Info Why are tables not taught in OW classes anymore?

[boulderjohn]

I'm guessing the added conservatism of always rounding to a deeper depth and generally erring on the conservative side makes up for this in practical diving.

The beginning of the sentence should be the title of the process of doing multilevel dives with tables not designed for that purpose. I'm guessing you will often be OK doing that, but then, I'm just guessing.

 

[jale]

I've attached the MT92 tables, from DiveTables.eu.
Here is a comparison (for air) of the NDLs for those tables, and for several others.

Depth (ft/m)	MT92- Table 3	US Navy (old)	US Navy (new)	PADI RDP (DSAT)	DCIEM (1997)	IANTD (2004)	TDI/Buhlm (2005)
40/12	165 mins	200 mins	163 mins	140 mins	90 mins	125 mins
50/15	80	100	92	80	70	75
60/18	50	60	63	55	50	51	44
70/21	35	50	48	40	35	35	28
80/24	25	40	39	30	25	25	20
90/27	20	30	33	25	20	20	18
100/30	15	25	25	20	15	17	16
110/33	12	20	20	16	12	14	14
120/36	10	15	15	13	10	12	12
130/39	8	10	12	10	8	10	10
140/42	7	10	10	8		9	7

The USN tables have the longest NDLs (i.e., most aggressive) of any of the tables, while MT92, DCIEM, and the old TDI/Buhlmann tables vie for the shortest NDLs (i.e., most conservative).The RDP and the old IANTD are middle-of-the-road.
MY92 has a Table 7 used for an equivalent air depth, so that Nitrox of various mixtures can be used with the air deco tables.
MT92 also has a Table 8, used for calculating an equivalent depth for a multi-level dive. It is, in fact, simply a table way of calculating a linear average depth. In general, this is an incorrect procedures, as can be shown theoretically and also by comparing results using it to results using correct procedures. It is possible that the differences in the two procedures (correct and incorrect) will be small, but that is not guaranteed and it is not clear how to determine when you will get a "good" answer and when you will get a faulty answer.

• The theoretical argument against using this average depth to determine decompression needs is that it ignores the decompression that takes place on a single level dive when one ascends to the surface. That is, if the dive level is (say) 30m, and you ascend at 10m/min, then you have 3 minutes of ascent time during which you are off-gassing; that off-gassing is included in the table calculation. By ignoring that ascent off-gassing -- which you do by using the average depth procedure -- you think you are in a certain nitrogen status but in fact you are not: you have more nitrogen than you think because you have not done the off-gassing.
• The practical example of the error in the average depth procedure can be illustrated by using the table RDP compared to the eRDPml, which is designed to allow multi-level diving based on the same decompression model as the RDP; they are the same model, just with results presented in different forms. Assume your dive is to 30m/100 ft for 18 minutes (NDL is 20 mins). Your Pressure Group is then P. In that Pressure Group, a second level at 60 ft/18m has residual nitrogen of 39 minutes and NDL of 55 mins, so you think you might have 55-39=16 more minutes you can spend at your new depth. But a multi-level calculation with the eRDPml says you only are allowed 13 mins at that second depth. (This is because you actually have more nitrogen in you than the square-table allows for, since you did not ascend to the surface.) Using the average depth procedure suggested by MT92, your "equivalent depth" is 25m/80 ft. At that depth, your NDL is 30 mins, but you will have spent more than that (either 31 or 34 minutes) by using the average depth procedure of MT92.

Calculating safe profiles for multilevel dives using depth average is not a good practice. You can easly go into deco (violate your NDLs) and not know it.

The only saving grace on using MT92 procedures is that the NDLs of the tables are quite conservative, so violating those NDLs by just a few minutes may not actually put you into deco status if you were using a more aggressive dive model. So, you might be lucky. Luck is no way to plan a dive.

Hi
Kudos to you to go through the official doc of the MT92
But I dont get your point.
You say the table is calculating a linear average depth. Maybe but it is not said. They talk about "coefficient". You may be right but it seems to me that it is a bit more complex than that as I don't think the COMEX's guys made all these tests to end up with a simple average. These tables must, by national law, also be used by working professionnel divers.
But who knows ? And you might be right.
Then, your example of 30M-18min+18M-13/16min does give a 24M equivalence with MT92 and then you need 3min deco as the NDL is 25min. But I don't see the problem!
If you really insist on not going to the NDL, just choose a number of minutes which will keep you where you want to be!
You say "You can easly go into deco (violate your NDLs) and not know it". ??? Don't you plan your dives?
Anyway, the point is that multi-level tables exist and whatever they are safer than computer it doesn't matter as, like every tool, you must use them in the way they have been designed.
Now, I don't dive MT92 anymore but the experience I got using them, gives me a better awareness of my position in the water column.
The computer, like the gauge, are tools to validate my rough expectations and if they fail during the dive then there is no need for panicking!
I really do appreciate your "commitnment" (you started a few threads about tables-computer ) and you are right to show the weakeness of tables comparatively to modern computers but I don't see why it should be done so, if I may say, negatively.

 

[tursiops]

You say the table is calculating a linear average depth. Maybe but it is not said. They talk about "coefficient". You may be right but it seems to me that it is a bit more complex than that as I don't think the COMEX's guys made all these tests to end up with a simple average. These tables must, by national law, also be used by working professionnel divers.
But who knows ? And you might be right.

It is easy to check. Just look at the numbers.

 

[JMarc]

It is easy to check. Just look at the numbers.

Although the contribution of spending a time T at depth D seems equivalent (I've just looked at a sample of the entries) to spending a time T/2 at depth 2 D, spending a time 2 T at depth D is not equivalent to spending twice a time T at depth D (for instance spending 5 minutes at depth 9m contributes for 5, spending 300 minutes at depth 9m is contributing for 270, 30 less than the 300 a time weighted average would).

(Considering the track record of COMEX in diving -- AFAIK, they are still holding both depth records for sea and chamber diving --, my first reaction if I don't agree with them concerning decompression procedure is that I'm missing something).

 

[tursiops]

You say "You can easly go into deco (violate your NDLs) and not know it". ??? Don't you plan your dives?

The planning procedure is done before a dive. Planning a multi-level dive using incorrect procedures can put you into deco and you don't know it.

 

[tursiops]

Anyway, the point is that multi-level tables exist

I'm sorry, but MT92 does NOT provide a multi-level table. It provides a square-profile table plus a procedure to calculate an average depth, which is a linear averaged depth, nothing more. Their Table 8 simply saves you some time in calculating that average, it does nothing extra. If you use that average depth -- whether you use Table 8 to do it or some other method -- you raise your risk of DCS. It is that simple. I've provided both theoretical reasons and practical examples. I can explain it to you, but I cannot understand it for you.

 

[inquis]

And in practice it seems to be working, giving safe dive profiles that usually mimics the typical buhlmann computers with some conservatism

You keep coming back to this as the criterion for being "ok", but that strikes me as moving the goal posts. OK should be "within the limits of the underlying GUE algorithm," as that maintains whatever cushion they have deemed reasonable.

Your statement is the equivalent of planning a dive with BuhlmannGF x/80 but overstaying and surfacing while still in deco (based on x/80). Then saying this is "proper application of x/80" because GF x/95 says you're not in deco. You've clearly increased the risk profile.

 

[scubadada]

This thread is an interesting read. The posts regarding using tables for a multi-level dive by tursiops, boulderjohn, steinbil, lowwall, jale, Bigbella, et. al. could easily be a good thread on its own

 

[steinbil]

You keep coming back to this as the criterion for being "ok", but that strikes me as moving the goal posts. OK should be "within the limits of the underlying GUE algorithm," as that maintains whatever cushion they have deemed reasonable.

Your statement is the equivalent of planning a dive with BuhlmannGF x/80 but overstaying and surfacing while still in deco (based on x/80). Then saying this is "proper application of x/80" because GF x/95 says you're not in deco. You've clearly increased the risk profile.

No. Not equivalent. I'm not moving any goal posts. The comparison is meant to be between tables and computers, that's the whole point.

Follow along...

- Several posters say "YOU CANT USE DEPTH AVERAGING AND DIVE TABLES" unless you use a computer you are "flailing in the dark"
- I say: GUE has a method for doing this that seems to work and in my experience does not exceed the limits of a typical dive computer.
- I also say: I wonder if there are any conditions where this method is more aggressive than following a typical dive computer, because if there aren't any, this method is AT LEAST as safe. And if there are, I would like to know, so I can learn and take that into consideration.

This is why I'm comparing it to Buhlmann with a GFhi of 85, because that is what I (and many others) use when I dive with my Perdix.

There's only one goal post, and I'm not moving it. BUT, I am open to the possibility that there might be conditions where this method is more aggressive than what I have seen. Which is why I'm trying to ask sincere questions. To learn. Instead I keep having to fend off straw man arguments and non sequiturs.

 

[boulderjohn]

- I say: GUE has a method for doing this that seems to work and in my experience does not exceed the limits of a typical dive computer.

The scientific method at its finest.