Help understanding dive planning for new technical diver

[atdotde]

@Dave090 I guess you want to have a plan B for all of those (but usually just one at a time as long as there is no reason to assume they are not statistically independent, i.e. think of a common cause of losing your buddy and your deco cylinder), i.e. I would assume that it's ok to rely on your buddy's gas in case of losing either bottom or one deco gas (assuming that he/she brought enough).

 

[Kevrumbo]

Kevrumbo, thanks so much. That was exactly what I was looking for to help me understand this all. I did talk to my instructor but I think there is a bit of a language gape too. I think the problem is I understand the calculations, but I don't understand which contingencies need to be added to the calculations. Do we need to calculate in case you get separated from your buddy and lose a deco tank? Do you have to calculate one of you has a bottom gas failure and one of you has a deco failure? I don't know what the best practices/recommendations are for these and that is what is making it hard for me to understand the planning. Anyways, lets go through what Kevrumbo said. My questions will be in BOLD.

[QUOTE="For a stressed, emergency SCR, use 30 liters/min per ATA and the average depth in ATA from 45m to 21m, which is 4.3ATA. So you need 30 x 4 x 4.3 equals 516 liters. Multiply this by two divers sharing gas in an out-of-gas contingency and you therefore require 1032 liters Minimum Gas Reserve (MGR).

Since you're using "D11" liter tanks (that is "double Aluminium 11 liter/bar cylinders) for a total of 22 liters/bar, divide 1032 by 22 to give your SPG reading in bar pressure units: 1032 ÷ 22 = 47 bar. Round this up to 50 bar SPG remaining MGR pressure. Whoever reaches this value first calls the dive and the team starts a nominal ascent to the first deco stop at 21m.

So that seems like you did the TDI calculations here for what I proposed right. It still seems like a low amount of bottom gas to leave the bottom with though. We were turning at about 120 bar. In your calculations, you assume you have no failure with your deco tanks right? It is unlikely that you would have an out of gas bottom mix issue AND a deco tank failure though right? Although should you still plan for this? Also, should you also consider the unlikely situation that you and your buddy get separated (I read that somewhere also)?

"For a nominal SCR, let's use a novice tech diver rate of 22 liters/min per ATA as an example. Divide 22 by the total rating of your D11 cylinders, so 22 ÷ 22 = 1bar/min per ATA. Your 45m depth is equivalent to 5.5ATA and you want to spend 25 minutes at this depth, all bottom gas usable except MGR, open water with no rule of thirds, halves or 1/n conservatism required. Therefore you will use 1 x 5.5 x 25 = 138 bar of EAN25 bottom gas at 45m. Round this up to 140 bar. Add the 50 bar MGR from above so your total EAN25 bottom gas requirement is 190 bar. (A full fill of D11 cylinders is 207 bar).

Reserve gas calculation right? TDI says it is based on taking the total amount of gas you need for your assent (deco plus travel to deco stop), multiplied by 2, and add it to the bottom gas. But what I don't understand is when we do this calculation, we can assume that we get to our deco stop and have the gas there (no lost gas scenario)? If we lost the gas (and our buddy also, or if we were separated from them), we clearly would not have enough gas for the proposed dive. For example, with Vplanner there is the option for what your deco schedule is for a lost gas situation. For this dive, we would not have enough gas to get to the surface using just the bottom mix. Isn't that a problem?


As far as I can tell, my instructor did cover all of these issues but I wanted to get a much more indepth understanding of following TDI recommendations with planning contingencies and that is where I got mixed up. Thanks.

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From a practical sense, you can only cover one emergency contingency in gas management planning -or else you will have to carry with you an inordinate amount of reserve gas volume to cover ALL potential scenarios.

There is still a way to improve your odds of recovery to the surface if you and your buddy were unlucky enough to suffer two or more major failures of your gas sources (i.e. Losing both back gas and a deco bottle for example):

• Dive in a Team of Three (which is always recommended for technical diving in cave & wreck overheads);
• And/or have a Support Diver(s) rendezvous with you at your first deco stop with extra bottom and deco gas.
• Be sure all teammates have DSMB's to deploy from depth to signal surface support, in case of an individual separation/lost buddy scenario.

 

[Kevrumbo]

So that seems like you did the TDI calculations here for what I proposed right. It still seems like a low amount of bottom gas to leave the bottom with though. We were turning the dive at about 120 bar. . .

Did you simply turn the dive around at 45m with 120 bar, or begin your ascent to your deco stop at 120 bar?

Turning the dive around at 45m at 120bar makes sense for a 25 minute bottom time and a SCR of 22 l/min: Starting with 190bar at depth 45m, you consume 70bar in 12min going out; make the turn at 120bar and consume 70bar in 12min coming back; and reach near MGR pressure of 50bar reading to start your ascent to the EAN50 deco stop at 21m.

Otherwise if you began your ascent with 120bar after 25 minutes bottom time at 45m, you actually would have to have an excellent SCR of 11 l/min.

 

[Udi Freeze]

@Dave090 i’m suggesting contacting one of the experianced technical divers around you to give you some advice and practical knowlage. I will be more then happy to assist.

 

[Kevrumbo]

Below are some example pressure Surface Consumption Rate (SCR) values for a variety of common cylinders with an arbitrary volume SCR (also known as volume SAC rate or RMV) to help with Gas Management Planning:

Given an arbitrary nominal volume SCR of 22 liters/min per ATA (that's 0.78 cuft/min per ATA in US Imperial Units, a reasonable & achievable volume SCR for most novice divers, and exercise level for fit advanced divers with sustained active finning):

Cylinder Size | Pressure SCR
11L/bar tank (AL80): 2bar/min per ATA;
12L/bar tank (Steel HP100): 1.8bar/min per ATA;
13L/bar tank (AL100): 1.7bar/min per ATA;
15L/bar tank (Steel HP119): 1.5bar/min per ATA;
16L/bar tank (Steel HP130): 1.4bar/min per ATA;
11L Twins (Double AL80's): 1bar/min per ATA;
12L Twins (Double HP100's): 0.9bar/min per ATA;
16L Twins (Double HP130's): 0.7bar/min per ATA.

Given an arbitrary nominal volume SCR of 15 liters/min per ATA (0.53 cuft/min per ATA in US Imperial Units, relaxed with minimal finning for advanced divers):

11L/bar tank (AL80): 1.4bar/min per ATA;
12L/bar tank (Steel HP100): 1.3bar/min per ATA;
13L/bar tank (AL100): 1.2bar/min per ATA;
15L/bar tank (Steel HP119): 1bar/min per ATA;
16L/bar tank (Steel HP130): 0.9bar/min per ATA;
11L Twins (Double AL80's): 0.7bar/min per ATA;
12L Twins (Double HP100's): 0.6bar/min per ATA;
16L Twins (Double HP130's): 0.5bar/min per ATA.

Given an arbitrary nominal volume SCR of 11 liters/min per ATA (0.39 cuft/min per ATA in US Imperial Units, drift diving floating neutrally buoyant & going with the current):

11L/bar tank (AL80): 1bar/min per ATA;
12L/bar tank (Steel HP100): 0.9bar/min per ATA;
13L/bar tank (AL100): 0.8bar/min per ATA;
15L/bar tank (Steel HP119): 0.73bar/min per ATA;
16L/bar tank (Steel HP130): 0.68bar/min per ATA;
11L Twins (Double AL80's): 0.5bar/min per ATA;
12L Twins (Double HP100's): 0.45bar/min per ATA;
16L Twins (Double HP130's): 0.3bar/min per ATA.

The reason you convert from volume SCR to pressure SCR for the particular cylinders you're using is because your SPG reads in bar pressure units, not volumetric units.

You derive these bar pressure SCR's by dividing the volume SCR by the total metric cylinder rating of the tank(s) being used.

For example, given a volume SCR of 22 liters/min per ATA, with double Aluminium 11 liters/bar tanks (D11) for a total metric cylinder rating of 22 liters/bar: 22 ÷ 22 = 1 bar/min per ATA.

To convert to a Depth Consumption Rate (DCR), multiply by the depth in ATA that you plan to descend to: So if you're at 45m or 5.5ATA, your DCR is 1 x 5.5 = 5.5 bar/min.

Lastly, pick a convenient time interval like @Diver0001 's "bar per 10 minutes", so in this case it will be 55bar/10min. Hence checking your SPG at 10min intervals, you should expect to consume 55bar of gas. For example: Starting at 190bar at 45m, after 10 minutes you would expect your SPG to read 135bar (190 - 55 = 135bar); another 10min interval and your SPG should read 80bar; and then finally you should now know you better be heading up to your deco stop within 5 minutes with an SPG reading of 50bar (a total elapsed bottom time of 10 + 10 + 5 = 25min at 45m).

If at any point you find yourself consuming more than roughly 30% than you expected -because of physical exertion, or perhaps a leak somewhere on your Scuba kit- consider aborting the dive and ascend up to your deco stop depth.