Info Why are tables not taught in OW classes anymore?

[boulderjohn]

If I had the tables it would be my assumption that as I went up in depth I could spend more time at that depth. Assuming that I was just going up I would be fine and not riding the NDL and staying on the tables. If I went up and down then I would have to check and see whether that would be possible.
For example, on standard air(I don't have a nitrox cert yet) I do a standard OW dive to 60ft. Putting in a contingency because my buoyancy is bad/pretty fish/i made a mistake/something I look at the table for 70ft meaning the maximum amount of time i have is 40 mins. Theoretically(I think) If i went up say to 40ft I could get an extra 40 mins of bottom time. Of course I would run out of air but that is what I meant. If I am wrong please correct me, ill attach a picture to show what I mean.

**This isn't advice this is an assumption, I don't know if this would work properly or not.

As Tursiops said in the post above, the tables don't work that way. You can't do what you say you would do.

 

[inquis]

Reads 78 feet when the console mount and my wife's and another computer read 75 feet. It is fairly astounding how much difference that makes over 2 dives. At the end of the second dive of one day the one that reads deeper was 6 minutes into deco while the one that I believe to be correct had 15 minutes of no deco time remaining.

I think this must be a mismatch in configuration. Of course, I don't know what your profile was, but as an example: for a 78 ft NDL dive on EAN32 (41 min for Buhlmann+GF 40/85), a 1 hr surface interval, and another 24 minutes at 78 ft, I get about 12 minutes of remaining NDL compared to 15 minutes on the 75-ft computer.

Interestingly, if I set the deeper-reading computer to air (keeping the shallow one on EAN32), use the air-NDL time for the first dive (24 mins), and 27 minutes for dive #2, I get 15 minutes remaining on the shallow-reading computer and a 20 ft deco stop on the deeper-reading one with a 6 minute TTS.

On the flip side, this depth difference can be leveraged in some cases: I have a warm-water diving buddy that hangs about 5-7 ft shallower than I do, and he uses a Suunto computer (known to be relatively conservative for repetitive diving). This adjustment pretty well matches both our gas consumption and our NDL times for most of our Cozumel dive profiles.

 

[lowwall]

If I had the tables it would be my assumption that as I went up in depth I could spend more time at that depth. Assuming that I was just going up I would be fine and not riding the NDL and staying on the tables. If I went up and down then I would have to check and see whether that would be possible.
For example, on standard air(I don't have a nitrox cert yet) I do a standard OW dive to 60ft. Putting in a contingency because my buoyancy is bad/pretty fish/i made a mistake/something I look at the table for 70ft meaning the maximum amount of time i have is 40 mins. Theoretically(I think) If i went up say to 40ft I could get an extra 40 mins of bottom time. Of course I would run out of air but that is what I meant. If I am wrong please correct me, ill attach a picture to show what I mean.

**This isn't advice this is an assumption, I don't know if this would work properly or not.

As others have said, this is not how it works. But the good news is you underestimated the remaining time at 40'

Purely from a theoretical perspective, you actually could use the tables to calculate multiple level dives by treating each level as a separate dive with a 0 minute surface interval. So, using the PADI RDP, for your example of 70' and 40 minutes, you would be surfacing in pressure group T. Flip the card over and you will see that the Actual Bottom Time (aka no deco limit) for a 40' dive starting in pressure group T is 49 minutes.

Using the SSI tables for the same 70' and 40 minutes puts you in pressure group H, which means your NDL at 40' is 43 minutes.

 

[tursiops]

Purely from a theoretical perspective, you actually could use the tables to calculate multiple level dives by treating each level as a separate dive with a 0 minute surface interval. So, using the PADI RDP, for your example of 70' and 40 minutes, you would be surfacing in pressure group T. Flip the card over and you will see that the Actual Bottom Time (aka no deco limit) for a 40' dive starting in pressure group T is 49 minutes.

Using the SSI tables for the same 70' and 40 minutes puts you in pressure group H, which means your NDL at 40' is 43 minutes.

This is not a valid procedure. eRDPml PADI and Why you can't use the PADI RDP table for Multi-Level dives

 

[lowwall]

This is not a valid procedure. eRDPml PADI and Why you can't use the PADI RDP table for Multi-Level dives

It certainly is from a deco theory perspective. The fact that PADI decided to chuck in a little extra conservatism when they came up with their multi-level dive table doesn't change the tissue loadings.

Of course it won't be exact because of the missing ascent and descent, but none of these calculations are precise enough (depths in feet, time in minutes, 14 compartments for RDP, etc.) for it to make a noticeable difference. Take the example given. The real diver stops at 40' while the hypothetical diver spends approximately 1 minute* surfacing and descending. Adding sixty seconds at an average depth of 20' isn't going to change your NDL by more than a few seconds which is not enough to affect the rounded results given by the RDP.

* One minute because NDL in the RDP is based on an assumption of least conservative practices and it allows a 60fpm ascent and there is no required safety stop for this dive. So 40 second to surface. Since there's no descent speed limit, you could be back to 40' in well under 60 seconds if your equalization skills are better than mine.

 

[inquis]

Purely from a theoretical perspective, you actually could use the tables to calculate multiple level dives by treating each level as a separate dive with a 0 minute surface interval.

Unfortunately not. Your tissue loading is higher than expected since you didn't offgas during the supposed ascent & descent time. You can somewhat compensate by adding a pressure group, but even then it's easy to find cases where it still doesn't work when compared to an actual multi-level planner.

EDIT: We cross-posted, and I agree the deco theory is drawing a thin line through a broad gray area. The fact remains, though, that cheap computers are widely available.

 

[tursiops]

It certainly is from a deco theory perspective.

No, from a theory perspective it is exactly wrong. From a practical standpoint, it is only a little wrong.

Of course it won't be exact because of the missing ascent and descent

Agreed, and you chose an example, that minimizes the issues.

Adding sixty seconds at an average depth of 20' i

The off-gassing/on-gassing does NOT depend on average depth. the process is not linear with depth.

Read the references in my previous post; your suggested procedure is NOT correct, and appealing to the classic "well its all theory anyway" or the silly "because its just an approximation, another one won't hurt" is beneath you.

Why use the tables at all? Hey, they are just made up from some poorly tested model. [/sarcasm]

 

[Bigbella]

I really didn't know what I was missing, during those first fifteen years of diving -- entirely with the use of dive tables and a watch. We did plenty of decompression dives in those years; did young and terribly stupid things in hindsight -- repeated 50-60 meter dives, off Carmel, on air, immediately springs to mind; and none of those with whom I dove, back then, ever seemed the worse for it.

My first dive computer was purchased only in 1992, though I still carried a watch, and do so to this day; and while I don't wish to get into some post hoc argument about it, the very few whom I knew who eventually developed DCS, coincidentally did so while on computers.

I still think that there was and is more of a tendency and false security toward pushing the NDL limits with electronics. That was certainly the case with me and many other early adherents.

Yes, tables have severe limitations, especially in terms of multi-level diving, compared with the technology of today; but I am still here; still diving after going on forty-six years; and still remain able to feel my extremities . . .

 

[TMHeimer]

I really didn't know what I was missing, during those first fifteen years of diving -- entirely with the use of dive tables and a watch. We did plenty of decompression dives in those years; did young and terribly stupid things in hindsight -- repeated 50-60 meter dives, off Carmel, on air, immediately springs to mind; and none of those whom I had dived with, back then, ever seemed the worst for it.

My first dive computer was purchased only in 1992, though I still carried a watch, and do so to this day; and while I don't wish to get into some post hoc argument about it, the very few whom I knew who eventually developed DCS, coincidentally, did so while on computers.

Yes, tables have severe limitations, especially in terms of multi-level diving, compared to the technology of today; but I am still here; still diving; and able to feel my extremities . . .

Maybe tables vs. DC is like stick shift vs. auto transmission?

 

[Bigbella]

Maybe tables vs. DC is like stick shift vs. auto transmission?

I do drive a stick . . .