Importance of SAC rate in figuring 1/3rds

[plclark196]

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 ???

 

[Surelyshirly]

it is covered in the dis-similar tank sizes. When thirds are adjusted for dissimilar tank size, everyone is basing thier dive on the same volume of air no matter how much they are actually carrying. So if the plan is to turn the dive after using 26 cu then the person who hoovers the most will be the one to breathe up the 26 cu first and announce it is time to turn, no matter how much is left in the rest of the teams tanks.

it is covered in the gas plan.

it is why it is not appresicated when I carry my double 100's. I can breath off them forever but the others on my team are limited by the 1/3 of my gas and they use cave filled 104's or 120's for a reason.

 

[DA Aquamaster]

That is pretty much it...you use 1/3rd of the smallest tanks to ensure that anyone's third, not matter how large or small the tanks is large enough for the biggest hoover in the group and everyone turns when the first diver gets to what ever the turn pressure is for eac diver for the given volume of the smallest third on the team.

SAC really only becomes an issue for plannng purposes, but even then when setting up a circuit, it is pretty practical in terms of you either get to where yopu dropped the cookie upon eaching thirds on the set up dive, ir you don't. Still is is nice to know your sac so you can estimate how many psi or cubic feet it will take get x feet or how much will be used in X minutes.

 

[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 ???

Pretty much like it's already been stated, it's not as important in figuring 1/3's as it is in planning the dive.

1/3's are calculated based on the lowest volume of gas, whether that is determined by tank size, or fill pressure, or a combination.

SAC is useful in determining the distance that you can expect to travel on a given volume and can be used as a yardstick. Factors such as flow, stress, exposure protection, etc. can greatly influence this though, so it should only be used as a reference.

Typically, on an OC dive we check SPG at waypoints, distance markers, restrictions, turns, jumps, etc. Usually it's just to confirm what we already know. Once you get a good graps on your SAC you'll pretty much know what your SPG is gonna read before you even look at it. If you have a dive buddy you regularly dive with you'll probably start to know his as well.

With a good correlation of SAC/gas usage it will make it easier to plan dives and determine if it's within your gas limits, or if it will require additional gas to do safely.

 

[GDI]

While everyone here has focused on volume of gas consumed by the lowest cylinder. This does not really answer your question of how SAC rate in itself can be used for gas calcualations. When divers have the same size cylinders then SAc rates play into the gas calculation. Knowing your sac rates you can calculate which diver will consume the 1/3 of their gas the quickest (given equal work effort to get into the cave). Lets look at a dive plan just to show the point. Diver A has a SAC = 0.7 cu ft/min, Diver B SAC = 0.4 cu ft/min, both divers are wearing LP 80's (dbl = 6.06 baseline) and want based on their 1/3 rule. to do a cave dive where the avg depth is 60 ffw. (2.8 ata) Diver A is the controlling diver but diver B must ensure that he has sufficient gas for them both to exit the cave. Assume both divers have 2640 psi fill. (1/3 = 880 psi or 53 .3 cu ft). Diver A with the higher SAC rate would be turning the dive at 27 minutes or at 1760 psi. At this point of the dive Diver B should have 30.5 cu ft of gas used or remaining psi of 2136 psi.
So yes you can use SAC for setting turn pressures. So with this we know that if diver A was going to use 880 psi as a third then Diver B would only use 500 psi to match gas

 

[Surelyshirly]

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.

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

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

 

[Cave Diver]

So yes you can use SAC for setting turn pressures. So with this we know that if diver A was going to use 880 psi as a third then Diver B would only use 500 psi to match gas

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>

 

[DA Aquamaster]

I'll take the middle ground here... (edit: along with cave diver)

Whenever possible I try to be aware of the SAC's of the other divers on the team as well as the tank sizes being used. It gives me a good idea of who will be turning first as well as when they shoudl be turning and how much gas I should have when I anticipate their turning. If we get past that point and no one has turned yet, I may query the team about remaining gas. Similarly, if a diver looks like they are working harder than usual or are not as relaxed as they should be, I may have us all check gas midway to the turn point, just to see if everyone is still in the expected ballpark.

Thirds is not always as conservative as you want to be, so in some cases, I will turn the dive early, to ensure everyone has enough gas on the way out. For example, My buddy was still fairly new to her dry suit and still working a little harder than usual (and badly under finned). Normally our SACs are identical with less than 100psi difference per dive. On that dive the buddy was sucking a lot more gas than normal and looking tired, so I turned early as it was not going to get better going out as there was very little flow. Better safe than sorry and there is no sense pushing it all the way to thirds in situations where it is not prudent.

So yes...on the one hand, SAC does not really matter - thirds are thirds. On the other hand knowing the SAC of each diver on the team helps a lot and integrating that information into your general awareness of the dive is a real plus.

I also agree with the statement above that you shoudl get so familiar with your gas consumption that you can readily predict the reading you should see on the SPG. That not only helps you recognize when you are using more or less than planned but is also a great navigational aide. For example if it took you 5 minutes and 200 psi going from one check point to another, it should take you the more or less (taking into account any flow) the same 5 minutes and 200 psi going back out. If you pass the expected time and pressure, it is a big clue that you may have taken a wrong turn at a T or other waypoint and it lets you potentially correct that mistake before the gas gets critical.

Not seeing the expected reading on your SPG is also your first clue when the SPG fails and or the needle sticks. I had that happen once at 130' when the SPG said I had more gas than I should have had and confirmed a problem a couple minutes later when the needle had not moved.

 

[TSandM]

What DA Aquamaster said.

If you dive with the same people routinely, you get accustomed to their consumption in comparison with yours. If I'm diving with my regular buddies, I have a very shrewd sense of when the dive should turn, and as DAA said, if we go more than five minutes past where I think we should be turning, I might ask my teammates what's up with pressure, in case somebody has a case of cave awe and has forgotten to check for a while . . .

 

[GDI]

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>

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.