Dive Planning with a PDC or just riding it?

[rhwestfall]

Recent comments in some other threads got me to wondering...

On the subject of dive planning (as a sub-topic to tables vs. computers), there seems to be some differences of opinion as to what you can/should be doing regarding using a PDC and planning dives. What are you doing in regard to this? Were you instructed on a PDC?

I'll start with my approach (and I am a relic from the table era who was certified 25 years ago):

Pre-Dive #1:

Similar to the "old tables" way, I'll record my NDL times on a wrist slate for the planned depth, and the next 1 or two deeper intervals (we tend to dive with a hard bottom here as its mostly wrecks). Coupled with turn pressures, this and a planned time is the "light plan".

Dive #1:

A timer and depth gauge are used in conjuncton with my PDC, and time of dive and depth is monitored (PDC usually first, but also the back-up) as well as NDL on the PDC, and my SPG (no AI). Whicher happens first drives the dive...

End Dive #1:

Computer functioning? End of dive information recorded on slate (max depth, time). Once able, I will go into the dive planner on the PDC, and record depth/time for various depths in relation to dive location #2 on the slate as a snapshot in time. Now its lunch time!

Pre-Dive #2: Very close to splash time, see new NDL's for various depths in relation to dive location #2, similar to what was done pre-dive #1. Put those on the slate as well as planned time/depth, and turn pressure like before...

Dive #2... cycle to repeat itself as appropriate for multi-dives, and/or or multiple days...

Note: There are times where I will have a back-up PDC (the wife's if she isn't diving), and may use it rather than the bottom timer & depth gauge. For fun. I still may run a dive on tables ot my Wheel, just to see if things correlate.

What was your training, and what do you do?

 

[RJP]

I plan all deco dives with V-Planner, create a dive-specific table (including back-up and lost-gas plans) follow that table with a bottom timer, while running my regular computer as well. Assuming I'm relatively on schedule I stay in the water for as long as the more conservative of the two requires. If I shorten the dive significantly or it's otherwise easier than planned I may well follow the schedule allowed by the less conservative of the two.

Any subsequent dive is planned with V-Planner and executed in the same way.

Non deco dives:
- Wreck dives here in NJ are planned the same way to calculate gas needs and write a backup table, but I'll typically ride my computer. Most of these dives I've done many times and while I wouldn't say I'm complacent, my experience allows me to have a very good understanding of what each dive will be.

- Vacation warm water recreational dives. Planning is fairly rudimentary - what's the depth? What mix am I diving? How much gas do I have? I've got a pretty good idea of my SAC and NDL's. Figure rough plan in my head. Confer with buddy. Follow computer, modify plan accordingly during the dive. No need to create slates for such dives.

 

[tomfcrist]

Although tables are great for calculating square profiles, I see no issue with using a PDCs pre dive NDLs as a planning tool. Even when it comes to deco dives, there are PDCs that have that capability, so why not use it. That being said, using vplanner or the like is great for planning these dives days/months in advance to calculate gas requirements and the like. Very few divers that I know still use tables at all to plan dives. Most plan via the PDCs, as around here you rarely know which wreck you will be diving until the boat is underway....could be a 90' dive, could be a 130'dive, which is why most wreck divers in NC dive 28-30% nitrox.

 

[Dr. Lecter]

Training and practice differed depending on what dive is contemplated. At the very least, I'll know in advance what kind of gas requirements I'll have for a given time at a given depth, and if multiple gases are involved what the worst-case (lost all deco mixes) requirement is (have to deco out on backgas).

That said, if I drop on a 120' wreck with double 130s and an 80 of 80%, I'm not going to worry over whether I run up 10 minutes or 40 minutes of deco while I'm exploring (aside from the rec charter boat leaving me there ). If I'm diving a 230' wreck, everything is planned in advance and the plan is closely followed.

 

[rhwestfall]

um.... this is "Basic Scuba Discussions" - not advanced... you guys are throwing out deco, doubles, mixed gas, v-planner, etc...and have 2000-3500 dives between the three of you. I'm more interested what the industry is doing with OW/AOW Recreational divers. Trying to get them to think.... You guys are way ahead of that and likely scaring the poop out of the folks this was intended for.....

 

[tomfcrist]

um.... this is "Basic Scuba Discussions" - not advanced... you guys are throwing out deco, doubles, mixed gas, v-planner, etc.... I'm more interested what the industry is doing with OW/AOW Recreational divers. You guys are way ahead of that and likely scaring the poop out of the folks this was intended for.....

Um.....I believe two of the three replies directly answered your question, and yes we included deco in our responses. We did however address that in recreational no deco dives, computers can and are used for planning. There is nothing saying that an OW certified diver cannot use vplanner, or a predator/petrel as their PDC......I know more than a few OW/AOW divers that do just that. Far cheaper than constantly upgrading to a more capable computer everytime they further their diving education.

In short, there is nothing wrong with using a PDC as a dive planning tool. The plan mode NDLs are assuming a square profile anyway.

 

[Dr. Lecter]

Heh, the answer doesn't change much for NDL dives, though. If I'm doing a 120' wreck on a single tank with no pony, I'll know my NDLs in advance, adhere to them, and while I won't write them down I'll at least make sure my dive watch bezel is set so I know where I am if the computer fails. If things go badly sideways at the same time there's a computer failure (say a sliced LP hose due to environmental hazard I was somehow careless enough to strike), I can at least know roughly where I stand in terms of DCS risk as I make my emergency ascent. I find that sort of rigid dive planning too restrictive unless it's really necessary (very deep dive with significant deco), though, so I rarely dive a single tank without redundancy.

If I'm doing the same dive on a full set of doubles, though, I'll likely ride the computer as far into the NDLs (or even past them) as I have time to stay down before the boat gets pissed about my delaying them. This is a function of my being comfortable with determining an emergency deco profile in my head based off time and max depth if the computers fail, plus having the gas to do it several times over.

Gas is time, and time (plus knowing how to use it) allows for a good deal of play in the joints of a dive plan. If you don't have the gas and the knowledge, you'd better adhere to your computer and/or pre-dive plan.

 

[TSandM]

I was taught to plan my dive using tables, although when we did the Deep dive for my AOW, we checked our no-deco time on our computers.

Nowadays, I have the no-deco times for the various depths memorized, and I plan the dive based on the briefing about the dive profile. I then occasionally look at my computer with curiosity, to see what it thinks of what I am doing. So, in a sense, I am planning with tables, I guess.

 

[rhwestfall]

I have the no-deco times for the various depths memorized, and I plan the dive based on the briefing about the dive profile.

For a repetative/multi-dive day? Fascinating!

 

[RJP]

around here you rarely know which wreck you will be diving until the boat is underway....could be a 90' dive, could be a 130'dive, which is why most wreck divers in NC dive 28-30% nitrox.

Would there really be a change from a 90' wreck to a 130' wreck once the boat got underway? I can't imagine making that kind of change... or ever diving with such an operator again. We will certainly make a change to something more in-shore (shallower) based on sea/weather conditions - and agreement among the passengers - rather than heading way out to something deeper. If we call the need to change wrecks it will be similar depth, if passengers are cool with new site; for a swap for a shallower site, any passenger who wants his money back gets his money back. But we'd never change from a 90' wreck to something at 130' - even if customers all agreed and had appropriate gas on hand. If something went wrong and someone who planned to do two dives on a 90' wreck ended up dieing on a 130' wreck... there'd be big trouble, I'm sure.