Computer vs tables

[Charlie99]

Theoretical Dive: Wreck in 100'. Planned visits are the hull at 80', the deck at 60', and the bridge at 30'. For ease of calculations, plan for 10 minutes at each site. We also are assuming this is the first dive with no nitrogen buildup to begin with. Dive 1 is using a completely square profile, Dive 2 is using my 0:00 SI method. These use the RDP.

Dive 1: 80' for 30 min. = PG R (which is actually the NDL for 80'.)

Dive 2A: 80' for 10 min. = PG C
0:00 SI leaves 14 min. RNT at 60'
Dive 2B: 60' for 24 min. (14+10) = PG I
0:00 SI leaves 48 min. RNT at 35'
Dive 2C: 35' for 58 min. (48+10) = PG L

Credit is obviously being given for something, as at the end of the dives, there is a PG difference of 6. My question is whether this credit comes from somewhere we don't want it to or not. Can anybody with software/computer run this and see if it cries?

It's not that you are getting "credit" from anything. The PADI RDP table (and most other tables too) simply track one (and only one) compartment. The PADI RDP tracks the 60 minute compartment, and the different pressure groups correspond to specific levels of loading in the 60 minute compartment. 10 minutes at 80' loads the 60 minute compartment to the same level as does 14 minutes at 60', as does any other depth/time combo that corresponds to pressure group C. That's why using zero SI and RNT to start the next multilevel segment works.

The zero SI method you used above provides you with a good estimate of the loading in the 60 minute compartment at the end of the series.

Although the RDP tracks ONLY the 60 minute compartment, the NDL is determined by calculating the loadings of all compartments. The faster compartments that control dives deeper than 40' will reach their limit faster than does the 60 minute compartment ---- that's why for example, that the 30 minute NDL of an 80' dive only gets you out to pressure group R.

While the zero SI method you described correctly tracks the 60 minute compartment, you can come up with multilievel profiles that stay within the limits of the RDP yet are slightly outside the NDL limits on the faster compartments. That's why the PADI wheel has slightly reduced NDLs for multilevel dives. I've put together a table that has the reduced NDLs (PADI Wheel calls them MDLs), but in practice I've found that just using the PADI RDP with the zero SI method is close enough to do a sanity check pre-dive on a proposed multilevel profile, and then I use my computer realtime as I dive.

Charlie Allen

 

[Charlie99]

So, here is my dangerous statement: I'll trust in my computer. Knowing that I understand and am aware of what is going on enough that I can bail myself out when(not if) it dies.

As Ronald Reagan said "Trust but VERIFY".

If you don't have a clue as to what the computer SHOULD be saying, then your are unduly trusting it.

A wise diver knows enough about his decompression status to know about what the computer should say and therefore is aware if it is giving bogus data.

The DIR crowd, IMO, has gone one step too far, in that they purposefully cripple computers so that they no longer display deco info. Redundancy is nice, and I treat my calculations and the computer calculations as two separate bits of data that should more or less agree.

==============

I treat the depth readings of my computer in the same way. I pretty much know what depth I'm at before I look at the computer. Any big discrepancy sets off alarm bells. OTOH, having the computer display depth doesn't in my mind make me overly reliant on it, to the point where I learn to judge depth independently and then turn off the depth function.

 

[Stu S.]

Years of study and development went into the 12 compartment algorythm in my computer. And, the computer took a lot of work to develop, too. It measures temp, averages depth, looks at ascent rate, PO2, and a bunch of other things.

I am not smart enough to perform all the calculations that the computer is constantly doing.... and dive at the same time. It does a more complete job than I can. That's why I bought the thing.

Happy New Years / Stu

 

[TSandM]

It's true, I can't do the math the computer does in my head. But precision is not accuracy and accuracy within a model is not necessarily truth. Since the algorithms are based on mathematical models of an imperfectly understood system, I'm not sure that having something iteratively calculating assumed nitrogen loads in 12 compartments is actually going to end up being significantly better than my simple table/depth averaging/minimum deco approach to things. It's probably not worse, as long as you understand what it's doing and don't turn your brain off.

 

[gcbryan]

As David says, it's not more accurate to do it yourself. It's probably less accurate, especially if you lose attention and lose track. Which is why I used my computer for a long time before I began to trust myself. You don't do it yourself because it's more accurate. You do it yourself because it FORCES you to pay close attention to your dive.

My new computer, the Aladin Tec 2g, will plan a decompression schedule for me, using two gases. But I don't know what model it's using, and I don't know what profile it will generate. So, if I ever do mandatory decompression diving, I will use a tool I understand, whether that's a deco program I can tweak and adjust beforehand, or something like Ratio Deco that I can use beforehand and adjust as needed in the water.

I don't want anything I don't understand telling me how I should best complete my dive.

Not to pick on you Lynne but I'm just curious given the nature of this thread...why did you get the new computer? I also understand that the more involved deco schedules are usually prepared ahead of time and carried along for the dive but on a recreational dive if you weren't doing the averages in your head would you really not be paying close attention to your dive? I'm guessing you would be paying attention regardless wouldn't you?

 

[SparticleBrane]

The DIR crowd, IMO, has gone one step too far, in that they purposefully cripple computers so that they no longer display deco info.

And yet, I can't think of anyone else doing these kind of dives on a regular basis.

 

[Blackwood]

The main DIR objection to dive computers is that they supposedly encourage the diver to turn of his/her brain. So let it be written: "no computers." That's very unforunate because it eliminates one of the best tools available to learn about decompression.


I believe that most of the practical information I know about decompression came not from taking deco classes or from playing with VPlanner or from reading text books, but rather from diving with a dive computer.


The only way to have an reasonable expectation of your computer's computation is to have learned what your computer does. IMO, the best way to do that is to dive with your computer and watch how it treats various profiles. Watch how it changes the NDL display as you ascend, or as you stop, or as you descend. Watch what happens when you dive a square profile. Watch what happens when you don't. Pay close attention to your profile, and pay equal attention to the result of the math your computer does based on your profile. If you do that, you will know what to expect when you've spent 10 minutes at 120, 15 minutes at 70 and 30 minutes at 30. But if you don't know both your profile AND your computer, your expectation is unreasonable.

Personally, I'd love to see an in-depth decompression class that involves the principles of ratio deco/depth averaging AND dive computers. One or the other is good, but putting those tools together allows for a potent understanding of decompression mathematics.

 

[SparticleBrane]

Not to pick on you Lynne but I'm just curious given the nature of this thread...why did you get the new computer?

Well according to this thread, it was a Christmas present from her husband.

 

[Charlie99]

And yet, I can't think of anyone else doing these kind of dives on a regular basis.

A wise diver uses the right tool for the job. My dives are quite different than the ones you linked to. Those dives are quite different than the dives that are being discussed in this thread.

Only a fool thinks that a certain procedure, technique, or equipment is optimal for all dives.

 

[RS]

Thanks for the example.