Help understanding dive planning for new technical diver

[lermontov]

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).

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.

Just a word of caution- the TDI books are reviewed and updated now and again and there are errors and contradiction in some of the calculations. The important thing for you to do is get a decent planner with option of entering data for deco bottles (sub surface is free) that way it becomes easier to see what gas in in what quantites you need to take

nb in the example you gave it mentions at the top of the page 176 about deco cylinders

The adv trimix book covers planning a bit more and if you can get a copy off a mate then read it for the gas planning section - if you plan to do more deeper dives its a good idea to use a system that will overlap into more complex dive planning later - Ill suggest that theres no perfect system but start with a solid one and adapt it to suit as you grow

It does seem confusing when you have information overload after doing a course but stick with it and practice manually calculating on paper until its clear in you mind. break the dive down into sections e.g. descent time-bottom time -ascent time -deco time then work out which sections need how much gas and how much reserve. once you can do that with clarity then you can allocate those volumes to which bottles

If your competent with excel spread sheets you can spend hours manipulating the numbers - and its very sobering to see how a few minute too long on the bottom can have huge implications when you get over 50m

 

[divezonescuba]

The PADI technical diving training record has specific entries for a students completion of each practical application of dive planning. The instructor must sign off on each entry and the student must initial it as well. There are also tests including dive / gas planning in the 40/45/50 sequence.

I would be surprised that TDI would not have something similar. A TDI representative once claimed that TDI was the PADI of technical diving? Not sure what he meant by that?

 

[RyanT]

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

Yikes, I sure hope this was your perception as the student and not the mindset of your instructor. After all, are you only going to dive that site from now on?

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?

Absolutely. One possibility is that a Kraken tentacle could grab me and pull me into the wreck, on the way in, I hook my arms over the edge of the wreck and we have a two hour stalemate until I win and get free. The probability of that happening is pretty low, so I'm going to leave it off my planning list. Of course I was being a bit silly and indeed, there are dives and instances where things can go sideways in a manner you never saw coming. However, there are some basic planning protocols that will cover the vast majority of things you may encounter, and it is important to be able to to some competent planning for the dives you expect to do.

 

[RyanT]

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).

I would also add that the scenario you describe is on that is easily planned for. Carry enough back gas to get you and your buddy to your first deco stop and then you both carry 2x the deco gas. As you go further into tech and dive deeper and longer, planning for multiple failures should become routine. Of course, you can reach a point where you just can't carry enough reserve, but I don't do those dives.

 

[Kevrumbo]

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).

. . .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.
. . .

A preliminary and initial planning calculation exercise based on the above parameters:

Take no more than 1 minute to problem-solve a contingency at 45m depth.

From 45m operational depth to your Eanx50 deco switch depth at 21m, it will take 2.4 minutes to ascend this distance at a rate of 10m/min. Let's round this up to 3 minutes. Total time plus the 1 minute above is 4 minutes.

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.

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).

For a deco SCR, let's use a conservative 20 liters/min per ATA. Divide 20 by the 5.5 liters/bar cylinder rating of your EAN50 tank, so 20 ÷ 5.5 = 3.6 bar/min per ATA. Your average depth from the 21m deco stop to the surface is 2.05ATA (that's a relatively conservative and deeper average value since we don't know your exact deco stops profile) . Therefore your average depth consumption rate (DCR) will be 3.6 x 2.05 = 7.4 bar/min. With a full fill of 200 bar and depending on what Deco software & algorithm you use, you can have up to 27 minutes (200 ÷ 7.4 = 27 min) of a particular ascending deco profile with the EAN50 gas you have in a 5.5 liter/bar cylinder. (With a 11 liter/bar cylinder of EAN50, you have enough deco gas to cover yourself and your buddy should he lose his deco bottle).

Finally, with EAN25 bottom gas, your ppO2 will be 1.36 ATA and gas density 6.63 g/L at 45m working depth: recommended bottom gas ppO2 should be lower at an average of 1.2 ATA; and gas density should be lower than 6 g/L to moderate work-of-breathing/CO2 retention with stressed physical activity and exertion contingency (i.g. kicking against a very strong current at 45m depth). In other words, you should be using a standard Helium Trimix blend of 21/35 if you anticipate heavy current conditions at depth. . .

 

[atdotde]

I made a 5min tutorial video showing how to do the gas planning for your dive in Subsurface:

 

[Schwob]

I made a 5min tutorial video showing how to do the gas planning for your dive in Subsurface:

No time this second, but thanks and will watch this weekend!

 

[Dave090]

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.[/QUOTE]

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)?

[QUOTE="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).[/QUOTE]

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.

 

[Dave090]

I made a 5min tutorial video showing how to do the gas planning for your dive in Subsurface:

@atdotde, that was awesome. Thanks so much. It helps a lot. But I guess my question is just generally still what are the "suggested guidelines" for contingency planning (lost gas, lost buddy, lost bottom mix) as I said in my comments to @Kevrumbo. But maybe @RyanT is correct. I should just carry an extra deco tank.

Thanks everyone!

 

[RyanT]

@Dave090, you've been given some good advice here and I certainly understand your trepidations. There are a lot of moving parts to get your head around when you first start down this road. You don't necessarily have to carry an extra deco tank, perhaps just a bigger one. Carrying one AL 80 is probably easier than carrying two AL 40s. Also, it depends on you and your buddy's SAC rate as well. For my buddy and I, we could do a 45m dive for 25 minutes with a single AL 40 giving almost 2x the necessary deco gas. Given any additional stress, we might come up a few minutes short on the deco gas, but we would have ample back gas that we could use to finish out deco, albeit inefficiently. For a longer dive, I would opt to carry more deco gas.