New divers and computers

[Charlie99]

The simple answer is that you do a bunch of repetitive dives with a surface interval of 0:00.

Here is your dive plan: Dive #1 = 80' for 10 mins, SI = 0 min, Dive #2 = 60' for 15 mins, SI = 0 min, Dive #3 = 40' for 20 mins.

1. Find your pressure group (PG) for 80' for 10 mins.
2. With this PG and a surface interval (SI) of 0 mins, find your new PG.
3. Calculate your residual nitrogen time (RNT) from the new PG.
4. Add your RNT to your actaul bottom time (ABT) from dive #2 for your total bottom time (TBT) for dive #2.
5. Using your TBT at 60' for dive #2, calculate your PG.
6. With this PG and SI of 0 mins, find your new PG.
7. Calculate your RNT from the new PG.
8. Add your RNT and ABT for dive #3 to get your TBT.
9. Is your TBT for dive #3 less than the NDL for 40'?

This process is much easier if one uses only the backside of the RDP (the one normally used to find residual nitrogen time; the one with PG columns and rows for Depths). The dive depth + time to PG table 1 on the front side is identical to the Table 3 on the back, except for a couple entries with 1 minute differences because of how rounding was done (or a typo by PADI's).

For your 3 level dive of 10 min @ 80', 15@ 60', 20 @ 40' the process is as follow:

1. 80' row, 10 minutes is PG C.
2. Move up to the 60' row, staying in Pressure Group C column. The 14 in this cell shows that one reaches PG C after 14 minutes at 60'.
3. Add your planned 15 min at 60 to the 14, getting 29 minutes total equiv 60' bottom time.
4. 29 min in 60' row is PG K
5. Move up to 40', PG K where you will find a 48. That means that, as far as the 60 minute HT compartment, your dive to this point is equivalent to 48 minutes at 40'.
6. Add your planned 20 minutes at 40' to the 48, to get 68 minutes total eqivalent 40' bottom time.
7. That puts you into Pressure Group P.

One can use the pressure group P as your ending pressure group for calculating repetitive dives later that day.

As noted in my post above, one should really be comparing the total bottom times not to the NDL values of the RDP, but to the ML values, which are a few minutes less than the RDP NDLs.

 

[Charlie99]

I'd love to, but it is not realy the responsible thing to do. It was taught in my advanced course in the deep and wreck diving sections.

What agency? What table?

There are various pitfalls of this method since the table tracks one and only one comparment. I wonder if your instructor explained this and how that can result in decompression obligations for a profile that stays within your multilevel planning method.

 

[Somethin-Fishey]

Yes, we were off the tables by literally a couple of minutes but lots within the computer's NDLs.



I understand I should always know where I am with regards to the tables - but presumably, on some dives using a computer you will be off the tables and still be very safely with NDLs, correct?

Instructor discretions aside. I don't think I have ever dove in 80ft or deeper and been "on" the tables. That is over 100 dives, computers give you much more flexibility. The tables assume you jump in the water at depth and hold onto a rock at the bottom for the entire time and then slowly come up, do your 3 minute stop and exit the water. I was shocked when I saw my first high resolution profile from spearfishing. I went up and down over 30 ft several times. When you are in 110' and vis is 100' you have no real idea how much you are moving around.

I can GUARANTEE you I know the tables front and back, inside and out, including the wheel, and I would NEVER tell someone to get a computer to avoid the tables, it should compliment tables. For me it is also my dive log, except for fundives in Cozumel, Belize etc... I take my real log for the divemasters to sign. I always print out 10 or so dives before I go on a trip to prove I have more than 4 dives lifetime.

 

[Somethin-Fishey]

What computer do you dive that gives you 30 minutes at 100'?

I use a Cochran EMC-16 and I routinely get 60 minutes in 100ft of water. Most of the time I run out of air at about 50 minutes, but I still have about 10 minutes of NDL.

 

[sewcopp]

This process is much easier if one uses only the backside of the RDP (the one normally used to find residual nitrogen time; the one with PG columns and rows for Depths). The dive depth + time to PG table 1 on the front side is identical to the Table 3 on the back, except for a couple entries with 1 minute differences because of how rounding was done (or a typo by PADI's).

For your 3 level dive of 10 min @ 80', 15@ 60', 20 @ 40' the process is as follow:

1. 80' row, 10 minutes is PG C.
2. Move up to the 60' row, staying in Pressure Group C column. The 14 in this cell shows that one reaches PG C after 14 minutes at 60'.
3. Add your planned 15 min at 60 to the 14, getting 29 minutes total equiv 60' bottom time.
4. 29 min in 60' row is PG K
5. Move up to 40', PG K where you will find a 48. That means that, as far as the 60 minute HT compartment, your dive to this point is equivalent to 48 minutes at 40'.
6. Add your planned 20 minutes at 40' to the 48, to get 68 minutes total eqivalent 40' bottom time.
7. That puts you into Pressure Group P.

One can use the pressure group P as your ending pressure group for calculating repetitive dives later that day.

As noted in my post above, one should really be comparing the total bottom times not to the NDL values of the RDP, but to the ML values, which are a few minutes less than the RDP NDLs.

:huh: Charlie this may be thought of as a small point, but befiore you start throwing around acromins like we've all done the same training by the same orgniazations (I realize that more people have been trained in some than others) try typing out what the the acromins are before assuming (We all know what that word breaks into) everyone knows everything. Generally it is accepted that the first usage of an acromin in a typed document is written out the subsequent usages can then use the acronim alone.

That would possibly help in the understanding of many additions to forums here. I try to do this when I remember.

REM. THE WORLD IS NOT PADI.

I don't use nor have I ever used the PADI system. How would you explain this using US Navy, ACUC, NAUI, SSI, SDI/TDI tables. The world is reading these entries.

Sue

 

[Charlie99]

I don't use nor have I ever used the PADI system. How would you explain this using US Navy, ACUC, NAUI, SSI, SDI/TDI tables. The world is reading these entries

I have no idea if ANYBODY has done the calculations and/or brute force simulations needed to pick appropriate table limits for multilevel dives using the USN/ACUC/NAUI/SSI/SDI tables. PADI/DSAT (Professional Association of Dive Instructors / ???) have done the simulations and generated the multilevel planning tool called the PADI Wheel. My tables are based upon that same model and rely upon the multilevel simulations and testing done by PADI.

RDP = Recreational Dive Planner, the PADI square profile table based upon the DSAT deco model/m-values.
ML = multilevel limit, kind of equivalent to an NDL
NDL = no-decompression limit. The time at a given depth that will cause one or more compartments to reach the model limit.
PG = pressure group. Same terminology is used by some agencies. Others, such as perhaps the US Navy use the term "repetitive group".
PADI pressure groups correspond to a given level of assumed N2 loading in the 60 minute halftime compartment. A corresponds to loading increase from sea level N2 saturation level of 12% of the difference between sea level N2 and the limit for the 60 minuter compartment of the DSAT decompression modle. (This limit is referred to as the "M-value"). B is 21% of the way to the limit, C is 26%. Each pressure group after than increases by 3% of M value, up to Z which is 95% of the M value of the 60 minute halftime compartment. The repetitive groups of USN/NAUI/YMCA and related tables are similar, but are based upon the 120 minute halftime compartment, and IIRC, each reptitive corresponds to a 2fsw increase in ppN2 in the 2 hour compartment. And of course, they only run A to N for no decompression dives.

Some divers will plan multilevel dives just using average depth. Some divers do a multilevel plan by treating the levels as multiple dives chained together with 0 minute surface intervals. Perhaps you have another basic method. You posted that your instructor taught a multilevel dive planning method using a square profile table. I haven't heard of any agency teaching multilevel diving except PADI, using the Wheel. I have also been told that GUE DIR-F instructors will allude to, but not actually teach, multilevel diving through the use of depth averaging and a variety of tables.

That's why I'm so curious as which agency taught you multilevel planning on a square profile table?

 

[sewcopp]

:life: I haven't heard of any agency teaching multilevel diving except PADI, using the Wheel. I have also been told that GUE DIR-F instructors will allude to, but not actually teach, multilevel diving through the use of depth averaging and a variety of tables.

That's why I'm so curious as which agency taught you multilevel planning on a square profile table?[/QUOTE]

Charlie I've done most of my training with ACUC. Who as far as I can tell so far, have some of the most conservative tables on the go. It suprises me that people can do an Advanced SCUBA course and no multilevel training, or a deep specialty with no multilevel training.

To this point in my diving I found that the remainder of the SCUBA equip. (suit, regs. with Guages, BCD. mask, fins, snorkel, and tanks) was more improtanyt to my getting in the water. (I don't know about where you live but we have lots of water in Newfoundland with no dive shops close by, <100 km. so we tend to own our gear) I purcchased a computer just recently because I am doing training with SDI/TDI and they require a computer. So far I've a few dives on the computer but my buddies have had gear problems (water temp. < 32 F) so I have no repetative dives on the computer. No multilevel stuff either. A lot of the cool deep sites aren't readily available with the snow around (maybe next month). Also the wrecks off BEll Island are a little expensive for the budget right now.

So far I have more info. and I'm still more comfortable with the tables than someone else's programing. But I'm sure that will improve with more dives. I also don't have the download hardware (Infrared reader) for my computer yet so I can't see everything the dive computer is recording for me.

Life's all about learning and growing after all.

At least if my dive computer dies on a dive I won't have to end (ruin) a good day of diving.

Sue

:11:
PS The post about spear fishing makes me thankful you can't do that in Canadian waters. If you don't watch your profile while your diving your putting your life in the hands of a little box strapped to your wrist

 

[brutus_scuba]

Yes, we were off the tables by literally a couple of minutes but lots within the computer's NDLs.

It is my understanding that even when diving with computers everyone in the group must have a computer. Say my dive buddy and I were diving, and I was using a computer and my dive buddy wasn't using a computer that we should dive the tables and not my computer just because even though youre depths and time intervals may be very very close they are not exact, and dive tables, are always the safe alternative.

If you werent' all wearing computers then you definatly should have dove to the tables. Maybe I'm too conservative and other dives in here will disagree with me as I realize it is a very conservative thing to do but that's my two cents.

 

[vondo]

That's my thinking...I don't want to be a 'lazy' or unsafe diver, but I want to get the most out of my experience. I plan to keep learning and praticising all my skills, including use of tables. If I buy I computer, I'll lean towards a more conservative one, and I'll use it in conjunction with the tables.

The other argument made above, about being able to use the computer to track my dives over time, is another excellent one.

I'm beginning to feel better about a potential computer purchase!

For what it's worth, I bought a computer before buying a reg or BCD and I certainly don't regret it. Like another poster here, my first dives after certifcation were fairly deep, multi-level, and with about one hour SIs. A rental computer was included instead of a depth gauge. Taking the computer readings and assuming square profiles, I was past the NDLs by the first or second dive.

As others have pointed out, looking at the dive profile, figuring it out as a square profile or a multi-level profile will *increase* not decrease your understanding of what's going on in your body. In addition, let me make two other points where a computer can be *safer* for a new diver, even if within the tables:

1) It will track your maximum depth. The rental gear I certified on didn't do this, so I either had to check often and remember or trust someone else to do the same to know exactly how deep I'd been. With everything else going on, this wasn't easy.

2) A computer can keep you on your dive plan. You can set your maximum planned depth and have it sound an alarm if you exceed this.

Finally, I haven't seen anyone else say this yet, so "Get a Nitrox computer." They are just a bit more that air-only downloadable models and if you stick with diving you will want it eventually. Also, if you buy a computer before a reg, you will be forced to buy a wrist computer, which seems to be favored here.

Enjoy

 

[Meng_Tze]

Look at it this way:

Computers have their place in diving. They are accurate in determining where you are ina dive, calculating what time you have left at any given point (NDL/Deco/recalculation of profile) and make things easy for a number of people. Nothing wrong with that. There are many divers who dive this way and live by this. Only thing I would recommend is to have two identical ones for redundancy.

Tables are also very good, if you understand them and can work with them. The issue is that you have to use your brain if something changes, or be able to rework things quickly on backup. In a stressful situation it can add delays, miscalculations...

Some people run computers and have tables as a backup. they rely on the computer and fall back on tables. Key to this is that you have to keep up with 'training your brain' to use tables and do bail out calculations.

There is no right way or wrong way of doing things, just make sure you dont hitch a ride to he chamber.

If there are any other ways of running a profile, I'd be interested.