Another Tables vs. Computers Thread

[TheRedHead]

Aren't some of the newer computers like Sunnto D-9 taking into account deep stops? I would argue that it would be very difficult for a new diver to plan a multilevel dive (and know exactly what is happening at any depth) than to follow a computer. My intention is not to advocate blindly following a computer, but to point out that it would be difficult for new divers to understand what they are doing and why they want to do it.

Another thing which happens is that some divers dive without computers and go into deco and aren't even aware of it. My buddy and I were in Florida last week diving the Adolphus Busch on doubles using EAN32 and there were 2 other divers diving AL80s on air with no computers. I don't know if they got narced on the wreck, but at 30 minutes of BT at 92 to 105 feet, these guys were still there! We ended our dive and signaled to them to follow us up the line. We made deep stops according to our plan and then a 5 minute hang at 15 feet and they barely made it to the surface before they went OOG. I would think this type of diver would benefit from a computer to tell them when to leave a deep wreck. I'm not saying that this is right, but simply pointing out reality.

 

[lamont]

Aren't some of the newer computers like Sunnto D-9 taking into account deep stops? I would argue that it would be very difficult for a new diver to plan a multilevel dive (and know exactly what is happening at any depth) than to follow a computer. My intention is not to advocate blindly following a computer, but to point out that it would be difficult for new divers to understand what they are doing and why they want to do it.

I agree completely. I pretty much blindly followed my computer for the first 20 or so dives just because I didn't have enough of an attention budget to be pondering deco theory while down there. Staying above 60 though typically means that you won't slide over into deco and don't have to worry a lot about what the computer is saying -- newbies mostly need to watch depth, time and gas and work on their buoyancy and ascent rates.

Another thing which happens is that some divers dive without computers and go into deco and aren't even aware of it. My buddy and I were in Florida last week diving the Adolphus Busch on doubles using EAN32 and there were 2 other divers diving AL80s on air with no computers. I don't know if they got narced on the wreck, but at 30 minutes of BT at 92 to 105 feet, these guys were still there! We ended our dive and signaled to them to follow us up the line. We made deep stops according to our plan and then a 5 minute hang at 15 feet and they barely made it to the surface before they went OOG. I would think this type of diver would benefit from a computer to tell them when to leave a deep wreck. I'm not saying that this is right, but simply pointing out reality.

Probably is that with divers who aren't situationally aware to being with, they could still ignore the computers beeping at them. Arguably you shouldn't be at 100 feet if you're going to blow a dive plan like that. Certainly, having a computer can help limit the damage (hopefully they would have noticed after a minute or two of beeping), but at the same time I think this could highlight how computers can be used as a crutch to let people do dives that they don't have the situational awareness skills for in the first place. They really are a double-edged sword.

 

[Scuba]

However, the reverse is NOT true. I frequently dive profiles that violate the table limits but are well within the limits of the model. Multilevel dives with a maximum depth of 100', and a total dive time greater than 60 minutes are pretty common in my dive log. A dive profile like 10minutes@100', 10@70', 20@50', 20@ 40' would clearly be within the model limits and within the NDL of my computer.

OTOH, a 100', 60 minute dive is clearly beyond the limits of all air tables, and that is how that multilevel dive must be treated on square profile tables such as the PADI RDP, and the USN Tables and the derivatives used by NAUI, SSI, YMCA, and others.

10 min.@ 100'.......Table Limit 20 min.
10 min @ 70'..................T L 40 min
20 min @ 50'..................T L 80 min
20 min @ 40'..................T L 140 min

There is no table limit violation at any specified depth.

This was not a 60 min dive to 100'.

While the statement in bold is correct, the limitations imposed by a square profile dive tool can not be extrapolated as applicable to a valid measurement of a dive outside it's scope, a multilevel dive conducted within the models multilevel limits, for which a different measuring tool must be used. A case of using an inapplicable tool for the measurement being made does not invalidate the measurement or the tool.

 

[MikeFerrara]

10 min.@ 100'.......Table Limit 20 min.
10 min @ 70'..................T L 40 min
20 min @ 50'..................T L 80 min
20 min @ 40'..................T L 140 min

There is no table limit violation at any specified depth.

This was not a 60 min dive to 100'.

No but if you use some software to check, I'll bet it comes out pretty close to being a 60 minute dive to 55 ft.

While the statement in bold is correct, the limitations imposed by a square profile dive tool can not be extrapolated as applicable to a valid measurement of a dive outside it's scope, a multilevel dive conducted within the models multilevel limits, for which a different measuring tool must be used. A case of using an inapplicable tool for the measurement being made does not invalidate the measurement or the tool.

Actually, within a reasonable depth and time range it can be approximated by extrapolation of sorts.

 

[Thalassamania]

Some backgrond on pre-computer repetitive dive calculations:

Repetitive Group (Graver) Method

Dennis Graver further popularized an already widely used table based multi-level calculation in the late 1970s, although this method had been around for at least twenty years and had been discredited in the mid 1960s. The concept behind it is disarmingly simple, one assumes that a given repetitive group represents an absolute quantity of nitrogen, no matter what the dive profile involved in taking up the nitrogen was. This permits the 'sidewise' use of the U.S. Navy Table B-1. The previous dive is simply transferred up the repetitive group column on Table B-1 to the new depth. The diver is assumed to have been at that new depth for the time indicated by the intersection of the repetitive group column and depth row. The diver could continue on until no-decompression time was exhausted. A variation on this technique is used in commercial diving and is called 'Repping Up'.

Despite the sound appearance of the technique, problems develop when dives permitted by this procedure are checked against the U.S. Navy Model. Once a dive is reduced to a repetitive group letter all information except that contained in the 120-minute compartment is thrown away. As a result, Graver suggested that this technique should not permit the entire table to be used, and should be limited to 'less strenuous' dives. Karl Huggins demonstrated a multi-level dive permitted by Graver's method (120 for 15 then 90 for 5 then 70 for 10) that resulted in a 101.5% M0 value for the 20 minute compartment. This value would likely result in bends and is lost when another 'Repping Up' calculation is made.

Repetitive Group/Residual Nitrogen Time Method

This method is similar to the Graver method but the secondary levels of the dive are considered to be repetitive dives without a surface interval. The residual nitrogen time for the secondary levels are obtained by taking the repetitive group produced by the primary level to Table B-3 and comparing the repetitive group to the secondary dive depth to yield a 'corrected' residual nitrogen time, which could be used to calculate the amount of no-decompression time remaining at the secondary depth.

While more conservative than the Graver method, there is still the same basic problem. Since there is no 10 minute surface interval, nitrogen pressures in the 40 minute compartment can build up to unacceptable level which is undetected by the 120 minute compartment that governs the repetitive group designation.

Half and Half Method

This is a patently unsafe procedure that should never be used. It is to spend one-half of the no-decompression limit at a given depth and then ascend and spend one-half of that depth's no-decompression limit at the secondary depth. There is no technique for computing a repetitive dive group after such a dive although some 'native' divers suggest using the repetitive group letter of a dive to the no-decompression limit of the secondary depth.

U.S. Navy Multi-Level Diving Procedure

In order to accommodate the needs of U.S. Navy Combat Swimmers who must remain hidden and use equipment that provides up to 12 hour of life support the U.S. Navy developed a technique that permits 'repetitive dives' without surfacing. The concept is that instead of having a minimum 10 minute surface interval at the surface the diver has a 30 minute 'surface interval' at a depth shallower than 20 feet. According to sources at the EDU, this technique has been tested and is considered safe.

Other Methods

Other techniques have been developed over the years. Most often they have been empirically determined at dive resorts that have a deep wall adjacent to a shallow reef. Profiles that have been used with safety over the years become part of the lore and continue to be used. There are said to be profiles that have been used for over 10,000 man-dives without incident.

 

[Charlie99]

10 min.@ 100'.......Table Limit 20 min.
10 min @ 70'..................T L 40 min
20 min @ 50'..................T L 80 min
20 min @ 40'..................T L 140 min

There is no table limit violation at any specified depth.

This was not a 60 min dive to 100'.

While this tangent is a bit of a hijack, I don't feel guilty hijacking 250+ post threads that should have died alread.

If you back up a bit, you'll see that we got on this tangent in response to a comment I made along the lines of "If you are within table limits, then you are within the model limits. OTOH, since table are crude representations of the model, you can be within model limits and be outside of the table limits".

The dive profile I posted was supposed to be a simple example of a dive that while within the limits of Workmann or DSAT models, is one that is outside the limits of either the USN table or the PADI RDP.

If you were to try and track that dive on the PADI RDP or the USN table, it would indeed be a 60 minute dive to 100' ----- provided you follow the instructions of the table.

------------------

How this does play back into the whole tables vs. computers discussion is that standard tables perform very badly as either pre-dive planning tools or as tools for tracking deco realtime during multilevel dives. Due to the square profile limitations of the table operating instructions, the tables are overly conservative when applied to multilevel profiles.

---------------------

10 min.@ 100'.......Table Limit 20 min.
10 min @ 70'..................T L 40 min
20 min @ 50'..................T L 80 min
20 min @ 40'..................T L 140 min

There is no table limit violation at any specified depth.

That is NOT an appropriate way of checking the validity of a multilevel dive profile.

How about:

15 min @ 100' ....... table limit 20 minutes
20 min at 70' ....... table limit 40 minutes
30 min at 50' ....... table limit 80 minutes
20 min at 40' ....... table limit 140 minutes

Per your style of analysis, it never violates the table limits. In reality, though,
this dive is into deco per the PADI DSAT model at 70' level, and goes deeper into deco at both the 50' and 40' levels. Most importantly, one would surface with signficant deco obligation. (I did the above calculations using a crude, non-standard, non-approved method of hacking the PADI RDP, but I'm relatively confident that anybody that cranks that profile through VPM, Decoplanner, etc. will come to the same conclusion). BTW, I do recognize that the above profile uses lots of gas, but for this discussion I'm assuming an endless supply.

 

[Charlie99]

Repetitive Group (Graver) Method

Dennis Graver further popularized an already widely used table based multi-level calculation in the late 1970s, although this method had been around for at least twenty years and had been discredited in the mid 1960s. The concept behind it is disarmingly simple, one assumes that a given repetitive group represents an absolute quantity of nitrogen, no matter what the dive profile involved in taking up the nitrogen was. This permits the 'sidewise' use of the U.S. Navy Table B-1. The previous dive is simply transferred up the repetitive group column on Table B-1 to the new depth. The diver is assumed to have been at that new depth for the time indicated by the intersection of the repetitive group column and depth row. The diver could continue on until no-decompression time was exhausted. A variation on this technique is used in commercial diving and is called 'Repping Up'.

Despite the sound appearance of the technique, problems develop when dives permitted by this procedure are checked against the U.S. Navy Model. Once a dive is reduced to a repetitive group letter all information except that contained in the 120-minute compartment is thrown away. As a result, Graver suggested that this technique should not permit the entire table to be used, and should be limited to 'less strenuous' dives. Karl Huggins demonstrated a multi-level dive permitted by Graver's method (120 for 15 then 90 for 5 then 70 for 10) that resulted in a 101.5% M0 value for the 20 minute compartment. This value would likely result in bends and is lost when another 'Repping Up' calculation is made.

The "Graver" method is essentially how the PADI Wheel works. Note that the violation that Karl Huggins found was relatively small. A slight reduction in the NDLs would have fixed that problem. Indeed, the PADI Wheel introduces a slightly reduced set of NDLs, called the Multilevel Limits or ML.

Unfortunately, for reasons unknown, PADI chose to only teach multilevel dive planning using the cumbersome and expensive wheel, rather than simply adding the rules and ML values to the flat table RDP. I've posted such a flat table equivalent in my photo gallery, and have also posted a few times the excel spreadsheet used to generate it.

 

[RICoder]

I am not sure if this has been discussed already, so forgive me if it has.

There is significant difference (contrary to what I have read in some posts) between the models that define tables and computers. Many new computers use an RGMB model, which takes into account dissolution of microbubbles over time by tissue compartment (or some such thing). Thusly, being within one model does not equate to being within the other, no matter how many times you say it.

As to the religious/political/subjective discussion on which to learn...I say table first. Even if you learn them both at the same time, you should know the table.
1) Planning a dive on a computer is a bit complicated, and different from computer to computer.
2) Table planning is fairly simple, and pretty much an international language.
3) Tables are conservative, far more so than a computer is, and this leads to 4...
4) Computers vary in conservativeness (is that a word) from manufacturer to manufacturer

...and for the love of pete...PLEASE stop talking about computers failing. That's gotta be the biggest urban legend going in the community. Yeah, they can fail, so can my reg and my bc and my gauges...but the way it is talked about you'd think the things just burts into flame every other dive.

 

[Thalassamania]

Unfortunately, for reasons unknown, PADI chose to only teach multilevel dive planning using the cumbersome and expensive wheel, rather than simply adding the rules and ML values to the flat table RDP. I've posted such a flat table equivalent in my photo gallery, and have also posted a few times the excel spreadsheet used to generate it.

LOL

 

[TheRedHead]

While more conservative than the Graver method, there is still the same basic problem. Since there is no 10 minute surface interval, nitrogen pressures in the 40 minute compartment can build up to unacceptable level which is undetected by the 120 minute compartment that governs the repetitive group designation.

I believe this is the model SSI uses and the tables I used in OW and for many of my early dives. Of course, people do take SIs so there is another table to use to calculate your new pressure group going into the 2nd dive. How does this affect your argument?