Are dive computers making bad divers?

[CAPTAIN SINBAD]

The problem I see is that this is defined to a narrow range which the air limits are around the NDL limits for most divers. 25 min at 100' is a fairly long dive recreational dive near most normal air limits. Add in the differences with normal changes in depth and you may get off-gassing benefits and not bend (or you still may bend). This, however, is taking advantage of certain normal diving behaviors in diving to be safe, not decompression limits.

Accordingly I could say that I will dive the following within the 'new suggested territory':

100' for 26 min and ascend to 60' for 9 min. Overall time 35 min ave 80'. Add in descent/ascent times ~5 gives 40 min total time, INCLUDING ascent (which is not the number presented in the table (BT)) which is the NDL for 80'.

Multi Deco comes up with: 1st deco is at 30' for 1 min, 2nd deco stop is at 20' for 19min for a total of 20 min deco and a runtime of 59 min.

MultiDeco 4.10 by Ross Hemingway,
ZHL code by Erik C. Baker.

Decompression model: ZHL16-C + GF

DIVE PLAN
Surface interval = 5 day 0 hr 0 min.
Elevation = 0ft
Conservatism = GF 30/85

Dec to 100ft (1) Air 60ft/min descent.
Level 100ft 24:20 (26) Air 0.85 ppO2, 100ft ead
Asc to 60ft (27) Air -24ft/min ascent.
Level 60ft 9:00 (36) Air 0.59 ppO2, 60ft ead
Asc to 40ft (37) Air -24ft/min ascent.
Stop at 40ft 0:30 (38) Air 0.46 ppO2, 40ft ead
Stop at 30ft 1:00 (39) Air 0.40 ppO2, 30ft ead
Stop at 20ft 19:00 (58) Air 0.34 ppO2, 20ft ead
Surface (59) Air -20ft/min ascent.


This most definitely is not a safe way of diving even with your new 'suggested territory'. Adherents to this philosophy may not be getting bent only through luck and definitely not by following recognized decompression theories.

BTW, the profile I presented is similar to one I have dove several times. When going to the props on the Spiegel Grove with a rec diver, I will follow the upper deck at ~80' and then descend to the props at ~135 for a tour then head back up. There is little to no time to tour the rest of the depths even with a computer. We get no where near the times an 'average depth' calculation would give. Following this philosophy will get you bent.

Firstly, you are absolutely correct that the profile you are describing above will put you into deco. However the agencies that propogate depth averaging instead of computers do not do depth averaging that way. I will try to present their perspective as best as I have understood.

The way UTD teaches computerless depth averaging is in 5 minute increments and it is a rolling average. It requires taking a depth reading every 5 minutes and averaging it with the last reading. The descent does not matter unless you take a very long time to get to depth. If we were to apply this method to your Spiegel Grove dive then you will drop down and land at the top of a wreck at 70' and you decide to swim at that level until you reach a spot where you want to descend.

70' - 5min = 70'average depth, 5 min run time. (You can subtract runtime from NDL i.e 50 – 5 = 45. This is the number you will be seeing if you had a computer in your hand).

100' - 5 min = 85' average, 10min RT (30 – 10 = 20 is fairly close to the number you will see if you had a computer in your hand)

90' - 5 min = 87' average, 15min RT

120' - 5 min = 105 ave, 20min RT

80' - 5 min = 92' ave, 25 min RT basically this is 90' for 25min

This dive profile will be on Nitrox 32 because that is the mix standardized for all dives below 60 feet. Keep in mind that the method described above is the only way UTD trains its divers to dive. They are strictly forbidden to carry computers and tables because these have "No Decompression Limits" and the founder of UTD, Andrew Georitsis believes that all dives are decompression dives so there is no such thing as a No Decompression Limit! All dives must end with a mini staged decompression called Min-Deco. Min-Deco is a modified deco-stop that is supposed to clear any deco obligation acquired due to the inaccuracies of depth averaging or other factors. Instead of taking a 5 minute “safety stop” UTD folks will start taking small (1 min) deco stops that begin from half the depth of their dive and progress upwards to the surface in 10 feet increments. So lets say that after doing a dive to a 100 feet we will normally do a 5 minute safety stop, these guys will do the following stops:

50 ft – 1 min

40 ft – 1 min

30 ft – 1 min

20 ft – 1 min

10 ft – 1 min

I have tried to read on this more from DAN and I am personally not convinced that taking shorter deep stops all the way up will reduce the any decompression time you may have accumulated due to inaccuracies of depth averaging or anything else. If someone has any opinions or research on staggered deep stops then please share… This is what be interesting to read:

Alert Diver | Deep Stops


Similarly the Min-Deco Limits are also not the same as NDLs. Below is the Min-Deco table for Nitrox 32 as taken from the UTD manual:

60 ft – 60 Mins (Min Deco Limit)
70 ft – 50 Mins (MDL)
80 – 40 Mins (MDL)
90 – 35 Mins (MDL)
100 – 30 mins (MDL)
120 – 20 mins (MDL)
130 – 15 mins (MDL)

As you can see for most of the shallower depths min-deco limits mimic air table. This may be the reason why a whole more of them are not getting bent diving without computers.

In the end … I am not Andrew Georgitsis so take my words for what they are worth.

 

[LowVizWiz]

I have two computers. I know how to dive air and nitrox with them. I understand all the little warnings and symbols. I know how they handle mild deco. I know their quirks. But I have never planned a dive on them...

So you CAN execute a dive with both computers you own, correct? How did you learn to do that ?

Those are both rhetorical questions since it does not matter how you learned. The point is that you learned.
Would you blame the computer if somehow you weren't able to conduct the dive with the info the dive computer provided you yet you decided to do the dive and got hurt?

 

[boulderjohn]

Firstly, you are absolutely correct that the profile you are describing above will put you into deco. However the agencies that propogate depth averaging instead of computers do not do depth averaging that way. I will try to present their perspective as best as I have understood.

The way UTD teaches computerless depth averaging is in 5 minute increments and it is a rolling average. It requires taking a depth reading every 5 minutes and averaging it with the last reading. The descent does not matter unless you take a very long time to get to depth.

As you know, I had the same training from the same people. One of several reasons I left that group was because even if depth averaging were legitimate, there is such a high probability of divers making a computational error.

Keep in mind that the method described above is the only way UTD trains its divers to dive. They are strictly forbidden to carry computers and tables because these have "No Decompression Limits" and the founder of UTD, Andrew Georitsis believes that all dives are decompression dives so there is no such thing as a No Decompression Limit!

As I mentioned in a previous post, one of our UTD divers was carrying a computer in gauge mode when he and his buddy got DCS, so we were able to look at their dive profile and see how much of an error they had made with their depth averaging.

By the way, everyone knows that every dive is a decompression dive, so this is no great revelation on Andrew's part. When a dive is "no decompression," that means there is no MANDATORY decompression stop, so the diver should be able to go directly to the surface without a stop if needed. PADI has changed its terminology for such dives to "No Stop" to try to deal with that confusion.

I have tried to read on this more from DAN and I am personally not convinced that taking shorter deep stops all the way up will reduce the any decompression time you may have accumulated due to inaccuracies of depth averaging or anything else. If someone has any opinions or research on staggered deep stops then please share… This is what be interesting to read:

Alert Diver | Deep Stops

The article is more than 5 years old, and there has been some important research on deep stops since then. Two of the expert participants in this article, Mitchell and Doolette, graced ScubaBoard with their presence during a very heated debate on deep stops only a few months ago. The trend in thinking is definitely moving away from them, based on recent research. The thinking now is that time spent on deep stops is adding to on-gassing in the slower tissues that does more harm than the deep stops do good.

 

[packrat12]

The article is more than 5 years old, and there has been some important research on deep stops since then. Two of the expert participants in this article, Mitchell and Doolette, graced ScubaBoard with their presence during a very heated debate on deep stops only a few months ago. The trend in thinking is definitely moving away from them, based on recent research. The thinking now is that time spent on deep stops is adding to on-gassing in the slower tissues that does more harm than the deep stops do good.

BoulderJohn, This is the article: REDISTRIBUTION OF DECOMPRESSION STOP TIME FROM SHALLOW TO DEEP STOPS INCREASES INCIDENCE OF DECOMPRESSION SICKNESS IN AIR DECOMPRESSION DIVES

Linked below to a PDF (Sorry about the caps but that is the title)
http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA561618

The original proposition that Captain Sinbad had proposed was that simple averaging would give back 'lost minutes' as stated but did not indicate Ratio Decompression:

True but hold on ...

There is a simple method called Depth Averaging that magically gives you those "lost minutes" that your computer will eventually arrive to after "real time depth calculations. A mathematical average is defined as the sum of all the numbers on a list divided by the total number on the list. In case of the wreck which starts at 80 and ends at 100, we will add 80 + 100 and divide that by 2 to get to 90 feet. From memory, I will know that I can now stay for 30 minutes instead of 25..

And NetDoc commented on UTD's method:

This has to be one of the most horrid inventions ever created to deal with NDLs. It relies on the most problematic instrument that every diver owns: their brain. This eventually convolutes into ratio deco which is indeed the most horrid of inventions. Don't fall for the hubris that maintains that this is 'simple' and 'easy'. That only applies to getting bent.

This thread is based on a Basic Scuba discussion. I would consider UTD's Ratio Decompression an advanced/technical discussion. While I am rudimentarily familiar with Ratio Decompression, I do not have anywhere near the background to discuss it other than to know that I do not use it and never will.

I commented on the specifics presented as they WOULD get a diver bent that did not understand that the presented information was for Ratio Decompression and not simple averaging of the tables.

 

[boulderjohn]

This thread is based on a Basic Scuba discussion. I would consider UTD's Ratio Decompression an advanced/technical discussion. While I am rudimentarily familiar with Ratio Decompression, I do not have anywhere near the background to discuss it other than to know that I do not use it and never will.

I agree that Ratio Deco is an advanced topic, but although the term has been tossed around a bit in this thread, what captain Sinbad sdescribed is what UTD (and GUE) call Minimum Deco, or mindeco for short. Ratio Deco is a way of determining an ascent strategy for decompression diving, but Minimum Deco is a way of determining an ascent strategy for shallower recreational dives. The only links between Ratio Deco and Minimum Deco is the agency that teaches them, the fact that they are often taught in the same class, and the fact that they do not use computers. Unfortunately, the term Ratio Deco is often used inappropriately to describe both.

 

[packrat12]

... what captain Sinbad sdescribed is what UTD (and GUE) call Minimum Deco, or mindeco for short. Ratio Deco is a way of determining an ascent strategy for decompression diving, but Minimum Deco is a way of determining an ascent strategy for shallower recreational dives.

If that was so, then why did Captain Sinbad present this as a simple air NDL table on post #250 which is where the 'average depth' discussion originated as I quoted above?:

While I dive with a computer, I have the following NDLs memorized in my mind. I remember this table listed below just like I remember my mothers name.

DEPTH(ft):TIME (mins)

60:60
70:50
80:40
90:30
100:25
110:20
120:15
130:10

As I stated, I do not have more than a fuzzy understanding of Ratio Deco and will also state I have no understanding of Min deco. I do not care to learn either as I do not believe they have any theoretical backing to them and I will much more readily trust computers which are reliable. I also do not believe that the 'lost minutes' will be recovered as Captain Sinbad described in post #250 which would cause issues to OW divers who do not understand that the tables do not work that way. All of my comments have revolved purely around post #250 as I believe the information relayed is incorrect and potentially hazardous, especially in the Basic Scuba area.

In the past, computers may have been unreliable enough to justify trying this in one's head. With the modern, very reliable inexpensive computer, there is no reason to do this. You need redundancy, bring 2.

 

[Steve_C]

Would you blame the computer if somehow you weren't able to conduct the dive with the info the dive computer provided you yet you decided to do the dive and got hurt?

There are no ordinary circumstances where I would ignore my computer and do a regular dive where the computer information is needed.

If I were dumb enough to ignore my computer when it is acting reasonable and necessary, I would not blame it.

I would only ignore it if I knew it was not really needed. I know that in my local quarry on one tank I am not going to exceed NDL so computer is really optional there. So if I show up at the quarry and both computer batteries are dead, or my buddy just dropped a tank on both computers, I would dive.

 

[oldschoolto]

OK.... This VEO 3.0 Is very nice... But, I can't see how this is easier... Way too many button push combinations to remember... Dive planning is WAY EASIER with tables.... I'll give it a fair chance to prove it's self....

Jim...

 

[Basking Ridge Diver]

OK.... This VEO 3.0 Is very nice... But, I can't see how this is easier... Way too many button push combinations to remember... Dive planning is WAY EASIER with tables.... I'll give it a fair chance to prove it's self....
Jim...

Agreed - although I have a VEO 2.0 - the only time I use it is in between dives - up north we wait about 1.5 hr on SIT or longer and I have a fair idea of how that will impact my next dive - down in FL or SC they seem to use 1.0 hr on SIT. So I just push the numbers on Dive 2 and beyond to get a feel of the next dive depth and time - square profile.... It is more for my peace of mind - than actual planning the next dive.

 

[PfcAJ]

If that was so, then why did Captain Sinbad present this as a simple air NDL table on post #250 which is where the 'average depth' discussion originated as I quoted above?:



As I stated, I do not have more than a fuzzy understanding of Ratio Deco and will also state I have no understanding of Min deco. I do not care to learn either as I do not believe they have any theoretical backing to them and I will much more readily trust computers which are reliable. I also do not believe that the 'lost minutes' will be recovered as Captain Sinbad described in post #250 which would cause issues to OW divers who do not understand that the tables do not work that way. All of my comments have revolved purely around post #250 as I believe the information relayed is incorrect and potentially hazardous, especially in the Basic Scuba area.

In the past, computers may have been unreliable enough to justify trying this in one's head. With the modern, very reliable inexpensive computer, there is no reason to do this. You need redundancy, bring 2.

Eh, I know you say you're not interested in learning about RD, and that's fine, but your reason for not wanting to learn about RD is faulty. There is 'theoretical backing', at least the way GUE teaches it.

Ratio deco is simply the correlation between deco time and bottom time as generated by a decompression algorithm. That's it. The same algorithm your computer uses. For instance, if 30mins at depth results in 30mins of prescribed decompression, and 20mins at depth results in 20mins of deco, the 'ratio' is 1:1. As you move deeper, that relationship shifts. Eventually 30mins at depth = 60mins of deco, a 2:1 ratio.

Its not nearly as mysterious as some folks would suggest. It CAN bite you, however, and I don't feel that it should be used as a primary planning tool. That's a discussion for another thread.

"Minimum deco" is just the max time at depth before a decompression algorithm gives you more than 1min stop times.

Both RD and MinDeco are based on outputs from decompression algorithms. Nothing fancy.