Mismatched SAC rates and Rock Bottom

[uncfnp]

In another thread a discussion of diver pairs with significantly different SAC rates and different sized tanks threatened to hijack the OP.

http://www.scubaboard.com/forums/basic-scuba-discussions/459232-3-5-minute-safety-stop-6.html

As an alternative, I have started this new thread.

Any thoughts on RB in mismatched buddy groups?

 

[Bob DBF]

I wouldn't see any problem with mismatched buddy groups, although someone would come back with significantly more air as the airhog/small tank may limit the dive. I usually dive a larger tank than my buddy to minimize that problem.



Bob
--------------------------------------------
I may be old, but I’m not dead yet.

 

[lowviz]

...//... Any thoughts on RB in mismatched buddy groups?

Yes, highly mis-matched SAC's are due to one of the two divers having a very low
SAC. This allows them to downsize their gas load, making their kit lighter. The
problem with this is that half of the rock bottom gas reserve is not yours, it
belongs to your buddy. This can severely limit your dive when using a small
tank.

With very small tanks, diving thirds can easily violate RB if you
wish to get back to an upline or follow the bottom to shore. The good news is
that while you cannot use a pony bottle to extend a dive, it does count in RB
calculations. Thus, you can dive thirds as you seem to have worked out with your
dive buddy as long as you carry his reserve somehow. I'll probably take some
flack for this, but his reserve can be split between your backgas and your pony.


So the problem for you is to grind through the mis-match calculations
and make sure that you carry enough gas for both of you. Your dive buddy
obviously has to do the same. You guys dive ponys so you may find that not much
changes while coming up to speed on this concept. And yes, RB is a DIR thing.
'Thirds' is good enough for everyone else.

A good DM will usually make
some feeble attempt to not pair up vastly differing SAC's so that thirds will work.

 

[Atom]

If the RMV are widely different (let's say 0.4 cuft/min for diver A and 0.8cuft/min for diver B), it can still work with diver A using a smaller tank since diver A will take longer to hit the RB pressure. Even if diver A has a RB pressure much higher than diver B.

Rule of the thumb, if diver B consumes more gas than diver A, to make sure diver B is not the one calling the dive, diver B needs a tank n times bigger than diver A for diver B, n being the RMV of diver B divided by the RMV of diver A. Of course they both need to know their RB pressure because their pressure will be different.

This all assumes that divers are calculating a rock bottom pressure appropriate for their combined consumption and they're aware of their own cylinder RB pressure (since they'll be different).

 

[Adobo]

If you are diving with a buddy, the proper way to calculate rock bottom/min gas is figure out how much gas a stressed out diver needs to do everything that needs to be done and then ascend all the way to the surface. Then take that amount of gas and double since you are reserving gas for your buddy as well.

Take this example scenario...

1) you are at 100ft well away from the anchor line when: (3 minutes)
- your buddy signals to you that he is out of gas. Likely, he is stressed out (and you will be too very shortly).
- You donate a reg to your buddy.
- You have to give your buddy a moment to gather himself. You need to set the tone as a relaxed tone (and not add any unneeded stress to the situation)
- Once your buddy is calm, you clean up the crap (dangling regs, cameras, whatever else that can create another issue on the ascent)
- In my case, I would consider deploying an SMB. That assumes I am with a squared away buddy and we have plenty of gas

2) Then we begin our ascent at ~30ft per minute.To keep the example simple, let's say you do this until we get to 15ft. (3minutes)

3) When there, let's say that the situation is calm enough that you decide to do a 3 minute safety stop. (3 minutes)

4) From there, you take a leisurely 1 minute to get from 15 ft to the surface. (1 minute)

The total time elapsed from the time your buddy signaled that he was out of gas to the time you get to the surface is 10 minutes.

Let's say that the average depth across those 10 minutes is 50ft/2.5ATA.

Let's say that the average stressed out diver breathes about 1 cubic foot of gas per minute.

To have enough gas to execute steps 1-4, you need (10minutes * 2.5ATA * 1cu ft * 2 divers) = 50 cubic feet.

50 cubic feet means different PSIs on different tanks. For example, for an Al80, it is 2000psi. For an HP100, it is 1700psi.

It doesn't matter whether or not you and your buddy breathe at the same rate. The only thing that matters is how fast you and your buddy are going through gas in that gas sharing scenario. If both of you are calm and relaxed, a SAC of 1 cu ft per diver is plenty. If however, one or both of you are sucking down gas due to stress or whatever else, you are going to have forget about steps that are optional. Like for example, don't dawdle at 100ft deploying SMBs.. get off the bottom. Don't bother tidying up a camera. Ditch it if you have to. You have to make certain assumptions in your rock bottom calculations. If those assumptions are wrong, you will have to adjust accordingly.

Does that make sense?

BTW, the example I described above is just that. An example. Its not a recommendation of what you should do in that situation. You will need to decide for yourself what steps you want to take in that scenario, figure out how long those steps will take, figure out what SAC rate is appropriate for a stressed out diver, etc.)

 

[lowviz]

@ Atom:

Two divers with mis-matched SAC's and dis-similar cylinders will make the problem of calculating a proper turn a bit complex. You obviously know how to do this if you have the tables as the tables give you no clue as to how to use them.

For anyone interested, the idea is this:
Assume an 'out-and-back' dive. Both divers start with a given amount of gas. Each diver determines how long he/she can go outbound before having to stop and return with his/her buddy as an added load on the return leg. Note that the time it takes to the turn point can be found by: (fill_cuft - turn_cuft)/SAC. But we don't know turn_cuft. However, we know that we need to return with our buddy and if it takes the same time for the return leg, then this time would be: turn_cuft / (SAC + SACbuddy). So both divers can equate these two calculations and then solve for turn_cuft. Then, each diver converts his/her personal turn_cuft into psi in his/her cylinder. Thus, each will hit the "no added stress" RB turn at the same time, there will be different psi's for each diver, but they will hit their turns at the same time. Having done that, they can refine their plan by factoring in some amount of reserve for sorting out an issue, currents, stress, slower swimming due to gas sharing, etc.

So each diver starts by finding this:

turn_cuft = [1 / (SAC / ((SAC + SACbuddy) + 1))] (fill_cuft)

Then converts turn_cuft into turn_psi for his/her cylinder.


BTW, [1 / (SAC / ((SAC + SACbuddy) + 1))] is the "SRF" that your tables give you.

 

[tracydr]

In another thread a discussion of diver pairs with significantly different SAC rates and different sized tanks threatened to hijack the OP.

http://www.scubaboard.com/forums/basic-scuba-discussions/459232-3-5-minute-safety-stop-6.html

As an alternative, I have started this new thread.

Any thoughts on RB in mismatched buddy groups?

I'm listening. I dive a smaller tank, too, because of my shorter body and low SAC. In warm water I dive an AL 63, in cold water an HP 80, which isn't that much smaller than my buddy's LP 85, except that he has a plus rating, so he can get a good fill and our local shops rarely give me more than 3,000 psi on my 3442, giving me less than my 80 on my fill. But, I like the size of my HP 80 tank and the way it fits my spine, its even smaller than my AL 63. The AL 80 or LP 85 hits me in the head and thighs, which is not very comfortable. Maybe if I convert to sidemount I will find them more comfortable.
Most people don't understand why you would want to dive a smaller tank but when its hitting your head and legs it can be a real irritant, especially when I can get the same dive off a smaller tank. However, if I need to share air, it is an issue.
If I'm acting as a DM or anticipate a difficult dive, I will go with a larger tank, despite the discomfort. More air is always better.
I do need to learn to dive sidemount soon, as I cannot see me physically ever being able to dive doubles and I'm getting to the point where I'd like to have two tanks for various reasons.

 

[Atom]

@ Atom:

Two divers with mis-matched SAC's and dis-similar cylinders will make the problem of calculating a proper turn a bit complex. You obviously know how to do this if you have the tables as the tables give you no clue as to how to use them.

For anyone interested, the idea is this:
Assume an 'out-and-back' dive. Both divers start with a given amount of gas. Each diver determines how long he/she can go outbound before having to stop and return with his/her buddy as an added load on the return leg. Note that the time it takes to the turn point can be found by: (fill_cuft - turn_cuft)/SAC. But we don't know turn_cuft. However, we know that we need to return with our buddy and if it takes the same time for the return leg, then this time would be: turn_cuft / (SAC + SACbuddy). So both divers can equate these two calculations and then solve for turn_cuft. Then, each diver converts his/her personal turn_cuft into psi in his/her cylinder. Thus, each will hit the "no added stress" RB turn at the same time, there will be different psi's for each diver, but they will hit their turns at the same time. Having done that, they can refine their plan by factoring in some amount of reserve for sorting out an issue, currents, stress, slower swimming due to gas sharing, etc.

So each diver starts by finding this:

turn_cuft = [1 / (SAC / ((SAC + SACbuddy) + 1))] (fill_cuft)

Then converts turn_cuft into turn_psi for his/her cylinder.


BTW, [1 / (SAC / ((SAC + SACbuddy) + 1))] is the "SRF" that your tables give you.

Dissimilar RMVs with dissimilar tanks sure make things interesting when planning.

There's also the fact that many people have never calculated their RMV, how to account for that? I use a generous RMV for my RB table, but the RMV of less frequent buddys is an unknown.... I fudge it by increasing the reserve necessary I have to admit it's far from scientific.

I made the tables myself in excel, maybe I should clean the sheet and attach it. They're for a direct ascent with a safety stop and some time added at depth to fix the situation before ascending.

 

[lowviz]

...//... I fudge it by increasing the reserve necessary I have to admit it's far from scientific. ...//...

That is what it is all about, understanding, guessing, and planning. "Scientific" just isn't up to the task....

 

[uncfnp]

So, since 99% of my dives are drift, excluding Bonaire, I'll start with a direct ascent calculation for a RB based on a no added stress dive at 100 feet:

Combined SAC of 1.12

1.12 x 4 = 4.48 x 1 min for preparation = 4.8
1.12 x 2.3 (43 feet average) = 2.58 x 3 mins = 7.74
1.12 x 1.45 = 1.62 x 3 mins = 4.86
Plus 2 cf for 1 min ascent from SS = 19.4 cf

When Eric dives rented
AL 100's that's 633 + 200 (reserve) = 822
Al 80 730 + 200 = 930

My HP 80's 826 + 200 = 1086
Al 63 904 + 200 = 1104

That's very interesting. I had not looked at it this way. Think I'll try calculating a RB with a stress factor next...

i might try Lowviz's out and back too once my eye's refocus. Have I mentioned I HATE math?