Importance of SAC rate in figuring 1/3rds

[DoubleDip]

PLclark196,
Sir,
You are planning to take formal cave training before leaving the daylight zone, correct?

 

[GDI]

Great questions and points here for discussion

we focus on it because the reason for thirds is that 1/3 of your gas is in reserve for the diver with the worst SAC rate. Therefore you have to have his/her gas in reserve at all times....doesnt matter what your SAC rate is.

"His or Her Gas" - No My Gas Reserve That I am willing to share You have to be able to exit the cave and still deal with problem should they arise. Size of cylinder has nothing to do with SAC rate, so you should always plan the higher SAC rate against the smallest cylinder. A diver can have larger cylinders and a better SAC rate. Conversley a diver can have a smaller cylinder with lower SAC rates. The dive needs to be planned with the ability to get ALL divers out. If you were at your max point of turn and in theory were to lose all gas immediately, those remaining thirds need be the reserve to get the supporting diver out and the needing OOA diver out. IF the OOA diver has a noramlly higher SAC rate that may be cutting it close. As for 1/3 'ds. One 1/3 to penetrate, 1/3 to exit, the final 1/3 is reserve and not necessarily for my team members.

You cant plan the dive on what some one else is doing....he is working harder but has bigger tanks so it is OK? cant work it that way.... something goes wrong he is still sucking more gas but out of your tank

AS others have mentioned here once you know the divers you are usually working with you get an estimate of where they would be in the dive as far as gas usage. AS a cave instructor I do not have that luxury all the time and I see the students often working harder then they need to. There are times I work harder then I normally would The increased work effort could be caused by the larger cylinders or other equipment and even the environment and physiology of the diver from one day to the next. You are 100 % correct to say I cannot plan the dive on what someone does but if anyone diver of the team hits their 1/3s sooner and turns the dive sooner that is ok. Your SAC rate can add an additional factor in the dive plan as a means to verify not just how the dive is going but also as a condition to turn the dive now based on time and / or air. If you turned the dive dive based on time having not reached 1/3 ds is perfectly acceptable, but to ignore time may have its risk in air reserves. The rule of 1/3'ds is a safety guideline. One of which I would not by-pass or neglect. Dive controling fators are; Depth (because of gas blending mix limts), GAS/Air (becuase of breathing limits and rates),Time (because of decompression limits and Air limits to complete deco profiles).

cant say he is working harder so you can just plan more gas in reserve for him and less for you? what happens if you end up working a little harder yourself? maybe you dont notice you are distracted and sucking more air than usual while you get situated..... or you do something else to throw off your normal SAC rate? You plan to turn on the team member who hits their turn pressure first..... you dont ration gas per person

SAC adjustments are not a rationing of air supply. They go hand in hand with the rule of 1/3'ds. (880 PSi over 160 cu ft = 53.3 cu ft which = 1/3 of the gas at 2640 psi on LP 80's) I would not be planning more for him and less for me, I would be planning on 1/3's but adjusted for SAC rates if the cylinders were the same size for both divers. My planning is for the exit from the cave. In my scenario example I stated that both divers have the same size cylinder and here is a good guideline example of where SAC adjustment can be used, its all about exiting the cave.

 

[Surelyshirly]

either we are misunderstanding each other and saying the same things or totally disagree....cant figure out which

I have simply tried to state you plan thirds with equal amounts of gas planned for each diver and the diver who uses that gas first turns the dive with all divers exiting. divers with larger tanks will have additional air but that air will not be part of the gas plan.

therefore as i see it sac rate is not part of the plan, gas volume is

sac rates only play in when planning specific goal oriented dives....such as a certain place or time at depth, and then you match your gas to the amount necesary for the goal ...... I will admit I didnt convey that part well but did not think it was what was being asked...can be wrong

 

[Cave Diver]

I'm trying wrap my brain around figuring 1/3rds and just what to do with dissimilar SAC rates . Dissimilar tanks are covered in most cave diving manuals and workbooks but I've not found the discussion of dissimilar SAC rates. Does this enter the equasion of when to turn the dive ???

The starter of the thread asked about using SAC to plan a dive, and yes it can be done
However
I hear you and yes your SAC can very in a dive. IF you work harder, then you will turn sooner (all other factors and intelligence considered) and your time to turn less then planned for on time. IF you are doing a dive that would require deco then knowing your varibles and bringing them together is a plus. You can alwys turn the dive on gas, and/or time (and for any other reason) IF I were diving where a deco requirement was likely then knowing my turn time, my turn gas and mmy turn pentration mark all can contribute to a safer dive.

For divers diving at the intro level knowing their SAC rate and comparing that to NDL will help keep them out of increased risk. 1/3 are normally suffice.

No, he asked if you use SAC to turn a dive, so I answered accordingly.

I personally would not use SAC to set turn pressures, because SAC rates can vary wildly. I would use it more as a guideline to estimate at what point in the dive the turn may occur at, but it would be that, just an estimate.
YMMV

AS others have mentioned here once you know the divers you are usually working with you get an estimate of where they would be in the dive as far as gas usage. AS a cave instructor I do not have that luxury all the time and I see the students often working harder then they need to. There are times I work harder then I normally would The increased work effort could be caused by the larger cylinders or other equipment and even the environment and physiology of the diver from one day to the next. You are 100 % correct to say I cannot plan the dive on what someone does but if anyone diver of the team hits their 1/3s sooner and turns the dive sooner that is ok. Your SAC rate can add an additional factor in the dive plan as a means to verify not just how the dive is going but also as a condition to turn the dive now based on time and / or air.

I think we all pretty much agree on this and are stating the same premise, just in different ways. SAC is part of the planning process and may influence the turn point.

either we are misunderstanding each other and saying the same things or totally disagree....cant figure out which

I have simply tried to state you plan thirds with equal amounts of gas planned for each diver and the diver who uses that gas first turns the dive with all divers exiting. divers with larger tanks will have additional air but that air will not be part of the gas plan.

therefore as i see it sac rate is not part of the plan, gas volume is

sac rates only play in when planning specific goal oriented dives....such as a certain place or time at depth, and then you match your gas to the amount necesary for the goal ...... I will admit I didnt convey that part well but did not think it was what was being asked...can be wrong

We all seem to basically agree that SAC is part of the planning. It tells you if you have sufficient gas to safely complete the dive as planned. I think we mostly all agree that SAC can vary as well and that the dive should be turned whenever the established volume has been consumed, whether its 1/3, 1/6 or some other amount.

The variation in SAC between divers may either hasten that point, or prolong it. In the event of prolonging it, adequate planning should have been made to ensure sufficient gas for any deco obligations that may arise have been met as well.

Overall, a well rounded dive plan will take many things into consideration, as well as contingencies for unexpected events as well. SAC rates is just one part of it.

 

[GDI]

CD - good observation

It is another tool for the dive planning process

 

[Doc Harry]

Correct me if I am wrong, but Tom Mount of IANTD has a big discussion on using SAC to figure 1/3s, as well as some tables for figuring turn pressures based on SAC.

I think Steve Gerrard's new cave diving book also covers the topic.

Frankly, I find the whole issue confusing and prefer instead to base turn pressures on volume rather than SAC.

For me, it is a lot easier to convert dis-similar tanks to volumes and figure 1/3s that way.

 

[DA Aquamaster]

I agree that using volume is a lot easier with dissimilar tanks.

I also think everyone is promoting the same concept, just articulating it in different ways.

SAC can become important during a dive. For example if the team reaches a jump or gap, everyone on the team should assess whether they each have enough gas to make it worth the time and effort to put the jump or gap in as it makes no sense to place a jump reel then turn the dive 50 ft past the jump.

SAC is also important in terms of deco planning. First from the persepctive of knowing how much time at a given depth you could rack up on the dive with a given third - and in turn how much deco obligation you will acquire. If you know your SAC (and your team members) and the depths/profiles involved, you will have a good estimate of the deco obligation that will result and that is vital in terms of ensuring you have enough deco gas available. You may also find you simply want to turn the dive "early" based on time rather than thirds to keep the deco within more comfortable limits.

In addition to using SAC rate in planning for the deco gasses, you will also use SAC and deco obligation for lost gas planning. For example if you are conducting a typcial north Florida 32% nitrox dive with only O2 for deco, you need to ensure you have enough backgas remaining in your reserve third to accommodate the longer deco on back gas.

In either case you may find you need/want to turn the dive earlier than thirds to ensure you have adequate redundancy during the deco phase of the dive. At a minimum, SAC and gas planning will serve as a cue that you need more gas or redundancy if you want to push farther on a given third at deeper depths where deco obligations begin to add up.

Team size is also a consideration. I often dive with just my sig other and we are very well matched in terms of SAC, swim speed and overall level of aggression in diving and dive planning. But as a two person team we have no margin for error on thirds and a single failure at max penetration can put us very near the edge as there is then only a single third available in reserve - exactly enough to get us both out in a low flow cave and we will be sharing that gas on the way out. With a three person team with the same failire, the team has an extra third in reserve among the team so they have a bit extra to accommodate a delay in exiting.

My full cave instructor pointed out that when diving thirds with a two person team, or if diving solo, having a stage along as a buddy bottle makes sense as it provides an additional 1/3rd in reserve gas and provides many of the advantages of the three person team.

 

[Meng_Tze]

There are three things when it comes to gas planning: volume, plan (1/3's, stages or some thing like that) and SAC.

Volume will determine how much you have. Plan determines how you use the volume. SAC determined how fast you use that volume.

On straight forward simple dives, SAC does not really become an issue.

On more complex, multi faceted dives, it very quickly becomes a gating factor and will then start to drive the plan.... I.e, 'because I have a sac of x, I now need to have gas deposits here, and here and there in order to swim it all out'

 

[Doc Harry]

SAC can become important during a dive. For example if the team reaches a jump or gap, everyone on the team should assess whether they each have enough gas to make it worth the time and effort to put the jump or gap in as it makes no sense to place a jump reel then turn the dive 50 ft past the jump.....

Ya'll have some good points that I'm going to have to review in more detail.

The tables to which I refer strictly use SAC to determine turn pressures. I don't like that. You're right, gotta consider volume, SAC and plan.

Thanks

 

[Cave Diver]

Ya'll have some good points that I'm going to have to review in more detail.

The tables to which I refer strictly use SAC to determine turn pressures. I don't like that. You're right, gotta consider volume, SAC and plan.

Thanks

While you're reviewing, there is some good info on GDI's website: Genesis Dive Institute of Florida presents: Scuba Diving Tips