Info Why are tables not taught in OW classes anymore?

[Blackcrusader]

Making your dives that are close to the NDL on your computer safer:

You can do the equivalent of a Shearwater adaptive safety stop using any computer. Simply extend your safety stop to 4, 5, or whatever minutes if you come within 5 or whatever you choose minutes of the NDL. I did this kind of safety stop for many years

The guesswork has been taken out. With the SurfGF function on Shearwater, Garmin, maybe other computers, you can simply sit at your safety stop until your SurfGF has decreased to a level consistent with what you want. I have not surfaced with a GF greater than the low 80s since purchasing my Teric in 2019. I certainly do not push my NDL on most dives and my average surfacing GF is around 50.

Yup I set my safety stop to 5 mins on my Perdix and like Scubadada surface with a low GF.
This is the highest GF 71% I had on exiting any of my dives on my last vacation. Most were around 40%

 

[boulderjohn]

I thought I would add something that many people do not understand. As I said earlier, if you begin an ascent within NDLs, it does not seem to matter how slowly you ascend, as long as you don't go into deco. A long multilevel dive ends with the same safety stop and direct ascent as a square profile dive. That is what makes multilevel diving possible. A computer algorithm keeps track of your dive and tries to warn you before you overstay a depth and go into deco. Without a computer, you are guessing. Tables do not give you any such guidance.

What is missing in this discussion is why that matters.

A common misstatement on ScubaBoard is that all dives are decompression dives. In reality, there is a big difference between a dive with no required decompression stops and what is commonly called a decompression dive (one with required decompression stops). In an NDL dive, a slow ascent does not matter. Take your sweet time, do a safety stop, and head for the surface. Once required decompression is reached, it matters very much. The longer you tarry at depth, the longer and deeper the required decompression stops.

 

[steinbil]

I thought I would add something that many people do not understand. As I said earlier, if you begin an ascent within NDLs, it does not seem to matter how slowly you ascend, as long as you don't go into deco. A long multilevel dive ends with the same safety stop and direct ascent as a square profile dive. That is what makes multilevel diving possible. A computer algorithm keeps track of your dive and tries to warn you before you overstay a depth and go into deco. Without a computer, you are guessing. Tables do not give you any such guidance.

What is missing in this discussion is why that matters.

A common misstatement on ScubaBoard is that all dives are decompression dives. In reality, there is a big difference between a dive with no required decompression stops and what is commonly called a decompression dive (one with required decompression stops). In an NDL dive, a slow ascent does not matter. Take your sweet time, do a safety stop, and head for the surface. Once required decompression is reached, it matters very much. The longer you tarry at depth, the longer and deeper the required decompression stops.

This sounds very counterintuitive. Considering the line between "NDL" and "deco" is defined by the choice of algorithms and gradient factors (or equivalent conservative settings), not to mention all of the other factors that affect the efficiency of decompression, it makes no sense to me that there should be a hard line between the two. It sounds more logical that there would be a gradual shift and that similar strategies for reducing risk in the ascent phase would apply.

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?

 

[scubadada]

...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?

The key to the no stop dive is staying out of deco on the ascent. As long as you do that, you can make a direct ascent to the surface and arrive on or below your GF high. Shearwater computers do not take a safety stop into consideration as it is "not required". A safety stop will decrease the surfacing GF below the GF high. The relative safety of this dive would seem to depend on the GF high that you are diving.

 

[steinbil]

The key to the no stop dive is staying out of deco on the ascent. As long as you do that, you can make a direct ascent to the surface and arrive on or below your GF high. Shearwater computers do not take a safety stop into consideration as it is "not required". A safety stop will decrease the surfacing GF below the GF high. The relative safety of this dive would seem to depend on the GF high that you are diving.

Of course I understand this, but I've heard many times that riding the NDL limit on they way up is more aggressive (and not recommended) than making an ascent after hitting the NDL on a square profile or a multilevel dive that doesn't go to the limit of NDL at every depth. It also seems logical that riding the NDL would lead to a higher loading of nitrogen, especially in the slow tissues, and that you would indeed increase risk of DCS to some degree. Considering then that the diving within the NDL limits is no guarantee to avoid DCS, it's a moving target that we don't fully understand, it seems strange to say that it makes no difference.

 

[johndiver999]

The dive tables that I am familiar with specify the assumed ascent rate. If the ascent rate is significantly reduced, particularly during the deeper portions of the ascent, then the tables would be violated.

Obviously, if you do the same (on a No-deco or even a deco dive) then the computer knows this and will make the necessary adjustments with respect to the generation of any stop or the extension of existing ones.

For example, I have seen my computer kick over from a nodeco to a deco dive (which requires a short stop) many times when I have initiated my ascent with negligible no deco time remaining. I often stop around 50-40 ft from a deep recreational dive for around a minute and when I do that, sometimes the computer clicks over from the "caution" to the "red" zone or if I am already in deco, I see no reduction in deco stop time associated with my slowed ascent, Sometimes it might even spank me for another minute of deco for the delay. I don't generally view this as a problem for myself because I carry some redundant air and always clear my required deco and generally pad the shallow stop with more time than the computer demands.

Perhaps this is outside of the basic diver discussion, but the point I am trying to make is that with tables, an excessively slow ascent rate can put you into an unknown situation which is outside of the table assumptions, unless of course you were to add the extra time spent on ascent to the actual bottom time.

This is one of the significant advantages of tables versus a simple computer.

 

[EFX]

Of course I understand this, but I've heard many times that riding the NDL limit on they way up is more aggressive (and not recommended) than making an ascent after hitting the NDL on a square profile or a multilevel dive that doesn't go to the limit of NDL at every depth.

Riding the NDL on the ascent is impossible unless you have multiple tanks staged at various shallow depths on the way up and you have a ton of patience. This is because the NDL time is not linear on the ascent. I've had a few minutes left of NDL only to go shallow and find my NDL is 23 min and then at 30 ft it's at 99 min. Actually it's much higher than that; the computer's display maxs at 99. If you want to see what the NDL is ascending to shallower depths up to 999 minutes run my Dive Excel spreadsheet.

As @boulderjohn pointed out earlier there is a big difference between ascending on NDL dives and ascending on deco dives. The latter has a ceiling which cannot be exceeded and because of the half-times of tissue compartments time is required to decompress to even go 10 ft (3 m) shallower. Ascending too slow will add deco time to slower compartments which take over at shallower depths. NDL dives have no such penalty because of a direct ascent to the surface. Fast ascents without stops are therefore possible which don't load up the slower tissues.

 

[inquis]

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

Not to speak for @boulderjohn, but there was no mention of risk in his statement. Simply mandatory deco vs. no-stop ascent according to wherever you want to draw that line (presumably in accordance with your risk tolerance). It's easy to stay in the no-stop area when ascending. It's easy for mandatory deco to increase once you've passed that point.

My personal take on the risk aspect: certainly there is more total gas absorbed in your tissues when you ride the NDL up vs. the square profile, but that's not necessarily more risky. My understanding is the limits, in fact, vary in order to yield comparable levels of risk for the various tissue types when they are at their respective limits (and by extension, at the conservatism-modified limits).

That said, I'm talking about surfacing with the controlling tissue at the same relative degree of tolerated supersaturation on the two dives. Whether this happens will depend on whether your computer includes predicted off-gassing during ascent as part of its NDL calculation and whether you take a safety stop. Assuming you make a safety stop, the differences in controlling tissue time constants on the two dives would result in surfacing with a lower degree of supersaturation for the square-profile (i.e., less risk).

OTOH, if you watch your surfGF (available on some computers) tick down to 70% (or whatever) while on the safety stop, the risk will be comparable from either profile. Like much in diving, "it depends".

 

[steinbil]

As @boulderjohn pointed out earlier there is a big difference between ascending on NDL dives and ascending on deco dives. The latter has a ceiling which cannot be exceeded and because of the half-times of tissue compartments time is required to decompress to even go 10 ft (3 m) shallower. Ascending too slow will add deco time to slower compartments which take over at shallower depths. NDL dives have no such penalty because of a direct ascent to the surface. Fast ascents without stops are therefore possible which don't load up the slower tissues.

My point was simply that I would expect that a very slow ascent from depths within NDL limits increases your tissue loading similarly to how they would increase in a dive with deco obligations. The difference between a deco dive and an NDL dive is where you draw the limit of supersaturation of the leading tissue, so I would expect that there is some sort of relationship between the amount of tissue loading and risk of DCS whether the you define the dive as an NDL dive or a deco dive - especially since the same dive could be interpreted in both ways, depending on where you draw the line.

Edit: This may very well be wrong, I'm not pretending to be an authority on this. I'm just trying to explain my thought process, in case there is something for me to learn about this.

Not to speak for @boulderjohn, but there was no mention of risk in his statement. Simply mandatory deco vs. no-stop ascent according to wherever you want to draw that line (presumably in accordance with your risk tolerance). It's easy to stay in the no-stop area when ascending. It's easy for mandatory deco to increase once you've passed that point.

He said "if you begin an ascent within NDLs, it does not seem to matter how slowly you ascend, as long as you don't go into deco". I interpret that as meaning it does not affect the risk.

 

[BoltSnap]

A common misstatement on ScubaBoard is that all dives are decompression dives. In reality, there is a big difference between a dive with no required decompression stops and what is commonly called a decompression dive (one with required decompression stops).

Do really want to get into this AGAIN?

Just because you call it "misstatement" doesn't make it so and doesn't make your argument valid at all.

I am not going to get into this AGAIN just because I don't think it is proper to hijack this thread like what happened in previous threads