The Rule of Thirds.

[detroit diver]

It's an ascent rate issue. If you make a 1 min stop at each of those depths, you assure that you are not rising at a rate faster than 30fpm. It's on the conservative side, but I know that I offgas much better when I do it this slow.

I figure the cf required using a SAC factor of 1.0. That assumes a fairly stressful level-something you might encounter during an OOA or other type of emergency. You must factor 2 people into this equation-yourself and your OOA buddy/new found friend So, 2 cf per stop (as I mentioned above) x 9 stops/travel =18cf. If you're diving an AL 80 (that really has 78cf, I think) with a starting pressure of 3000 psi, then the math looks like this:

3000 / 78cf=39psi/cf x 18cf = 700psi needed for an OOA ascent.

Getting back to the boat with 500 psi is going to possibly do 1 of 2 things-either bend 1 or both of you, or eliminate one or both of you. This might sound extreme, but when the S*it hits the fan, everything else falls apart also. Why not just make the calc right in the first place?

If you're diving something smaller, then you'll need more gas (ie a 72cf tank with 3000 psi start will need 800cf for rock bottom), and visa versa. With my double 104's, I can get away with 300 psi for obvious reasons-2 tanks and larger volume.

 

[jeffsterinsf]

detroit diver once bubbled...
9 cuft/person

1 to sort things out at 100 ft.
1 for ascent to 70 ft
1 for 1 min at 70 ft
1 for 1 min at 60 ft
1 for 1 min at 50 ft
1 for 1 min at 40 ft
1 for 1 min at 30 ft
1 for 1 min at 20 ft
1 for ascent to surface

Errr, what about the increased ambient pressure here? Your average depth is about 50' resulting in about a 1.6 bar overpressure, or 2.6 bar total pressure. You should then multiply the 9 cuft you quoted by 2.6 to get about 23 cuft (at 1 bar) per person. Assuming there are two of you breathing off one 80 cuft (at 3000 psi) tank, that would be nearly 900 psi per person, not including a reserve.

Yes?

1 cuft/min may be a good SAC estimate for two stressed divers (each), but that means that I need 1800 psi + 500 psi reserve = 2300 psi to satisfy "rock bottom" requirements if I have a typical AL80.

I agree with UPs leading questions, if you are within NDL and are willing to forgo the 3 min at 3 m safety stop, you should be able to surface in 3 min from 100' in recreational, open-water situations. That cuts the requirement to 1 min of "hey, its ok, let's get it together" time, and 3 min of ascent time, a total of 4 min to surfacing. 4 min * 3 depth factor * 1 cuft/min/person * 2 people = 24 cuft. That would be about 900 psi + 500 psi reserve = 1400 psi.

If I were to get to 15' in this situation, and we were both calm and collected, and at a safe exit point. I'd probably spend any remaining gas to the 500 psi level for a safety stop.

What does this mean for dive planning for a dive to NDL on an AL80? If I am at a 0.5 cuft/min SAC and diving at 100', that would be about 2 cuft/min at that depth. At 100' the NDL is about 20 min, or 40 cuft consumed. From above, I need another 24 cuft for fastest ascent in an OOA situation, and (80*500/3000) 13 cuft for reserve; total of 77 cuft. Throw in a few more for the descent, and I'm at the limit of the tank, beyond it if the tank was at 2800 or so instead of 3000 at the start of the dive. Higher SAC? This is where dive planning comes in...

 

[Uncle Pug]

Yes... you do need to take into account increased usage at greater ATA... but you do not need to add a 500psi *reserve* on top of that.

The concept of Rock Bottom replaces the concept of *reserve*.

If you want to pad your Rock Bottom with extra gas for a reason then give the reason.

The old notion was to have 500psi reserve... but you were never told what it was really for or how that figure was derived. What we are trying to do here is educated ourselves as to what is really needed and why.

Travel time between stops can be rounded into the stop by figuring 20 seconds travel and 40 seconds at the stop... this gives you a 30fpm ascent rate.

If you figure a 30 fpm ascent rate to your first stop at 30' then in DD's example to go from 100' to 30' should only take 2:20 not 5:00... safe ascent does not mean dallying around... if you don't need to do the stops then don't do them.

 

[The Chairman]

had something to do with the number of helpings at the Dinner on the Grounds last Sunday at church... :tease: OK, OK, so I viloated that on the Pecan Pie... shoot me! Maybe the rule of halves would have been more appropriate... half for me and half for the rest of you clowns! Rock bottom? That was that tub of 'nilla ice cream after I slathered it all over my fourth helping of Pecan Pie... :tease:

 

[jeffsterinsf]

Uncle Pug once bubbled...
...

If you want to pad your Rock Bottom with extra gas for a reason then give the reason.

...

Reason: Sticky or otherwise inaccurate gauges

I'll consider backing down to a couple hundred psi. With a 6000 psi FS gauge, 2% of full-scale is 120 psi. Bleeding off a cylinder to nothing and watching the gauge doesn't really tell me that, for example, 800 psi on the gauge is not 680 psi or 920 psi.

That 120 psi difference in the wrong direction could be the difference between a calm surfacing and both of us needing to unexpectedly and rapidly increase ascent rate. I know I've got about three or four good breaths left when the valve is shut off, and imagine would have more warning when the tank actually goes dry, rather than being shut off at pressure. However, I'd rather have the gas in what could be a very stressful situation. In the scenario, your buddy just went OOA three or four minutes ago -- having it happen again could freak them entirely.

Regrettably, I haven't seen any absolute accuracy claims on any of the SPGs. I believe that ASME B40.1-1991 Grade A is 2/1/2%FS and Grade B is 3/2/3%FS.

 

[Uncle Pug]

jeffsterinsf once bubbled...
Reason: Sticky or otherwise inaccurate gauges

Time to replace that gauge... or at least find out if your fears are based in fact.

 

[Aquamaniac]

Pressure gauges, in fact all gauges are manufactured to a tolerance.
That does not mean that because it has a 10% tolerance it will be out by 10%. It just means that it COULD be out by as much as 10%. In actual fact it could be as accurate as +/- 0.5%, its just that the OEM wont promise or warrant it.

When fitting out machinery we have a standard practice, very similar to blue printing.
We buy the gauge and test it in OUR lab. If its within OUR criteria, we use it. If its not we return it. More often than not, purchased pressure gauges are within 1-2%. Better than most can read (due to paralex error). Even though it may be ASME B40.1-1991 "B", it can still be very accurate.

When I bought my SPG, I tested it and found it to be ok. If it were out by more than 2% I would have asked the LDS for another, I would have kept asking for them until I found one that I liked. (The LDS has no problem with my pedantic checking.)

Dave

 

[AquaTec]

Uncle Pug once bubbled...

Dave mentioned some of them... dives where it is important to surface at the point of entry.

Should an NDL open water OOA occur at the furthest point the option to ascend directly exists but could prove dangerous.

If the rule of thirds plus rock bottom has been followed there is nothing to prevent the safe movement of the buddy team back to the exit point while air sharing.

The idea that an ascent must be immediately initiated just because one diver is OOA is commonly taught... but I am suggesting that it is not necessarily the best course of action if ascent would put the divers in harms way.

Proper planning and the use of Rock Bottom gas reserve with the rule of thirds gives the divers the ability to egress to the point of safe ascent.

This can only be done if Rock Bottom is held in reserve.... just the rule of thirds will not accomplish this.

Doug, semantics aside, surely there are *recreational* dives that you can imagine that would call for exit at point of entry as the best course of action.

Dave has already made some good suggestions... I will add one more... a current dive in Deception Pass.

Having enough gas for the trip in and enough for both divers’ trip back out plus the gas necessary to ascend safely is good planning.

Half in and half back with just 500 for ascent does not make for happy campers if things go sidewise. Not only does one have to contend with boat traffic... the walls on the inside are shear and prevent egress.

Another thing I would like to address... 500 psi is not enough for the average rec. diver and buddy to make a safe ascent from all depths. What amount would you figure for rock bottom from 100'?

PUG - wow i am in late on this one. I have been out diving all day [in a swimmming pool though]

My understanding of this thread may have been wrong, basic scuba. my undertanding of basic scuba wound not include diving in overhead enviroments or requiring decompression. I believe that i can take the deffinition one step further. recreational divers shalll alway have direct access to the surface without the requirements of any stops.

you are discussing dealling with an ooa diver, so a safety stop at any depth is not required you need to mearly make a controled direct asscent to the surface, the acceptable ascent rate would be 60 feet per minute.

so a 100 foot dive for less than 20 minutes in an ooa situation a direct ascent to the surface at 60 feet per minute is an exceptable procedure. keep in mind durring our normal diving we add in several procautions, such as an ascent rate of 30 ft/min and a safety stop at 15 feet and maybe even the rule of thirds.

but as a recreational diver myself and my buddy should be able to make a controled asscent to the surface from 100 feet in about a minute and a half. so you guys can do the math but if you figure a sac rate of 1.0 times 2 divers = 2.0 down to 4 ata = 8 with an average of whatever............ i would say we can make it to the surface on less than 700 psi


i do agree with where you have taken this thread, that some diving does require a rule of thirds or more. but i also agree with you that we should keep tech and rec seperate, i get the feeling that most people on this baord have gotten their tech training from this board.,

 

[Uncle Pug]

AquaTec once bubbled...
you are discussing dealling with an ooa diver, so a safety stop at any depth is not required you need to mearly make a controled direct asscent to the surface, the acceptable ascent rate would be 60 feet per minute.

...that old way of thinking is wrong.

An OOA does not necessitate immediately heading for the surface... and certainly not at 60fpm.... at least for the well trained and properly equipped recreational diver.

Furthermore... the 15' *safety stop* is not something you need to omit (but rather replace to be even more progressive.)

A better proceedure is to always do a minimum deco of 1 minute 30',20',10' since all dives of any meaningful duration are in fact deco dives.

And furthermore an adequately trained and equipped buddy team will have no trouble making the OOA ascent in a normal controlled manner at 30fpm making the stops mentioned above.

Times are changing.

i do agree with where you have taken this thread

the destination was always the goal.

 

[Aquamaniac]

AquaTec once bubbled...

......... i get the feeling that most people on this baord have gotten their tech training from this board.,

Hey! I take offense to that, MOST people on this board, and in particular this thread were diving when Bill Gates was playing on his Radio Shack TRS 80.

Dave