Mismatched SAC rates and Rock Bottom

[Kevrumbo]

For starters, figure out Rock Bottom using the teammate with the higher Surface Consumption Rate (SCR). Pad this value by an extra 30 to 100% as needed for more conservatism due to physical exertion, environmental conditions such as current, diver skill level (novice/beginner) etc.

Example:

18m Beginner/Novice OW Limit ~

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 22 litres/min by 11 litres/bar)- using an example NDL air dive to 30m (4 ATA) 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 (!!!) -note a starting full 11L cylinder (AL80) is 200bar (3000psi)!
--->obviously then, two novice divers on single 11L tanks should not be diving to 30m for any significant length of time. . .

________________
Here's another Emergency gas planning example & exercise, for a dive to the Basic Open Water Limit of 18m/60':

Emergency "Stressed" 28 litres/min SCR (Surface Consumption Rate, also referred to as SAC), with 18 meters (2.8 ATA) depth NDL, and with one minute stops every 3 meters to surface:

2.8 x 28 x 1 = 78.4
2.5 x 28 x 1 = 70
2.2 x 28 x 1 = 61.6
1.9 x 28 x 1 = 53.2
1.6 x 28 x 1 = 44.8
1.3 x 28 x 1 = 36.4
1.0 x 28 x 1 = 28

Sum Total: 372.4 litres gas needed to ascend to surface for an emergency contingency.

Divide the above total by the metric tank rating of the Scuba cylinder in use; for this example let's use the AL80 tank again which has a metric rating of 11 litres/bar. So 372/11 = approx 35 bar.

That's 35 bar to get you to the surface --to get yourself and sharing gas with an Out-of-Air Buddy you will need at least twice this amount: 35 x 2 = 70 bar. Therefore your Rock Bottom Reserve is 70 bar showing on your SPG --if there is no emergency air sharing contingency at that instant, just continue the dive but start a easy nominal ascent to the shallower depths between 5m and 9m. Finally, be at your 5m safety stop with your buddy with no less than 50 bar showing on the SPG.

Your usable gas for the dive is your starting pressure subtracted by the Rock Bottom Reserve --so 200 bar minus 70 bar equals 130 bar usable. Let's use 20 bar of this 130 to get squared away in good trim & buoyancy on the descent to 18m which leaves 110 bar usable. Now if your dive plan calls for returning to near the vicinity of your original point of entry (like a beach dive for instance), then turn the dive back around when you use half of 110 bar or 55 bar consumed.

So for a nominal dive with an SCR of 22 litres/min on a 11L (AL80) tank, your resulting pressure SCR will be 2 bar/min [22 divided-by 11 equals 2bar/min]. Your depth in meters, which converts easily to ATA (simply divide-by-10 and add 1) becomes your multiplier depth factor for your 2bar/min pressure SCR.

18 meters depth is 2.8 ATA (divide 18 by 10 and add 1 equals 2.8); your 2bar/min SCR at depth -or Depth Consumption Rate (DCR)- now becomes 5.6 bar/min. [2.8 times 2bar/min equals 5.6 bar/min]. So 10 minutes at depth 18m on an AL80 (11L/bar) tank in nominal conditions, you would expect to consume 56bar of gas (10min times 5.6 bar/min equals 56bar) and your SPG reading to show a delta down of 56bar. . .

Okay, you splash in to start the dive with 200bar and a 70bar rock bottom. You use up 20bar on descent --initial exertion, inflating your wing etc. -so you now have 180bar with 70bar rock bottom at present depth 18m, a net usable of 110bar (180 minus 70 is 110). Finning out for 10 minutes, an easy swim looking at all the cool marine life around -- you already know by the end of this 10 minute interval that you've used up 56bar (see previous paragraph above), so you decide to turn the dive. You should now know you have roughly 50bar of usable gas left before encroaching on your Rock Bottom Reserve, and realize that you must start a nominal ascent within the next 10 minute interval. Alright, so while turning back, you see a turtle and decide to chase it for a few minutes, huffing & puffing on your reg, until it dives down below your operational depth of 18m; you immediately check your SPG and it reads 70bar --bingo! Rock bottom has arrived so start ascending to the shallower depths . . .or if your Buddy just happens to blow his tank neck O-ring at that instant --you know you've got plenty of breathing gas margin for a controlled air-sharing Emergency ascent profile as described above. . .

Be aware that again depending on environmental conditions and physical exertion (cold water, stiff current, long surface swim, heavy workload at depth etc), you may have to reserve a greater Rock Bottom Reserve --perhaps as high or even over 100 bar on the 11L/AL80 tank to be conservative. (Even better just abort the dive, wait another day or find another easier site to dive!)​

 

[Kevrumbo]

@ 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.

A good article and follow-up to Lowviz above, and finally a clear explanation to finally motivate figuring out Unequal SAC/RMV Rates between team members, and an overview on how to match/handicap gas consumption:

Gas Matching
Gas matching is an important part of the dive planning process and an essential component of any pre-dive safety checks. If you and your team have a bad day and end up having to share air it could make the difference between a comfortable swim out of the cave and a rather frantic race, worrying that your gas reserves might not last. Gas matching is nothing more complicated than adjusting the size of your reserve to compensate for factors that might affect it. We are basically making sure that the reserves we plan are actually large enough to support our exit in an air sharing emergency.

So, when do we gas match? I am sure others could add to the list but here are a few examples for guidance.

Any time when in a team of two divers, the diver with the largest RMV is also using the largest cylinders.

Anytime when the team has to negotiate restrictions.

Anytime, even in a team of three or more, where there is a breathing rate difference of more than double between two divers.

When don’t we need to gas match?

In a team of three or more divers (apart from the breathing difference mentioned above)

A team of two divers using the same sized cylinders (exception as above)

What does all this mean? Let’s look at them more closely.

If a diver with a larger breathing rate uses larger cylinders (which, let’s admit, is quite common) then if something goes wrong at the point of furthest penetration and it is the bigger breather who needs to support the smaller breather then his larger cylinders plus the smaller breather should mean ample gas to swim home. If it is the other way around however, then it is quite likely that the amount of gas in reserve in the smaller breather’s smaller cylinders might not support both of them out of the cave.

When negotiating restrictions with all other things being equal, it should take the same time to travel in through the restriction as it does out. However a considerable amount of extra time might be needed when divers are sharing air through a restriction, hence the need to check that the reserves are going to be big enough.

If you have go someone on the team with a very large SAC then it would be a good idea to check the numbers to ensure the reserve volume is sufficient to support them out of the cave if you have to donate gas.

For my imperial friends we have the delights of what are known as ‘dissimilar tank calculations’. I am going to leave that for a separate short article. Those who have to use these will no doubt understand!

How do we do the maths for gas matching. One way is to cheat a little and get a gas matching table. IANTD certainly have them and they are easy to use.

Another way is to do some simple maths just to make sure the reserves are large enough. It mainly boils down to adjusting your turn pressure, turning sooner than would be expected, using less gas on the way in and out and keeping more than 1/3 in reserve.

As an example if my buddy has an SAC of 24 litres per minute and mine is lower at 12 lpm I can look at a chart at mine and my buddies SAC rates and quickly read the SAC Ration Factor (SRF). The SRF is nothing fancier than a new percentage of my starting pressure at which I need to turn. The usual 1/3 is roughly 66%. Looking at the SRF for mine and my buddies SACs I need to turn earlier at 76%. So for a start pressure of 230 bar I will turn at about 175 bar, thus making sure my reserve third is big enough to support my buddy if needed. I didn’t do the maths, I just flipped the chart over to check the numbers where my SRF crosses my start pressure.

If you dive with the same team then it is easy to pop the numbers down in your wet-notes, either from the a chart or from sitting down with a calculator!

Why is this important? Well if you don’t match gas, this could happen.

Diver A has an SAC of 12 litres per minute and is diving on twin 12s charged to 200 bar

Diver B has an SAC of 15 litres per minute and is diving on twin 15s charged to 200 bar.

They will reach their turn pressure at the same time [applying un-modified Third's]. If, at that moment Diver B has a catastrophic gas loss and needs to share gas with Diver A…..well Diver B will have used 2000 litres of gas going in [i.e. One-Third of 200 is 66.67 bar consumed of Diver B's gas supply; 66.67 bar multiplied by 30L/bar -or same as 15L/bar twinset- equals 2000 litres of gas going in]. Diver A will have used 1600 litres [i.e. One-Third of 200 is 66.67 bar consumed of Diver A's gas supply; 66.67 bar multiplied by 24L/bar -or same as 12L/bar twinset- equals 1600 litres of gas for Diver A]. Diver A will need the same volume to get out, 1600 litres leaving his 1/3 in reserve at 1600 litres. Diver B still needs 2000 litres of gas so that is a 400 litre shortfall. This isn’t going to have a happy ending.

If on the other hand the dive team match gas, and calculate that Diver A needs a reserve big enough to match the gas volume needed by his buddy…..well Diver A just needs 2000 litres as a reserve, leaving his usable gas at 2800 litres, 1400 litres for the dive in, 1400 litres for the dive home. Depending on the depth of the dive that extra 200 litres from each leg of the journey now put aside for the reserve might only be just a few minutes further in but puts the gas plan back in to the realms of being safety first.

1400 litres from the starting pressure means Diver A turning the dive at 145 bar rather than 135 bar.

All of this has been simplified slightly so it does not take in to account deeper cave diving with a significant decompression obligation or any other multi-stage extended range penetration dive. That is probably better left to the classroom as part of a Deep Cave Diver or Multi-Stage Cave Diver course but the main principles are very similar in concept.

Category: Tec Diving Stuff

 

[Kevrumbo]

Ok, Here's a comprehensive gas plan/turn pressure exercise for a planned wreck penetration at 30 meters for 30 minutes. In this case, the plan is complicated by dissimilar tank sizes and unequal breathing rates between the Buddy Pair.

Given Diver A:
12L twinset (total 24L/bar); 230bar fill; nominal SCR (Surface Consumption Rate; also known as SAC rate/RMV) of 9.9L/min;
Diver B:
15L twinset (total 30L/bar); 230 bar fill; nominal SCR of 19.8 L/min (twice as much as Diver A).

Solution:
First start with Minimum Gas Reserve -Rock Bottom Calculation (assume emergency stressed 30 L/min SCR for both divers) to reach Open Water O2 Deco Stop at 6 meters, after exiting wreck:

4.0 ATA x 1min x 30L/min = 120 L
3.7 ATA x 0.3 min x 30L/min = 33.3 L
3.4 ATA x 0.3 min x 30L/min = 30.6 L
3.1 ATA x 0.3 min x 30L/min = 27.9 L
2.8 ATA x 0.3 min x 30L/min = 25.2 L
2.5 ATA x 0.3 min x 30L/min = 22.5 L
2.2 ATA x 0.3 min x 30L/min = 19.3 L
1.9 ATA x 0.3 min x 30L/min = 17.1 L
1.6 ATA x 2 min x 30L/min = 96 L. [Two minutes to switch to O2 Deco bottles].

Total: 392 L. Total MGR for 2 Divers: 392 x 2 = 784 L;

Diver A Pressure MGR: 784 L divided-by 24 L per bar = 32 bar
Diver B Pressure MGR: 784 L divided-by 30 L per bar = 26 bar, (both actual SPG readings or remaining "Rock Bottom" minimum gas reserve in tanks).

Diver A Usable Gas: 230 - 32 = 198 bar (198bar x 24L/bar = 4752 Liters)
Diver B Usable Gas: 230 - 26 = 204 bar (204bar x 30L/bar = 6120 Liters).

Tank Match Starting Reference is to Diver A since he has the lowest and limiting amount of usable gas (4752 Liters).

Since Diver B has higher SCR than Diver A, use SRF Formula and apply adjustment factor to Diver A:

SRF = 1/([SCRa/(SCRa+SCRb)]+ 1) = 1/([9.9/(9.9+19.8)]+ 1) = 0.75 adjustment factor.

Apply SRF of 0.75 adjustment to Diver A's Usable Gas: 4752 L x 0.75 = 3564 L (148 bar). 148 bar adjustment subtracted from 198 bar usable equals 50 bar consumed, so 50 bar consumed from 230 bar Total Fill Pressure is a Diver A Turn Pressure of 180 bar remaining reading on the SPG.

50 bar consumed of Diver A's 24L/bar twinset is 1200 Liters; tank match this reference volume to Diver B:

1200 Liters of Diver B's 30L/bar twinset is the consumed pressure for Diver B of 40 bar out of 230 bar Total Fill (or Diver B turns the dive with 190 bar remaining reading on SPG).

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New exercise -->Diver B still has twice the SCR, but both Divers have the exact same 12L/bar twinsets or 24L/bar total, 32 bar MGR and Total Usable Gas of 4752 Liters. What are the Turn Pressures?

Solution: Diver A is the same SRF adjusted value of 50 bar consumed with 180 bar remaining turn pressure, but Diver B just takes straight Third's of his Usable Gas of 4752 Liters (198 bar); Therefore Diver B has 66 bar consumed out of 198 bar usable and turns the dive with 164 bar remaining reading on the SPG (that is, 66 bar consumed out of 230 Total Fill).

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New exercise -->What if Diver A is the "Hoover Vacuum" compared to Diver B, with dissimilar tanks (Diver A with 12L/bar twinset or 24L/bar total; Diver B with 15L/bar twinset or 30L/bar total).

Assuming same dive, Rock Bottom & twinset parameters above, what if Diver A had 19.8L/min, and Diver B had 9.9L/min (i.e. Diver A now has the higher SCR)?

Solution: Since Diver A has the higher SCR, apply SRF adjustment to Diver B's Usable Gas: 6120 L x 0.75 = 4590 Liters (153 bar).

Now tank match 4590 Liters to Diver A:
Critical Check -->Is this amount less than Diver A's Usable Gas of 4752 Liters? Yes. 4590 < 4752.

So then we can use this amount 4590 L (153 bar); 153 bar adjustment subtracted from 204 bar usable equals 51 bar consumed, so 51 bar consumed from 230 bar Total Fill Pressure is a Diver B Turn Pressure of 179 bar remaining reading on the SPG.

51 bar consumed of Diver B's 30L/bar twinset is 1530 Liters.

Apply the 1530 Liters turn volume to Diver A:

1530 L of Diver A's 24L/bar twinset is 64 bar consumed out of 230 bar Total Fill. Diver A turns the dive at 166 bar remaining reading on the SPG.