Bottom Reserve value of 138 bar for two experienced divers, and round it up to 140 bar for each team member on 11 litre tanks. Since a full 11 litre tank is 200 bar, subtract the Rock Bottom of 140 from 200, and you only have 60 bar to use for the dive. (Be aware that you will consume gas on the descent and probably not have an ideal full 200 bar by the time you reach your operational depth; if for example you arrive at depth with 150 bar showing on the SPG, then subtracting the Rock Bottom Reserve of 140 from 150 leaves only 10 bar to use for the dive. . .)

Of this 60 bar, if you must return to upline, anchorline or shore entry point, use the rule of Thirds: so one-third of 60 bar is 20 bar. Therefore after a delta of 20 bar (subtract 20 from from an ideal full tank of 200 bar for a SPG reading of 180 bar), you must turn the dive and start your return. (If you only have 10 bar to use, then Thirds only yields a delta of 3 bar --you might as well abort and begin your return immediately if for some reason you arrive at operational depth with a SPG reading that's right around your Rock Bottom Reserve. . .)

In conclusion then, the above is how you would conservatively utilize both concepts of Rock Bottom Reserve and Rule-of-Thirds. These two calculations both pre-dive and possibly revised during the actual dive real time on-the-fly become vital, once you start into the realm of technical diving and overhead environments.

 

[TSandM]

The bottom line is that you want enough gas for contingencies, for whatever kind of dive you are doing. The ugliest contingency most of us will have to face is a catastrophic gas loss -- either through freeflow, or because the person we are diving with, or someone else in the group, has not planned or monitored their gas well, and has run out.

If you are on a drift dive, where the boat has chucked you in the water and will pick you up wherever you surface, all you need is enough gas to get two divers to the surface from where the emergency occurs. That's "rock bottom", and for most drift dives, it's going to be a lot less than a third of your tank. "Rock bottom" also changes as the dive proceeds, if you are multileveling -- what you need at the deepest point isn't what you need at your safety stop, and if you execute the dive right, you'll get to use all but the 500 psi minimum reserve. That gives you a lot more bottom time than thirds does, but still keep enough gas around to deal with problems.

If you are shore diving, you'd like to get back to where you started. But on most shore dives, it isn't critical; you can surface offshore and just face an annoying swim in. If you reserve your "rock bottom", enough to get you and your buddy to the surface from any point in the dive, you can use half the remaining gas to go out, and half to come back; if you end up sharing gas, you'll just surface and deal with the swim. This will also generally give you a lot more bottom time than planning thirds would do.

Thirds came out of cave diving, where many times, you have NO option at all but to return to your entrance. There, you have to plan on supporting a buddy through the whole exit. For that reason, you use ONE third of your gas going in, on the theory that you then have TWO thirds to get you and your buddy out. It's not conservative; if you actually HAD to support a buddy all the way out, the increased consumption due to stress, and the slower exit due to sharing gas, would probably land both of you short. Many cave divers don't actually ever dive to full thirds.

In basic open water diving, you should NEVER be in a position where you simply cannot use the surface as an option. The closest would be if you are diving from an anchored boat in strong current, where you would not be able to swim back to the boat, and the boat cannot come get you. In that case, you need more than thirds, because if you use a third of your gas leaving the anchor, you have to get back to the anchor AND make your ascent.

So, outside of cave diving, thirds can be either excessively conservative or not conservative enough. If you use it routinely, you'll short yourself some safe dive time on a lot of recreational dives, and on the more advanced ones, you may be rolling the dice a bit. All gas planning is risk management; we all choose our tools.

 

[DaleC]

Hah hah, Lynne you posted while I was writing!

Seeing as this is the basic recreational diving forum...

Rock bottom for recreational diving is better (IMO) as it looks at the actual dive one is doing and the actual SAC rate of the divers involved. It is a dynamic form of dive planning that changes with depth, divers and dives.

For example, if one is doing a 100' dive on a wreck down an anchor line the same as doing a reef dive with a hard 30' bottom.

If diving thirds in the first case, what happens if your sac rate is .4cuft/min and your buddies is 1cuft/min. If they need to share air will 2/3's be enough to return?

Does thirds remotely address any conditions in the second dive?

Thirds is an overhead technique that doesn't really translate into recreational diving as (by definition) there should be no real or virtual overheads. The exception being boat traffic or kelp I suppose. It doesn't take into account individual characteristics of the dive or your buddies SAC rate. I know someone will say that you turn when the smaller tank/higher SAC diver hits 2/3's but if that isn't you, you've just handed control of your reserve gas over to someone else. Unless you dive with someone you know and trust that may not be a wise idea.

Some people suggest thirds because, for simple rec diving, it is so conservative that most people, most of the time, won't have a problem. However, the problem occurs when people know that plan is too conservative and cheat on their reserves without having a more realistic gas plan to fall back upon. Is anyone seriously turning a straightforward 30' dive at 2000psi (assuming an Al 80) and surfacing with 1000psi? Not with me they aren't.

 

[TSandM]

Just to follow up on DaleC's post . . . where you REALLY need to calculate thirds, you have to do it in volume and not in pressure, and take into consideration the size of the various tanks being used. The math gets complex. True thirds has no place in recreational diving, and there is a reason why people who do technical or cave diving tend to standardize on a common tank size.

 

[lowviz]

...//...where you REALLY need to calculate thirds, you have to do it in volume and not in pressure, and take into consideration the size of the various tanks being used. ...//...

Yes, thirds can be less than rockbottom for a really small tank.

...//... The math gets complex. ...//...

Baloney.

...//...True thirds has no place in recreational diving, ...//...

Thirds plus rockbottom works just fine and doesn't require math any more advanced than multiplication and division...

-the rockbottom gas reserve is dynamic and fades as you get closer to ascent. It doesn't really limit good dives but greatly improves dives gone bad. If one doesn't know how much gas it takes to get oneself and a buddy to the surface safely, one really needs another sport.

 

[Kevrumbo]

Just to follow up on DaleC's post . . . where you REALLY need to calculate thirds, you have to do it in volume and not in pressure, and take into consideration the size of the various tanks being used. The math gets complex. True thirds has no place in recreational diving, and there is a reason why people who do technical or cave diving tend to standardize on a common tank size.

Uh Lynne . . .your SPG reads in pressure units, does it not?

For a buddy team with dissimilar tank sizes the point is to derive a usable turn pressure for your own tank, AFTER calculating a normalized volume of Rock Bottom and/or Thirds for the teammate with the smallest tank. The arithmetic is fairly easy and straight forward (in metric of course):

Example:
12 litre/bar tank multiplied by 180 bar equals 2160 litres
15 litre/bar tank multlied by 150 bar equals 2250 litres

Therefore the 12 litre tank has the smaller volume at 2160 litres.

Calculate thirds of smaller volume tank's fill pressure of 180 bar, and you get 60 bar delta turn pressure. 12 litres/bar multiplied by 60 bar equals 720 litres volume (the normalized value with respect to the smaller tank).

Hence , the calculated turn pressure for the larger volume tank is:
720 litres divided-by 15 litre/bar equals 48 bar.

(How objectively simple was that?)