Why you can't use the PADI RDP table for Multi-Level dives

[Joneill]

Yes.

Only some of the excursions in the thread.

It is not that they are complicated, it is that they are not applicable. You can easily go through incorrect procedures (as several have posted) but that does not give a valid answer for multi-level dives.

That may be true, but tables are still more complicated and far less convenient to use than a dive computer for multiple, multi-level dives!

 

[steinbil]

In general, I would expect this to be true any time the later part of the dive was significantly shallower than the earlier part of the dive. You are spending significant time shallower than the average off-gassing from the early part of the dive. If the dive had portions deeper than the average just prior to the final ascent (possibly a violation of the GUE procedure), you would expect the opposite result, possibly dangerously so.

If you run the dive of 1a with the 30m and 24m segments reversed (same average depth), your Surface GF hits 100% 3min into the 30m segment, and at the end of the dive you have a SurfaceGF of 119%.

While this is not really allowed using this method, I tried incorporating a deeper-than-average stage of a multilevel dive to see:

Dive 7
multilevel 30m/15m/27m avg 24m- 50 minute bottom time

	depth​	duration​	runtime​
➘	30m	2min	2min
➙	30m	14min	16min
➚	15m	2min	18min
➙	15m	15min	33min
➘	27m	2min	35min
➙	27m	15min	50min
➚	0m	6min	56min

SurfaceGF% 88

At this point it's very close to the square profile. It's interesting to see that depth averaging would still work, especially considering the conservative padding that would happen on a practical dive. So as long as you start at the deepest depth, and the profile is somewhat sane, it seems quite useful within the recreational limits.

 

[dmaziuk]

Just to be clear, are you saying that riding the NDL from depth is as safe/risky as a square profile to the same depth at NDL?

It's less risky than if you planned your square profile to some chi-squared-average-depth and then "accidentally" went a little deeper a little longer.

 

[steinbil]

It's less risky than if you planned your square profile to some chi-squared-average-depth and then "accidentally" went a little deeper a little longer.

Way to derail with a topic that was banned from this forum. Please take it to an appropriate forum, and I'd be happy to explain it to you, since you seem to have trouble comprehending this simple procedure.

MOD EDIT: MOVED

My edit:
Now that it's been moved to an appropriate forum, let me know if you actually want to discuss this perfectly safe method, or if you were just trolling...

 

[dmaziuk]

Oh, I just find it amusing that someone would worry about relative risk of "square" vs. "not square" NDL ascents while at the same time be fine with some rule-of-thumb ratios that some guy said were "perfectly safe" when he did it.

 

[steinbil]

Oh, I just find it amusing that someone would worry about relative risk of "square" vs. "not square" NDL ascents while at the same time be fine with some rule-of-thumb ratios that some guy said were "perfectly safe" when he did it.

I'm not a UTD diver. I don't use AG's ratio deco. Do you just pull this out of your behind?

 

[dmaziuk]

Deco averaging: bad, depth averaging: good. Got it. Moving on.

 

[JMarc]

On the subject of MT90 tables multi-level procedure, I wrote:

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).

Looking at the tables again, the use of the procedure seems indeed to be a time-weighted average. The discrepancy I identified can all the explained by rounding (in the example above, if the 5 minutes are instead 4m30s rounded up, the 270 is then explained).

Trying to derive mathematically a profile having an issue, I instead found out that a multi-level dive with decreasing depth always leads to less load in all compartments than a dive of the same total time at the time weighted average depth (the proof is not hard, but probably out of scope for this thread). That's probably what I was missing and the COMEX didn't when the established the table.

Yet I don't recommend that procedure:

1. No other tables publish it. I presume the MT90 designers did some validation additionally to the mathematical derivation that wasn't done for other tables.

2. Computing the time weighted average depth while under water is not easy and error prone. What works for underwater workers with a surface support team and a planned dive profile with depths and durations determined by the work needing to be done is not always valid for a recreational scuba diver.

3. We now have dive computers which are tracking our real profile far better than we could follow a planned one even in an ideal situation.

 

[JMarc]

Answering in the advanced forum as my message has been moved here.

Are COMEX schedules derived from a strictly exponential model? -- Assuming the exponential model and the same starting Pinsp, I am not sure time T at depth D is exactly equivalent to T/2 at D*2: I think the latter should result in a slightly greater loading. But that aside, if you assume a linear-exponential model, your result may be very different, and I am not sure about sequential model either.

As far as I know, the MT tables are based on an haldanian model. I don't know which additional adjustments they may have done.

What I've a proof for is that spending a time T1 at depth D1 followed by a time T2 at depth D2 where D2 < D1 result in less load than spending T1+T2 at depth (T1*D1+T2*D2)/(T1+T2).

> I am not sure time T at depth D is exactly equivalent to T/2 at D*2

Consider T/2 long enough to reach a saturation level bigger than the one reachable at depth D. Due to the fact that we have to take into account the atmospheric pressure, it is plausible that there is some combination of T, D and half time of the compartment where the two scenarios give the same load, but that should be exceptional.

 

[dmaziuk]

Consider T/2 long enough to reach a saturation level bigger than the one reachable at depth D. Due to the fact that we have to take into account the atmospheric pressure, it is plausible that there is some combination of T, D and half time of the compartment where the two scenarios give the same load, but that should be exceptional.

That's one possibility. I was thinking of gas being driven by delta-P: say, a 5-minute tissue compartment with the starting Pamb and Pinsp of 1 atm:

1. dive to 40m: Pamb is 5, dP is 4. The TC will on-gas to 1/2 of dP in 5 minutes, i.e. in 5 minutes its Pinsp will be 1 + 4/2 = 3.

2. Dive to 20m (D/2). Pamb is 3, dP is 2. The TC will in 5 minutes on-gas to Pinsp = 1 + 2/2 = 2. Now in the next 5 minutes (T*2) it will again on-gas to 1/2 of dP: Pinsp + (3 - 2)/2 = 2 + 0.5 = 2.5.