Help understanding dive planning for new technical diver

[Dave090]

I am a newly certified technical diver with TDI in Israel and had some questions about dive planning that are not really answered in the manual. We are doing open circuit, open water dives to 45 m with 2 11 Lt tanks (doubles) usually filled with EAN25 (no helium) and a standard deco bottle with EAN50. We would like to stay at our bottom time for 25 mins. I am trying to figure out how much gas I need to bring with me when considering contingencies (we don't have to get back to a start line).

The TDI manual gives examples for dive planning regarding calculating how much gas reserve you should have and what your turn pressure should be (I can email you exert of manual if interested). In their example they do a dive to 30 m on air for 45 m (Vplaner then gives them deco stops with a runtime at 93 mins). The book examples (pasted below) are then based on that hypothetical dive. The problem is that all these examples in the book assume you are not using an additional deco bottle like we are. I provided text below of what is in the manual (rules) and example calculations for their theoretical dive . But what I would like to know is how would I make similar calculations following their rules (see below) when I have a deco bottle? What "rules" should I be following to make sure I bring enough gas and follow a moderately conservative profile? For example, should my gas reserve calculation not be based on getting to the surface but to my gas switch? Should I factor the gas switch into calculating my turn pressure? Thanks so much.





So for calculating their gas reserve they use the following example.

"RULES
1) Begin by planning a dive to the desired depth for the desired time using only bottom gas.
2) Calculate the amount of bottom gas needed.
3) Calculate the amount of transitional and decompression-gas consumed.
4) Multiply the transitional and decompression-gas consumed by two and add it to the bottom gas. The sum represents the total amount of bottom gas that
must be carried for the dive. Note: A diver’s breathing rate should not vary on ascent if he properly monitors and maintains his buoyancy. However, just
as with other phases of the dive, it is possible for a diver’s breathing rate to increase on ascent if he works harder or experiences anxiety.
5) Ensure that the total bottom gas carried is also adequate for allcontingency schedules assuming no gas failures.

If we revisit our sample profile, the dive team needs 3564 L of bottom gas,198 L of transitional gas, and 1477.1 L of decompression-gas.
Total ascent gas needed for one diver = 198 L + 1477.1 L or 1675.1 L
Total ascent gas needed for two divers = 1675.1 L x 2 or 3350 L
Total bottom gas needed including reserves = 3350.2 L + 3564 L or 6914.2 L"

For turn pressure, they use the following example:
"RULES
1) Calculate the volume of ascent gas needed (decompression-gas + transitional gas) for the diver with the highest SCR to surface utilizing bottom gas for the entire ascent.
2) Multiply this number by two to establish the volume of ascent gas needed for two divers.
3) Convert volume to cylinder pressure

Example:
From the sample profile, the total ascent gas needed for one diver = 1675.1 L.
The total ascent gas for two divers = 1675.1 L x 2 or 3350.2 L.
The team will utilize double 15 L cylinders with a service pressure of 232 bar.
3350.2 L = 112 bar."

 

[w3dge]

If you're a newly certified technical diver, wasn't this covered by your instructor throughout the course? Seems bizarre that you would be left to figure this out on your own?

General rules at this level are;
Plan to reach your first gas switch using only 2/3rds of your back gas
In-case of lost deco gas, plan to be able to reach the surface, including deco time, on back gas
Plan to use no more than 50% of your deco gas.

 

[Pkishino]

Yeah, this is very strange that it wasn't covered. Which course exactly did you do?

 

[Diver0001]

It's true that TDI doesn't give the diver a "cook book" for these kinds of things. The IANTD books are more directive in this way. TDI's general message is that the planning must make sense. Their overriding message is that we are all "big boys and girls" and we need to take responsibility for our planning. That's certainly not much help if you are looking for "rules" to follow.

That said your instructor should have lead you though some generally accepted guidelines. These are "best practices" as opposed to "rules" and they were probably mentioned in your book (although I haven't read that book for a long time so I'm not 100% sure of that).

First things first
To start you need to think about a couple of things and make some decisions about principles before you start making any calculations at all. The most important of these is whether or not you want to organize the gas plan such that if

a) a diver had a complete loss of back gas at the worst possible moment
or
b) a diver had a complete loss of a deco gas at the worst possible moment
or
c) both of the above

Then how much safety margin do you want to build into that? With the dives you are doing it's perfectly feasible to address any of the above scenario's. On much more aggressive and deeper dives then having enough gas to cover all your bases in a worst-case failure situation isn't an option anymore and you need to start thinking about damage control as opposed to complete mitigation.

Secondly, get gasses organized
Personally I find it much easier to plan when all of the divers are diving on the same gasses. I'm not a big fan of mixed OC/RB teams or teams that are on very different deco schedules. So right there you have a "principle" that is part of my gas planning. Getting everyone on the same page makes contingency planning SO much easier because you can define one contingency plan that applies to all of the divers in the team.

I hope you see where I'm going with that.

SAC
I'm sure by now you know the importance of all of the divers knowing their SAC *on similar dives*. You'll need this to make calculations. Another principle applies here, which is the idea that your predicted SAC isn't the SAC you want to plan with, but the question becomes "how much higher" should your planning SAC be?

This could be where you are getting stuck. In my case, personally, I know that on almost all my technical dives to date my SAC in full gear is almost spot on 12l/m. So I plan with 20l/m. Why 20? This is completely arbitrary but is higher than the highest number I've ever recorded on a dive so I'm comfortable with that. In other words, using 20 is, to me, an acceptable risk boundary. You need to find yours and you need to know where your buddy's are before you can plan.

The thing about SAC and dive planning is that this is the key variable and the one thing in the whole equation likely to vary the most. Using SAC for planning is really like measuring to the millimeter and then cutting with axe. Another principle that I personally apply because of this is to "measure with an axe if you're going to cut with an axe". This is why I don't worry too much if 20l/m is good enough or if I could get away with 18 or need 21. It's also the reason that I choose to ignore the complications of working out average ascent/descent depths and rates... Realistically is this just fiddling in the margins and gives a false sense of accuracy. Again, if you are going to cut with an axe then my conviction is that it's ok to measure with an axe.

So what I do is in addition to calculating liters of gas needed, I'll pin that to "pressures" that I expect to be above at certain points in the dive (usually by remember bar/10min) and if I'm above that then I'll call the dive "early", which reduces both gas needs and deco obligations in order to put me back on my mental map of where I should be. This is important because your buddy's contingency plan will almost certainly depend on using some of YOUR gas and you need to be mindful of the fact that not all of your gas is your own.

Up to now this is all about principles and not about calculations. But this is where the big lesson is. The calculations are a side issue as compared to making these kinds of choices.

When are all gasses available?
Another principle you could apply to contingency planning is the step to get back to a depth where all gasses are available. With 50% deco gas you're "safety" depth is about 21m. That depth opens doors. Again this is cutting with an axe. This means that if a diver has a major loss of gas below 21m that you may first attempt to get back to 21m with the gas you have available on the bottom and then pause and possibly take DIFFERENT action at that point.

Case in point. We make a lot of dives similar to the one in your book. We use 28% or even air and go to 50m with 50% deco gas. The amount of deco we have is seldom more than about 30 min on these dives and we carry AL80 deco bottles, which will allow for at least an hour of dive time from 21m to the surface... more than enough to clear even the worst case deco obligations we could get.

In the event of an emergency, even one that causes one diver to lose all back gas at the worst possible moment, then getting back to 21m means that we suddenly have an hour or more of gas available to us that wasn't available before that.

Ergo... the contingency plan is also a navigation plan.

Other principles related to this were mentioned by @w3dge

Then come the calculations
So..... getting to the point of this post. I think calculations in and of themselves are usually so out of context that they really don't give you much of a handle on how to plan a safe dive. Calculations are part of it, of course. The "bar/10min" calculation I mentioned above is a "boiler plate" number that I could have only reached by making calculations but the starting points for a safe dive are not the calculations themselves but the principles and contingencies that underpin that.

R..

 

[TrimixToo]

Diver0001's post is pretty comprehensive. I will only add that, sooner or later, you *will* have a problem underwater, and that having enough gas to individually and collectively recover from *at least* one lost gas or significant unexpected delay at depth will suddenly matter. A lot. You can't just go back up and get more when this happens. Make sure you bring enough with you. Get some planning software, if you don't have it, that shows you gas requirements including lost gas scenarios quickly and easily, use it, and be cautious about the diving contracts you sign with yourself and your companions.

 

[Diver0001]

Diver0001's post is pretty comprehensive. I will only add that, sooner or later, you *will* have a problem underwater, and that having enough gas to individually and collectively recover from *at least* one lost gas or significant unexpected delay at depth will suddenly matter. A lot. You can't just go back up and get more when this happens. Make sure you bring enough with you. Get some planning software, if you don't have it, that shows you gas requirements including lost gas scenarios quickly and easily, use it, and be cautious about the diving contracts you sign with yourself and your companions.

A good example of this was a dive I wrote about recently in which I had 20-odd minutes of delays during an ascent from 50m due to a reverse block.

On this dive we were with 3 divers. I like three man teams a lot because you basically carry one entire dive's worth of gas as a reserve. In this case I managed it within the boundaries of my own reserve but it was nice to know that if the block was severe enough that it caused an hour of delays that even THAT wouldn't have been a problem....

It's all about planning.

R..

 

[boulderjohn]

I used to teach TDI, nd I, too, am mystified as to how you came out of the class without knowing this. It should have been a required part of the planning for every dive you did.

When I taught TDI, they used V-Planner in the course to show this kind of planning. You mention using V-Planner in your post. If you inputted your SAC rate, as the course requires, V-Planner will spit out an answer for you.

 

[Dave090]

Thanks everyone. We did use Vplanner (it does not spit out the answer to these questions though). Not in terms of the way the TDI manual calculates turn pressure and reserve gas needs. My instructor did go over this all to some degree but I just did not feel I had a good grasp of it all. Also, where we dive we are right by the shore, with lots of support so I think the instructor is a bit less conservative because if something were to happen someone would jump in the water and give us support (I don't love this answer though and want to understand how to plan when we are not in this situation). He also says it just depends on what you want to plan on (like do you plan that you lose your deco gas AND have your buddy's bottom mix fall at the end of the dive). There are only so many issues you can plan for after all right?

So what I am looking for is how to approximately plan with Vplanner (assuming a SAC of 20 L/min and 12 L/M on deco) and the TDI "general guidelines" on turn pressure and reserve gas amount, what I should plan for theoretically with a 45 m dive with twin 11L tanks and a 5 L deco bottle. With what you all said in mind about plans and theory, this will just give me a general understanding of best I should plan these dives.

The TDI reading material really only gives you examples on how to plan for a dive without a deco bottle and does not really talk enough about contingency plans if you do have a bottle.

 

[ahereng]

The license for V-planner also works for Multideco, and Multideco can use Bühlman/Zhl16 with gradientfactors.

 

[kensuf]

When I was an instructor

Thanks everyone. We did use Vplanner (it does not spit out the answer to these questions though). Not in terms of the way the TDI manual calculates turn pressure and reserve gas needs. My instructor did go over this all to some degree but I just did not feel I had a good grasp of it all. Also, where we dive we are right by the shore, with lots of support so I think the instructor is a bit less conservative because if something were to happen someone would jump in the water and give us support (I don't love this answer though and want to understand how to plan when we are not in this situation). He also says it just depends on what you want to plan on (like do you plan that you lose your deco gas AND have your buddy's bottom mix fall at the end of the dive). There are only so many issues you can plan for after all right?

So what I am looking for is how to approximately plan with Vplanner (assuming a SAC of 20 L/min and 12 L/M on deco) and the TDI "general guidelines" on turn pressure and reserve gas amount, what I should plan for theoretically with a 45 m dive with twin 11L tanks and a 5 L deco bottle. With what you all said in mind about plans and theory, this will just give me a general understanding of best I should plan these dives.

The TDI reading material really only gives you examples on how to plan for a dive without a deco bottle and does not really talk enough about contingency plans if you do have a bottle.

Can you call your instructor and say to him that you feel like you didn't have a good understanding of this, explain the things you're not sure about, and ask him to review it with you again? Most instructors I know would be happy to do that.