A bunch of words on dive computers

[String]

My view on computers in that they are an incredibly useful tool provided the user understands the concepts behind it and can understand the information it presents and put it in context.

Id feel happier if new divers did say 30-40 dives without a computer just using tables and a watch before buying a computer. That at least would ensure they have an understanding of the principles involved and not just blindly relying on a computer to perform something they dont understand and present information they cant evaluate.

Edit:- forgot to add this isnt just for deco information, i met a dive abroad whose computer had died, diving without it he stated he didnt know how he'd judge his ascent rate without the computer telling him. Thats a pretty essential thing to know !

 

[SeanQ]

On edit (should've refreshed before posting):
SeanQ, I lent my wheel to a friend so he could learn how to use it - does that really work? Seems reasonable...

It makes sense to me. Just don't mix up the "theoretical time" with the "actual time". In the example I listed, the "theoretical time" was 81 minutes at 40ft which is equivilent to 47 minutes at 60ft.

The "actual time" was 60 minutes; 5 minutes past the NDL limit for a square profile of 60ft. The multilevel profile leaves us in at 40 ft with a 59 minutes left on the NDL limit. I know where I would rather be.

BTW, I just finished editing my first post because I made a mistake typing out the pressure group. This is why you shouldn't always listen to someone on the internet. It could, however, be the computer's method of revenge.


-EDIT- I was just eating dinner when this started nagging at me. I'm getting a feeling that I'm missing something but can't figure out what it is. Can anyone who knows more about it confirm it for me? In the meantime, don't go out and plan a dive based on my post.

 

[immersed]

happier if new divers did say 30-40 dives without a computer

That's an excellent idea.

I'm getting a feeling that I'm missing something

Yep - the 92 foot segment.
You'd have to plan maybe a 10 minute 100 foot/30 meter leg, followed by the aforementioned 30 minute 60 foot/18 meter leg, and the 30 minute 40 foot/12 meter leg.

If your theory works - I asked the friend who has my wheel and he verified the S ending group in your previous formulation - then you'd have:
100' for 10 minutes = E, RNT for 60 = 17
60' for 30 + 17 minutes = S, RNT for 40 = 85
40' for 30 + 85 minutes = X

 

[Charlie99]

-EDIT- I was just eating dinner when this started nagging at me. I'm getting a feeling that I'm missing something but can't figure out what it is. Can anyone who knows more about it confirm it for me? In the meantime, don't go out and plan a dive based on my post.

Your method of using the RDP as a replacement for the wheel to plan a multilevel dive did omit the MultiLevel NDLs, which are bit shorter than the RDP NDLs. For 40' the MDL is 128minutes, still well short of the 81 minutes equivalent of the dive you calculated. A dive to NDL rather MDL would go slightly into deco per the PADI/DSAT model, but not very far, and would probably still be a no-stop dive per the USN/Workman model.
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Take your RDP and plan a dive to 60ft for 30 minutes, then 40ft for 30 minutes.
According to PADI's RDP, after 30 minutes you're in pressure group L. Now you ascend to 40 ft. Pretend you have a surface interval of 0 minutes which leaves you in the same pressure group as before. Now switch over your third table which lists residual nitrogen times. Find the intersect of 40ft and L. According to the table you have 51 minutes of residual nitrogen time. For comparison, look at table 1 and see what time at 40 ft gives you a pressure group of L. A 60ft dive for 30 minutes is equivilent (in terms of nitrogen loading) to a 40ft dive for 51 minutes. Add the 30 minutes you want to stay at 40 feet and you get 81 minutes which places you in pressure group S.

It's a little bit simpler to just stay on Table 3 for everything. 60' 30min puts you into L column. Go up to 40' row to see 51 minutes. Add your 30 minutes to get 81equivalent minutes, which puts you in PG S.

I use a table similar to RDP for my multilevel planning. The numbers vary from RDP/wheel a minute or so because they are calculated from the base equations and PADI seems to have rounded off differently here and there. THE MDLs are shown on the table. The same way that a safety stop becomes mandatory if within 3 PG of NDL on the RDP, with the wheel or flatwheel, a safety stop becomes mandatory if within 3 PG of MDL at any level. There are a whole bunch of other rules such as 1) each level must be progressively shallower and also meet the additonal restrictions on the wheel, 2) any time spent between levels gets counted as time at the deeper level, 3) .... (see PADI wheel book for the rest, but they are commonsense).

When using the SI row, remember that you don't get to drop down a pressure group until your elapsed time is equal to or greater than the time shown. E.g. You exit in PG Q. 30 minutes later you are still in K.

 

[Charlie99]

If your [SeanQ] theory works - I asked the friend who has my wheel and he verified the S ending group in your previous formulation - then you'd have:
100' for 10 minutes = E, RNT for 60 = 17
60' for 30 + 17 minutes = S, RNT for 40 = 85
40' for 30 + 85 minutes = X

If you look at this dive on the wheel, or my "flatwheel" above, you can see that this slightly exceed no-stop limits in that you will exceed the 46 minute 60' MDL at the end of the 60' segment. This just means that you need to spend a bit extra time during your final ascent and safety stop after the 40' segment.

Whether it's a couple minutes inside the limits or a couple minutes outside the limits doesn't affect the safety anywhere as much as doing a nice slow ascent and hang after your 40' segment.

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In practice, although I have a general idea, I won't know precisely the depths and times that I'll be doing on the upcoming dive. I just do some general ballpark sets of profiles. After a while, you get used to some fairly standard sequences, AND HOW MUCH AIR YOU NEED TO DO THEM.

 

[Diving Lore]

I think one of the bottom lines is that a computer no matter how good it is should always be considered fallible and therefore a back-up be it another dive computer, dive slate with deco. info etc should always be considered.

 

[Trisha]

Curing computer dependency
1) Plan your dive; dive your plan.
2) Computers fail; carry a water-resistant watch for backup.
3a) Write the max depth limits on a dive slate (just takes a narrow portion) and carry it.

3b) Or memorize the max depths.
U.S. Navy dive tables are way liberal, but easy to remember. They work in a hard spot, just don't skate the edge with them --
Money rule (pretend you are using quarters, half-dollars, etc) - 50ft/100min, 100ft/25min, 25ft/500min (actually 595, but who makes $6 bills? Or wants to stay at 25 feet for just under 10 hours?)
120 rule - 60/60, 70/50, 80/40, 90/30
Add 100 - 50/100, 40/200, 35/300 (310), 30/400 (405)
Take away 5 - 100/25, 110/20, 120/15, 130/10
http://www.vnh.org/DivingManual/DiveManRev4ChangeA.pdf
pg 482 of 948 - Table 9-6, Unlimited/No Decompression Limits and Repetitive Group Designation Table

4) And finally, always turn on the air-integrated computer between your ears before entering the water :thinkingo