How to plan second dive of the day using a computer

[diversteve]

I went to the website for Dive Computer Training, and it said I can't do the training without an activation code, that I can't purchase the activation code, but rather have to get it from the store that sold me the computer. So I emailed the outfit in Spain that sold it to me to get it. We'll see how that goes.

Check with [user]DiveNav[/user] - he owns it.

 

[FPDocMatt]

Matt,

It would help other posters on the board if you were to post the make and model of the computer you bought. All computers are different. The fact it supports 16 compartments indicates it's a reasonably advanced model and so the chances are that it supports a dive planning mode. Be aware that this is still going to plan a square profile and will therefore predict the 'worst case' NDL. If you want to understand what the compartments means, you're going to have to dig deeper in decompression theory.

As another poster in this thread pointed out, I have a Mares Puck.

I don't know that the Mares Puck has the 16 compartments in its algorithm. The 16 compartments were mentioned in the Dive Computer Training's course, "Introduction to Dive Computers" as being part of the most commonly used algorithm.

I understand what the 16 compartments are. What I don't yet understand is why the "dominant" compartment is something other than the fastest or slowest compartment.

As for my original question regarding planning the second dive, this was nicely answered by two posters in this thread already.

Really, I appreciate all the information people on this forum have given me in response to my questions in recent weeks. But I don't appreciate being criticized for asking questions in a way that is something other than the way people like to be asked. If some of the people on this forum want people to only post in a certain fashion, and are willing to be rude in order to get them to do so, then all you're going to have on the forum are mean-spirited people who have very thick skins.

 

[Hatul]

As another poster in this thread pointed out, I have a Mares Puck.

I don't know that the Mares Puck has the 16 compartments in its algorithm. The 16 compartments were mentioned in the Dive Computer Training's course, "Introduction to Dive Computers" as being part of the most commonly used algorithm.

I understand what the 16 compartments are. What I don't yet understand is why the "dominant" compartment is something other than the fastest or slowest compartment.

As for my original question regarding planning the second dive, this was nicely answered by two posters in this thread already.

Hi Matt, The leading compartment is the one with highest nitrogen loading. If the computer has a nitrogen loading bar graph that's the one that it tracks. When you first descend the fast compartment takes up nitrogen the most, but when you ascend it also offgasses the fastest so you're left with some middle compartment being in the lead.

With regards to DiveNav computer courses you can buy them online. I bought mine for the Galileo Sol and I'm sure Alberto has one for the Mares Puck.

Adam

 

[lowviz]

...//.... you don't actually enter the SI. The computer is constantly tracking your nitrogen load (etc) while you're diving. It continues to do this during your SI, as you offgas. ....//.....

Matt, Dirty-Dog has it right.

Forget compartments for now. Your computer KNOWS where you were, what you did, and how long ago that was. Just learn how to enter dive plan mode. The plan that it gives you will depend on what it thinks your present loading is. You can check the plan at, say 100', and compare that to tables -but that is kind of an advanced thing. If the computer craps on a dive day, (it won't), just take a pass.

The whole idea is to know how long you can remain at your planned depth (at your present level of nitrogen loading) before you hit deco. Thus, diveplan mode. Tables, DC, past experience, who cares. The whole idea is to know how long you can remain at your planned depth before you hit deco. Oh yeah, it is important to know this BEFORE you splash...

 

[DevonDiver]

I understand what the 16 compartments are. What I don't yet understand is why the "dominant" compartment is something other than the fastest or slowest compartment.

The different compartments have varying times to absorb and release nitrogen. Specifically for release, they use a 'half-time' - the period of time that it takes for 50% of the starting nitrogen to be released.

When any given compartment reaches saturation, you are in 'deco'. That can happen quickly with the fastest compartments, but those compartments will also clear (sufficiently desaturate) during the course of a normal ascent (at pre-determined max ascent speed). This is why divers are recommended to not exceed a certain speed. PADI say 18m/60ft per minute. Most dive computers use 10m/32ft per minute.

The saturation level of a set compartment is described by its 'M-Value'. The Max amount of saturation that can occur (relative to ambient/surrounding pressure) without undue risk of bubbles forming.

This is why some divers will argue that there is technically no such thing as a 'no-deco' dive. In reality, you always complete deco, but recreational 'no-deco' limits just keep your deco obligation below the level that would permit sufficient desaturation during a controlled ascent. Thus, you can see the critical importance of controlled ascents

When a diver is sufficiently saturated that their slower compartments become saturated, then those tissues may not clear at a speed that allows sufficient nitrogen release to occur during a controlled ascent alone. Extra 'stops' are needed during the ascent to provide more time for nitrogen release. These are 'deco stops'.

When a diver has saturated multiple compartments, they begin to ascend and, in that process, actually clear their fast tissues. However, their slower tissue compartments are still not cleared. Thus, the slower compartments are now 'controlling' the dive.

Repetitive Dives. This is especially relevant if multiple daily/repetitive dives are conducted, as slower tissues retain nitrogen from one dive to another (called 'residual nitrogen' on the tables). After a couple of dives, these slower compartments can become heavily loaded, even whilst the faster tissues are not. Thus, medium-slow tissues can be the controlling factor from the start of the (repetitive) dive. This is one reason why a diver should never assume that some small deco will 'disappear' on ascent. After several dives, you may well be controlled by slower tissues that won't desaturate at a speed faster than your ascent rate.

Multi-Day Dives. When you start conducting intensive multi-day repetitive diving; for instance 4-5 dives per day on a liveaboard trip, then your very slowest compartments can soon become the controlling tissues. Day-after-day, you leave the water with slightly increased slow tissue saturation. Whilst you continue to desaturate overnight, the slowest tissues won't completely clear, so you'll be topping them up the next day, and the next. After a few days diving, you'll be waking up in the morning still heavily saturated in the slowest compartments and taking that nitrogen back into the water with you.

I remember graphing my dive computer profiles after my first intensive diving trip to Thailand, diving heavily very day for 2 weeks, to learn that I was entering the water in the morning with my slowest compartment already at 85% saturation. I would add maybe 5% per dive, lose a marginal amount in the surface interval and then lose about 5% overnight - a rough gain of 10% per day. As you can see, in my second week of diving, the slowest tissues were very much 'controlling' the dives. In this case, any over-stay beyond the computer supplied NDL would have rapidly generated long deco stops (to the point of being nearly exponential).

Residual nitrogen tracking. This reality differs from the PADI tables, that lead you to believe you are 'nitrogen free' after a surface interval of >6-8 hours. Those tables weren't meant for multiple deep dives or intensive multi-day diving. However, natural conservatism inherent with the tables (assuming some element of multi-level diving) and tables rules (don't repetitively dive below 30m) can help this becoming an actual health risk. Your dive computer WILL track that accurately though.

 

[TSandM]

You would think that the fastest compartment would be controlling, because it fills up the fastest -- and if you've spent six half-lives at that depth, the compartment is full. But the theory also says that those very fast compartments tolerate a greater "overpressure gradient" (meaning a nitrogen content higher than saturation at ambient) than slower compartments, before bubbling. So, since the fast compartments offgas fast and tolerate overpressure gradients, it's often something a bit slower that's actually "controlling", or demanding a certain stop or ascent rate. Very slow compartments rarely control anything, as even with multiple dives over multiple days, they never approach saturation.

Really, Deco for Divers is what you want. You have the education and the curiosity to want and to process the information. You'll enjoy the book, and come away with a MUCH better understanding of what your computer is doing, and what is theorized about what is going on inside your body.

 

[DevonDiver]

Really, I appreciate all the information people on this forum have given me in response to my questions in recent weeks. But I don't appreciate being criticized for asking questions in a way that is something other than the way people like to be asked.

I don't see a problem with anything that you've asked. That said, the advice that some have given - to read the manual - is prudent. There are many potential issues that can arise from not having a clear understanding of your dive computer function.

As an instructor/guide, I've encountered many divers who did fail to fully read and comprehend their dive computer manuals. They do so because it's very easy to use a dive computer - just jump in the water and go. However, their knowledge deficit comes back to bite them in the rare occasion when the computer doesn't behave as they'd expect it to. A particular example is if they accidentally go into 'deco' mode - they don't understand the information and instructions they are being given and this can be dangerous.

In that respect, there is a definite polarity between effectively using the computer as a functional tool and simply assuming that the computer is a wonderful device that allows you to abdicate a whole bunch of responsibilities for your diving.

A lot of computer based questions do get asked here on Scubaboard and the vast majority of them could be answered directly from the relevant manual. That doesn't mean that the questions shouldn't be asked. But it might mean that some, albeit relevant, responses might be more terse than you'd otherwise expect.

 

[redacted]

Very slow compartments rarely control anything, as even with multiple dives over multiple days, they never approach saturation..

I don't believe that is correct. I have done FGB trips up to 5 days. Generally speaking, my FGB dive are all in the 80 to 100 ft range. On my last 3-day trip, I did 5 dives on each of the first 2 days and 2 in the morning of the last day. Dive times, using an HP 100 with 32%, run 50 to 60 minutes with actual bottom times running 35 to 40 minutes. All dives are within NDLs. Longer dives tend to occur later in the trip for me when rest stops may be extended to 10 minutes or more to bring the Oceanic tissue loading graph back into the green (or close). Based on looking at those kinds of dives with DiveNaves SW, it sure looks like the controlling tissue compartment is up in the three slowest used in the Oceanic algorithm (320, 400, 480). In a 5 day trip, I would think even the 635 minute compartment in a 16 compartment model would start to get quite a workout since the maximum SI is less than one half-time and offgassing is rather shallow (less than 40 ft). I'm still hoping DiveNav will eventually pull out the limits so I can run that dive sequence.

Matt- You really need to spend some more time with your manual. With no experience with Mares dive computers, I pulled up the Mares Puck manual and in about 5 minutes figured out that you must press the button to turn it on, you press and hold the button for 4 seconds to activate the backlight which will turn off automatically after 4 seconds unless you press the button again, and the algorithm uses a 10 tissue compartment model. I do understand those manuals often contain an overwhelming amount of information for the first read. But when questions do arise, they are usually answered by a bit more reading. And it does get easier as your familiarity with the manual grows.

 

[RonFrank]

Remember dive computers are theoretical.

Your computer has a gauge somewhere that shows theoretical nitrogen loading. Mine has a graph that shows ^^^^^ up to five. This represents Nitrogen loading. Yours could be bars, or dots. Never allow this graph to reach the saturation point. In my case 5 ^.

Another thing your computer shows is time to NDL. If that goes to zero you have reached deco. This is not good for recreational divers.

Other handy things found on most computers:

Date
Time
Day
Dive Time
Current Depth
Ascent rate (often a graph)
Max Depth
Water Temp (lowest)
NDL
Time to NDL

Generally computers show the most critical value. So if you have 3 minutes until you hit NDL that is what is displayed.

The planning mode on my computer looks like:
depth time
10ft 60min
20ft 50min
30ft 40min
40ft 30min
................
up to 200ft

So that tells you for example based on you current nitrogen load you can dive 30feet for 40min. As you off gas that value will change. Maybe after 1 hour that value is 30feet for 160min. You will learn what you can do based on your remaining SI and nitrogen load.

Good Luck

 

[redacted]

If he asked, how do I read the depth on my dive computer? And my response was RTFM.. would that be deleted too?

Remember DD, this is the basic fourm. That should be RTUM. It is a User's Manual, not a Field Manual.