The old tables vs computer argument

[Blackwood]

It's pretty easy to memorize the first dive NDLs. Knowing the adjusted NDLs versus depth is a little more difficult. That's why I would write them on a slate.

I wasn't meaning adjusted NDLs. I was meaning all the columns for less time than NDL. I don't plan that way (i.e. with pressure groups, etc.), so to me it's useless information.

 

[Trace Malinowski]

Many people of every generation have a problem acception new things. Like a home computer for instance. When the desktop PC was becoming popular I just didn't see the importance or the value of them. Changed my mind though when Windows 98 came out. Another instance is the electric starter on a Harley. I thought that only sissies would use a button to start one. After about twenty years of cranking, hurting my ankle and cussing because I didn't get the gas/air mixture right I gave up and got one. Never be without either one now. I've only dived with a computer and never learned the tables. It's going to stay that way.

My personal evolution

Tables
I first learned to dive in 1982 using the US Navy dive tables which employ "No-Deco" and standard air decompression tables. Being able to use Navy tables allowed me to look at any table from any agency and use them.

Computers
I bought my first dive computer in 1990. It was an Aladin Sport. I was led to believe it was identical to the Pro, but it wasn't. The Aladin Pro would give you the amount of time you would be required to remain at each decompression stop, but the Sport would just give you something like STOP 10 and you would have to remain there until the indicator of a required stop was no longer displayed. You had no idea how long you'd be there, but knowing decompression tables gave me a pretty good idea so I kept the unit. I still have it. It still works. Although, my friend and I got around the $99.00 battery change fee, by soldering in a replacement battery ourselves. The display otherwise was large, simple, easy to understand. It logged your last 9 dives. Another downside was the strap design that couldn't be easily replaced if broken. It was water activated.

My next computer was a Dive Rite Bridge II for nitrox. There were 3 buttons that took a while to learn the sequences to press in order to navigate through the computer. The unit had a very easy to read display and even tracked the battery life. The problem was the computer was always "on" like a home PC is sleep mode when not in use and you could watch the battery drain rapidly as it sat on the shelf. I sold that to an ex-girlfriend who needed a computer.

After that, I had a Dive Rite Nitek III. You need the owner's manual to operate it. It is another difficult computer to use because there are so many possible variables to program the gases and gas switches. It tends to have a pretty severe "Nag" feature. By the time you program it, you can already have generated 3 sets of software tables.

Back to tables
A decade ago, and having been an instructor for 10 years, I learned the art of scuba diving as I presently know it. I learned that computers often force you to do things that will increase your risk of DCS due to their various ways of punishing the diver such as beeps, nags, and lockouts. The decompression algorithms in computers don't take into consideration the variables of a dive that one's own mind can and most all of them employ archaic Neo-Haldanean dissolved gas models and aggressive M-values or gradient factors. To make use of reduced gradient bubble models and varying permeability models, I turned to computer generated tables. These softweare programs made the dives safer by calculating just about every nuance from gas needs to OTU's and CNS% before you entered the water. I carried my plan, my bail out plan, and my worst case overstay in my pocket written in a Wetnotes notebook.

No more tables and no computer
My current evolution is no more tables and no computer. I use a methodology called "Ratio Decompression" in which I can extrapolate my ascent schedule in my head based on a ratio of 1:1 - 1 minute of deco for every 1 minute of bottom time for nitrox and trimix once the Rule of 120 or Rule of 130 has been exceeded and we are no longer on a slide-pause "direct" ascent schedule called "minimum deco." Diving this way allows the diver the ability to take advantage of the bubble model in the deeper section of the dive, a combination of bubble and dissolved gas models during the intermediate stops, and the dissolved gas model in the shallow stops. The diver may also take advantage of the oxygen window through the use of higher oxygen gases and/or inverting the hangs in the shallow stops. We ascend at 30 feet per minute through the deep portion of the dive, then slow to make mandatory stops in technical diving or safety pauses in recreational diving from at either 75% of depth or 50% of depth depending upon whether a dive is technical or recreational. I compare my average time at any depth to the baseline depth of dives over a given range and then determine my ascent profile. 50% of my stops have to be in shallow water & 50% in intermediate depths. Once I know the depth from my baseline, I can add or subtract my required deco time.

For example a technical dive might look like this after 40 minutes at 150 feet with 21/35 trimix and 50% oxygen:
10 ft. - 7 minute stop
20 ft. - 13 minute stop
30 ft. - 4 minute stop
40 ft. - 4 minute stop
50 ft. - 4 minute stop
60 ft. - 4 minute stop
70 ft. - Gas Switch to Nitrox 50 stop 4 minutes
80 ft. - 1 minute stop
90 ft. - 1 minute stop
100 ft. - 1 minute stop
110 ft. - 1 minute stop
120 ft. - PER MINUTE
130 ft. - FEET
140 ft. - AT 30
150 ft. - ASCEND

A recreational dive to 100 feet on Nitrox 32 for 30 minutes might look like this:
Rule of 130 = BT + DT < or = 130
100 ft + 30 min = 130 = minimum deco "direct ascent"

10 ft. - 1 minute stop
20 ft. - 1 minute stop
30 ft. - 1 minute stop
40 ft. - 1 minute stop
50 ft. - 1 minute stop
60 ft. -MINUTE
70 ft. - PER
80 ft. - FEET
90 ft. - AT 30
100 ft. - ASCEND

All this sounds complicated and I made it intentionally vague. You can learn this method in a class and learning it takes no longer than learning to use dive tables, dive planning software or learn how to use all the features of a dive computer.

The advantage is that I'm not locked into anything. I can change any portion of the dive at any time to meet the needs of the situation. If I want to stay longer than my bottom time, I can. If I want to multi-level a dive, I can. If the ocean is rough and I want to do my shallow stops deeper, I can. If there is an emergency and I need to ascend more quickly, I can. There is no run time to chase and no computer to say I can't do something.

Gas management is easily calculated in my head with my tanks multiplying the pressure on the SPG by "5" to get my cubic feet and then dividing by the number of ATA's to get the maximum amount of gas I have remaining at that depth.

I'm free of square profiles, run times, and computers punishing me for safety margins or extensions they haven't been programmed to consider.

Back to dive computers
Not likely. The most expensive computers cannot do what my brain can do, more easily, with a clearer understanding of why I'm opting to ascend a certain way and they can't do it for FREE!

 

[DiveNav]

My personal evolution

................................

Back to dive computers
Not likely. The most expensive computers cannot do what my brain can do, more easily, with a clearer understanding of why I'm opting to ascend a certain way and they can't do it for FREE!

Interesting post.

Question: How do you track bottom time and depth?
Don't you still rely on some sort of "computer" (digital device with a micro-controller inside) or you use "analog" gauges only?
For your info the MTBF of digital devices is much higher than their analog counterparts

Alberto

 

[Trace Malinowski]

Interesting post.

Question: How do you track bottom time and depth?
Don't you still rely on some sort of "computer" (digital device with a micro-controller inside) or you use "analog" gauges only?
For your info the MTBF of digital devices is much higher than their analog counterparts

Alberto

UWATEC or OMS digital depth gauge/bottom timer along with a Casio G-shock watch or other such device in stop watch mode.

Each safety or deco stop includes movement/ascent to the next level.

For example, if we needed to make 1 minute stops for a recreational dive, the leader would signal, "Up next level" after 30 seconds at a stop. The team would move 10 feet in 30 seconds bringing that stop to completion. Then, the leader would signal, "Level off. Deco. 1 minute." Everyone would hit their timer. After 30 seconds, the leader would signal, "Up next level," and the team would repeat the process. Normally, everyone displays the hand signals and sets the stop watch timers in unison, but ultimately the team leader will be primarily responsible for controlling the team's ascent.

If the digital device craps out, I have an analog. So, yes, there is computer technology involved in the devices, but they do not track or interfere with my dive profiles. Also, if they crap out, I can use an analog device on the next dive. The next dive I can make right after surfacing if I want without being locked out or done for the day. My depth may not be as accurate with an analog, but deco is an inexact science which is why we are able to move the profile around as we see fit.

 

[Blackwood]

Trace - I think he's asking you TRACK it, not MEASURE it.

The answer is: in his head. As I indicated above (without actually saying the words 'ratio deco'), it's the Average Depth that matters to this methodology, and the diver keeps a running, real time tally of depth throughout the dive.

If I'm at 150 for a few minutes, 160 for a few minutes, 140 for a few minutes, then my average is 150' for 3*"few minutes", which would oblige upon me 3*"few minutes" of decompression on EAN50 between 70' and the surface.

As Trace mentioned, the shape of the ascent profile is variable. He gave an example of a linear ascent in the 70-30 region, whereas me and my team generally s-curve it (his 4-4-4-4-4 could be 5-5-3-3-4) to extend the high PO2 stops.

Some devices (Uwatec Tec2g, VR3, maybe others) will tell you average depth during the dive, but most people I know who dive this way compute it manually (I back up my mental math with the 2g).



So, as I mentioned above, diligence is a required quantity.

 

[rstofer]

An interesting approach but too difficult for me. I'm too old to be doing that kind of thing in my head. My brain cells are dying at an amazing rate. Brain grenades in my youth, I suppose.

Another factor to consider: that approach doesn't have the validation of many multiple millions of dives. The tables have eased over the years to account for outliers; divers who still get DCS. The technical approach is primarily used by younger divers who are presumably in pretty good shape if they're humping doubles. Old people, like me, should stick with old methods.

Team leader? When I dive we're all in this separately. It is not in my nature to trust other people. I'll just continue to live or die based on my own decisions. It's worked out pretty well so far.

To be fair, I have no interest in technical diving while many others do It may turn out that my grandson progresses to that point when he gets older and I'm fine with that. For me, at my age, recreational diving with a computer or tables will work just fine.

Richard

 

[Blackwood]

An interesting approach but too difficult for me.

It's actually quite simple in application, though reading it for the first time can be a lot to digest. I have the advantage of being taught it by Andrew Georgitsis, who basically formalized the system.

that approach doesn't have the validation of many multiple millions of dives.

RD obligations are essentially envelopes of a few models (Buhlmann and VPM). By only using a select set of gases, one is able to recognize trends and apply them. Diving "best mix" or something won't necessarily work, and is outside of the scope of this simplification.

Team leader? When I dive we're all in this separately. It is not in my nature to trust other people. I'll just continue to live or die based on my own decisions. It's worked out pretty well so far.

As Trace mentioned, often the team will give the hand signals at the same time. It's everyone's responsibility to keep up. It's the deco captains responsibility to make sure everyone is on board.

If the deco captain starts making ridiculous calls, you can say "no", discuss it, or even take over if you want to. It's not like you just turn off your brain and start blindly following him (doing so can get you hurt, to which kevrumbo can attest).

To be fair, I have no interest in technical diving

The same methodology is easily applied to "recreational" diving via the 120/130 rules (that Trace mentioned), or by counting back from the 150' setpoint. In fact, as long as you stay within the Deco Limit, it's exceedingly simple: 30FPM to 1/2 max depth, and 10FPM thereon, stay out for 90 minutes and do it again. No pressure groups, RNTs, adjusted NDLs etc. to worry about.

 

[BabyDuck]

i totally respect someone able to do ratio deco. there's no way i could do that. it's tough enough to remember times & turn pressure. if i had to do math in my head the whole dive, i might as well not go because i sure won't enjoy or remember anything.

yay nitek duo & vr3! yay v-planner!

 

[GrandpaScuba]

Mild DCS...and SIX chamber rides? Something ain't right there.

Treatment table 6 calls for 3 20min pressurizations. If they did a table 6 and followed up with another table 6 sometime later, perhaps they interpreted that as "6 chamber rides".

 

[GrandpaScuba]

When I teach the tables in my Open Water Diver class, and the students have progressed to being able to handle any problem I throw at them, I give them one last excersize to do. I ask them to figure the table info for a 3-dive day that was a real 3-dive day I did in Roatan one year. They all get a puzzled look when they eventually realize that the dives I gave them can't be done according to the tables. I then launch into a discussion of dive computers.