Advanced Gas Planning

[Kevrumbo]

. . .
Btw (and may we all never have to do it for real!) . . .you gotta learn to do the gas calcs underwvater, quickly & decisively on the fly, for a Lost Buddy Search in an Overhead. . .

. . .:
Yes. That calc REALLY bites. Especially when it is someone close to you.............

Modified Thirds Planning and Calculating On-the-Fly Gas Needed to Do a Lost Buddy Search in a Wreck Penetration:

Suppose you originally planned a wreck penetration with Double AL80's (11L/bar Twinset), same SCR and twinset as your Buddy, starting with 200 bar at the entrance with an openwater Rock Bottom minimum gas reserve of 50 bar. So 200 minus 50 bar equals 150 bar usable for the penetration --Modified Thirds of this value is 50 bar (one-third of 150 equals 50), so you would turn-around for egress when you consume 50 bar of gas with an actual SPG reading of 150 bar. If you needed to do a gas-sharing emergency egress with your buddy at this point, you would together need 100 bar to get out of the wreck, with 50 bar Rock Bottom remaining to get both of you to the surface (or your Oxygen deco bottle stop of 6m).

Lets say you used up 30 bar already working against a current getting to the entrance of the wreck for a total of 170 bar pressure available --can you quickly recalculate Modified-Thirds?

No problem with bar pressure metrics: 170 bar minus 50 bar Rock Bottom yields 120 bar usable for the penentration; One-Third of 120 bar is 40 bar which is your new Modified Thirds turn pressure value. Therefore you would turn the dive when you consume 40 bar for an actual turn pressure SPG reading of 130 bar (170 bar minus 40 bar equals the actual turn pressure SPG reading of 130 bar).

What if you lose your buddy at this instance, at the farthest distance inside the overhead your Modified Thirds value allows? How do you calculate the amount of gas to do a Lost Buddy Search?

Easy! At your turn around pressure reading of 130 bar on the SPG, simply add your Rock Bottom value to your Modified Thirds value (50 bar Rock Bottom plus 40 bar Modified Thirds equals 90 bar); Put a line-arrow pointing the way out on your Primary Reel mainline that you've been laying, and take reference note of where you are inside the overhead at that exact point as well. Now go and search for your buddy with the understanding that you must be back at this line-arrow marker by the time your SPG reads this actual value (90 bar). So you would have from 130 bar down to 90 bar reading on your SPG, or 40 bar delta of gas to search for your buddy --if you were to do a straight line search down a long corridor inside the wreck for example, tactically you should use 20 bar out and 20 bar back to your line-arrow marker for a delta of 40 bar, and an actual end of search SPG reading of 90 bar-- you must start your egress whether you found your buddy or not when you use up this 40 bar delta of gas, at the line arrow marker, with the actual 90 bar final reading on your SPG.

At any point before your Modified Thirds turn pressure, for a lost buddy search, the final egress pressure is figured just by adding your Rock Bottom value to the amount of gas you've consumed on the penetration up to that point --for example you start with 170 bar on your SPG and you lose your buddy with 140 bar SPG reading for a delta consumption of 30 bar. 50 bar Rock Bottom plus delta consumption of 30 bar equals 80 bar. Place a line arrow, and now you've got from 140 bar down to 80 bar (a tactical delta search pressure of 60 bar) to look for your buddy, and be back to your line arrow to egress smartly when your SPG reads 80 bar.

At any point after your Modified Thirds turn pressure, all you need to do to figure out the amount of gas tactically available for a lost buddy search is always just your Modified Thirds value --in this case 40 bar. For example, you've turned the dive already on normal egress, and you lose your buddy with 120 bar actual reading on your SPG: Subtract the Modified Thirds value of 40 bar from 120 bar -which equals 80 bar- and it is this actual reading that you must have on your SPG when you get back to your line arrow to successfully exit the wreck with all your Rock Bottom still available to reach the surface.

Remember that on a lost buddy search, you will deliberately encroach and use up the Modified Thirds Reserve Value needed for an emergency gas-sharing egress contingency (and possibly use up some amount of Rock Bottom as well) --in other words, if you do find your lost buddy and worst of all worst scenarios he happens to be out-of-gas in a silt-out . . .well dea ex machina. I hope you're in a 3-person Team, somehow make it out and run into other divers on the outside who can donate gas & assist. .

[Note: the above gas plan is taken from wreck penetration dives on the HMCS Yukon (San Diego); USS New York (Subic Bay Philippines); HMAS Perth/USS Houston (Sunda Strait Indonesia); and various wrecks in Truk Lagoon. Depth 30m using twin 11L/bar tanks (double AL80's) and Oxygen deco.]

 

[ajduplessis]

Nothing to add after Kev's comprehensive response.

 

[Kevrumbo]

[A few corrections to post #40 (see bullet points below)]:

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). Therefore Diver A turns the dive at 148 bar plus 32 bar MGR which is 180 bar remaining reading on the SPG (that is, 50 bar consumed from 230 bar Total Fill Pressure).

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).

--------

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 132 plus 32 bar MGR which is 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); in this case, Diver B turns the dive at 153 bar plus 26 bar MGR which is 179 bar remaining reading on the SPG (or 51 bar consumed of 230 bar Total Fill). 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.

------

Notes: Added the MGR pressure values to the adjusted Usable Gas and Tank Match amounts to reflect the correct actual SPG Remaining Turn Pressure Readings...

 

[beester]

One word: MIN GAS Look it up! (which basically is what andy is saying... including his exposé on disimilar tanks (changing from pressure to volume for calculation... of course after you've identified the volume you can calculate back to pressure to make on the fly calculation and planning easier).

 

[Kevrumbo]

One word: MIN GAS Look it up! (which basically is what andy is saying... including his exposé on disimilar tanks (changing from pressure to volume for calculation... of course after you've identified the volume you can calculate back to pressure to make on the fly calculation and planning easier).

(Andy's "exposé" is only just a quick qualitative overview without any quantitative methodology). . .

Minimum Gas Reserve (MGR) also known as Rock Bottom Minimum Gas Reserve --is that amount to get you and a buddy gas sharing together either to the surface or the depth of the first deco stop gas switch.

Total Initial Fill minus MGR equals Usable Gas available which is used to figure Modified Third's Turn Pressure --with Dissimilar Tank Matching and SRF adjustment as needed (all of which is methodically explained in post #43 above). . .

 

[victorzamora]

Am I the only one curious about how bad the "worst sac" could be in technical diving, and how different that actually could be from the average?

I mean, in your example, one has almost twice the amount of gas the other one carries... (I'll also add that I expect everyone to carry roughly the same amount of gas, though I've been using 2 al80s when my buddy had a d12 on his back, was close enough)

I know of Full Cave divers with SACs hovering near the 1 ft3/min range. I'm a 6'6"/250 (1.98m/115kg), and mine hovers at the 0.55 ft3/min range. A friend I know hovers in the 0.45 ft3/min range. There are several instructors that scooter/deco/float in the 0.30 ft3/min range. So, bottom to top you're looking at a potentially 3x gas usage amongst divers that I know. Put Diver1 on a mediocre-to-bad day and get him to 1.2 ft3/min (a 20% SAC increase isn't hard) and you're talking about 4x the gas of many instructors.

Using the same tanks, my cave instructor could've easily done 2 dives to my 1. I think, upon comparison, there wasn't a single dive we did that I used less than double the gas he did....and I don't consider my SAC to be bad.

Note: My story isn't meant to be boastful or disrespectful about anybody. People have different SAC rates, and those vary day-to-day and dive-to-dive. I know mine fluctuates a lot depending on what dive I'm doing, and I have yet to dive with any Tech diver whose SAC had any real impact on my enjoyment of the dive.

 

[beester]

Of course it's easier to dive the same tanks in the team, makes for fast and easy calculation. However I've dived both in caves and wrecks with disimilar tanks within the team. (for example teammate diving double 18L set and me diving a a double 12 set and 1 80cuft bottom stage... which amounts to not much difference in total gas volume but some differences in calculating)

I have a friend who his quite different in body build (me being tall and wide, with big tidal long volume) and he being a bit smaller, but we have the exact same SAC rate in a lot of different circumstances... it's uncanning.

For sake of gasmanagement I just calculate using a sac of 20L/min which is higher than my highest (non stressed) sac for instance in cold water. Gives it that additional bit of margin and is easier than gestimating your actuale sac for the dive you are planning. (tropical reef withstanding... which is totally different).

 

[Kevrumbo]

Of course it's easier to dive the same tanks in the team, makes for fast and easy calculation. However I've dived both in caves and wrecks with disimilar tanks within the team. (for example teammate diving double 18L set and me diving a a double 12 set and 1 80cuft bottom stage... which amounts to not much difference in total gas volume but some differences in calculating)

I have a friend who his quite different in body build (me being tall and wide, with big tidal long volume) and he being a bit smaller, but we have the exact same SAC rate in a lot of different circumstances... it's uncanning.

For sake of gasmanagement I just calculate using a sac of 20L/min which is higher than my highest (non stressed) sac for instance in cold water. Gives it that additional bit of margin and is easier than gestimating your actuale sac for the dive you are planning. (tropical reef withstanding... which is totally different).

I'm tank matched with same AL80 11L doubles in either backmount or sidemount with my Truk Wreck Dive Guide, plus I carry an AL80 single 11L stage cylinder of bottom mix since my SCR (SAC/RMV) is usually twice that of my Guide. (Our 50% & 100% deco cylinders are dropped & left in the vicinity of the upline and Bridge Superstructure).

On Reel line penetrations, I'll usually leave it at the main tie-off at the entrance, or if the penetration is not too tight, carry & breathe it to half tank pressure and then drop & clip it off to the Reel Line. Then either myself or the dive guide turns the dive on backgas and then pick up the stage on the egress out. On full length hull & superstructure traverses & engine room circuits with nearby exits, I usually breathe the AL80 stage to empty and clip it off to a D-ring, and switch to backgas.

 

[beester]

On Reel line penetrations, I'll usually leave it at the main tie-off at the entrance, or if the penetration is not too tight, carry & breathe it to half tank pressure and then drop & clip it off to the Reel Line. Then either myself or the dive guide turns the dive on backgas and then pick up the stage on the egress out. On full length hull & superstructure traverses & engine room circuits with nearby exits, I usually breathe the AL80 stage to empty and clip it off to a D-ring, and switch to backgas.

So basically you use the cave or wreck protocol I use:
- Cave = Drop stages at MOD or drop them at half + a bit of reserve and switch back to them otw out. (makes sense if you are absolutely sure you'll exit the same way and makes it easier in tighter passages).
- Wreck= basically carry all your stages with you and breath them down before switching.

 

[Kevrumbo]

So basically you use the cave or wreck protocol I use:
- Cave = Drop stages at MOD or drop them at half + a bit of reserve and switch back to them otw out. (makes sense if you are absolutely sure you'll exit the same way and makes it easier in tighter passages).
- Wreck= basically carry all your stages with you and breath them down before switching.

Yes . . .an application of the Half Plus 15 bar Stage Cylinder Method.