New divers and computers

[Charlie99]

Generally speaking, the computer is putting you beyond the table because the table can only calculate square profiles, not multilevel dives, right? If we're talking about a computer that's as conservative as the tables, is there something else at play?

Short answer: MOST of the difference is indeed because the table can only calculate square profiles, with a bit of difference also caused by the table rounding up to the next 10' intervals, and rounding up to specific times/pressure groups on the table. There are some additional factors when doing repetitive dives.

The long version:

The major difference is indeed multilevel calculations rather than square profile. For several reasons, however, dives with a computer tend to be more agressive than those planned with a table, even if the computer and the table are based upon the same exact model.

One reason is that with tables there is extra (but highly variable) conservatism added in because you have calculated the dive as a square profile, but in reality not all bottom time is spent at max depth.

There are some additional, more subtle things in play when doing repetitive dives, caused by how that particular computer does repetitive calculations. Some computers, Oceanic for example, do what is called "surface control credit" where the faster than 60 minute halftime compartments are treated as 60 minute halftime during the SI. Other computers don't do this and therefore will be more liberal than tables for repetitive dives, even for square profile dives (as long as they are deeper than 40' or so -- for very shallow dives the 60 min compartment controls). This is because the repetitive groups on tables track one and only one compartment (60 min HT compartment for PADI RDP, 120 min for USN and USN-derived tables such as the older NAUI tables, YMCA, SSI, etc.). The computer tracks all compartments and knows which one is the limiting one for a planned repetitive dive.

Even computers with "surface credit control" will give more repetitive dive time for square profiles if you spend much time shallower than 20'.

Yet another subtle effect of diving with a computer and the resultant multilevel possibilities is doing what some refer to as "riding the NDL". This is where you go fairly deep until approaching NDL, then you go up 5 or 10 feet until once again NDL is approached. This process is repeated upwards until air consumption limits the dive.

What happens when "riding the NDL" is that, at first it is a fairly fast compartment that is loaded up near the limits (beacuase you are deep). As you rise, however, the compartment nearest to the limit becomes the medium HT ones, then eventually a slow one. You are still within the model limits, but right at them.
If it were a fast compartment at the limits, a proper ascent and a few minutes of safety stop time will take that compartment well away from the model/deco limit. OTOH, "riding the NDL" will put a much slower compartment right up to the limit, and it will take a very long time to get very much safety margin.

Like any good and powerful tool, a computer can both be very useful and very dangerous, depending upon how it is used. Unfortunately, right now the world seems to be mostly be divided into two camps. One blindly uses computers. The other blindly reject them.

Properly used, a computer can take care of the routine bookkeeping of tracking time and depth, while you do the higher level function of deciding upon the general overall depth-time profile of the dive and the ascent. As with any gauge, one does need to pay enough attention that failures don't go undetected.

 

[sewcopp]

Guys if you weren't taught to calculate miltilevel dives on a table I'm sorry it is possible and reliable. It's actually not realy any more difficult than calculating repetive dives. Just no SI. It might be something you want to look up for your own personal interest.

Yes computers using the same allgoritms as a table will give you more possible TBT. The tables can do multi-level but only to tens of feet at atleast 20 foot intervals. The table also can only calculate (round) times to the next depth and time interval on the table. Computer are constantly measruing and recording actualities so do not round up and therefore give more time.

Sue

 

[WarmWaterDiver]

Short answer: MOST of the difference is indeed because the table can only calculate square profiles, with a bit of difference also caused by the table rounding up to the next 10' intervals, and rounding up to specific times/pressure groups on the table. There are some additional factors when doing repetitive dives.

The long version:

The major difference is indeed multilevel calculations rather than square profile. For several reasons, however, dives with a computer tend to be more agressive than those planned with a table, even if the computer and the table are based upon the same exact model.

One reason is that with tables there is extra (but highly variable) conservatism added in because you have calculated the dive as a square profile, but in reality not all bottom time is spent at max depth.

There are some additional, more subtle things in play when doing repetitive dives, caused by how that particular computer does repetitive calculations. Some computers, Oceanic for example, do what is called "surface control credit" where the faster than 60 minute halftime compartments are treated as 60 minute halftime during the SI. Other computers don't do this and therefore will be more liberal than tables for repetitive dives, even for square profile dives (as long as they are deeper than 40' or so -- for very shallow dives the 60 min compartment controls). This is because the repetitive groups on tables track one and only one compartment (60 min HT compartment for PADI RDP, 120 min for USN and USN-derived tables such as the older NAUI tables, YMCA, SSI, etc.). The computer tracks all compartments and knows which one is the limiting one for a planned repetitive dive.

Even computers with "surface credit control" will give more repetitive dive time for square profiles if you spend much time shallower than 20'.

Yet another subtle effect of diving with a computer and the resultant multilevel possibilities is doing what some refer to as "riding the NDL". This is where you go fairly deep until approaching NDL, then you go up 5 or 10 feet until once again NDL is approached. This process is repeated upwards until air consumption limits the dive.

What happens when "riding the NDL" is that, at first it is a fairly fast compartment that is loaded up near the limits (beacuase you are deep). As you rise, however, the compartment nearest to the limit becomes the medium HT ones, then eventually a slow one. You are still within the model limits, but right at them.
If it were a fast compartment at the limits, a proper ascent and a few minutes of safety stop time will take that compartment well away from the model/deco limit. OTOH, "riding the NDL" will put a much slower compartment right up to the limit, and it will take a very long time to get very much safety margin.

Like any good and powerful tool, a computer can both be very useful and very dangerous, depending upon how it is used. Unfortunately, right now the world seems to be mostly be divided into two camps. One blindly uses computers. The other blindly reject them.

Properly used, a computer can take care of the routine bookkeeping of tracking time and depth, while you do the higher level function of deciding upon the general overall depth-time profile of the dive and the ascent. As with any gauge, one does need to pay enough attention that failures don't go undetected.

Charlie99,

Based on your post (bold font by me), can you explain the graphs in these articles, where computers other than Oceanic give more conservative repetitive profiles than Oceanic? Check the second article in particular - a 60 minute SI between Dive 1 and Dive 2, a 90 minute SI between Dive 2 and Dive 3, and another 60 minute SI between Dive 3 and Dive 4.

http://dive.scubadiving.com/images/200408GR_computer_charts.gif

http://dive.scubadiving.com/PDF/200505_divecomps_graphs_01.pdf

There's more stuff on the UK dive magazine web site of a similar nature.

Someone has the 'Flat Wheel' posted on this board if you search - for something other than dive computers or the PADI Wheel.

 

[Charlie99]

Based on your post (bold font by me), can you explain the graphs in these articles, where computers other than Oceanic give more conservative repetitive profiles than Oceanic? ...........

Someone has the 'Flat Wheel' posted on this board if you search - for something other than dive computers or the PADI Wheel.

I was comparing and contrasting dive computers vs. tables and giving the various reasons why dive computers can be more liberal than tables. It wasn't really meant as a comparison between computers.

I peeked at the Nitek Duo manual. It just said Buhlmann ZH-L16 model. The actual numbers on the graphs, however, show that they have added significant conservatism to the model. They may very well have also added "surface credit control". It's hard to tell without doing much more in the way of calculation than I'm willing to do.

When comparing tables vs. computers, the Oceanic series computers are an interesting case because they are based upon an unpadded, i.e. no extra conservatism added, DSAT model, the same one upon which the PADI RDP is based. Many other computers have signficant additional fudge factors added into their calculations.

You can find the flat wheel in my photo gallery. IF you want to see the calculation you can play around with the attached excel file. The majority of the table entries are calculated values. The NDL values are precanned, but can be calculated by replacing the 60 min HT and associated M-value with values for each the other compartments and finding where the m-value limit is exceeded.

http://www.underwaterbob.com/dive_computers.htm is another interesting review of dive computers. While it doesn't have the Nitek Duo or the other computers shown in the computer comparisons you linked to, it does show testing on both the Oceanic Versa Pro and the Nitek 3. More than likely, they behave similar to the respective computers of the same manufacturers in the articles and charts you linked to.

It shows that the Nitek 3 performance can best be emulated by assuming it is a ZHL16 model with gradient factors (conservatism factors) of 40/80. In other words, it has a large degree of conservatism added in beyond the simple "ZH-L16" description given in the manual. A simplified explanation of 40/80 gradient factor is that it starts off with M-values being reduced to only 40% of the base model, increasing that to 80% of the model values at the surface.

The Oceanic Versa Pro comes in gradient fact 96/97 on Decoplanner / ZHL16. That probably corresponds to really being 100/100 on the DSAT model --- the DSAT and ZHL16B M-values are pretty close once you compensate for the different halftimes chosen for the model.

 

[WarmWaterDiver]

I just found your post contrary to published info is all. Nothing secret nor especially new - this is why the Pelagic dive computers are often charachterized as 'liberal' versus most of the competing models.

Take the Atom in 2005 (especially comments on the second dive i.e. repetitive).

http://www.divernet.com/equipment/0605divertests.shtml#atom

The UK folks refer to a "Jeckyll and Hyde" personality for the Veo 250 in 2003.

http://www.divernet.com/equipment/computers2003/comps2.shtml

The Versa Pro was definitely rated as more liberal in 2002.

http://www.divernet.com/equipment/0602divertests.htm#versa

Even the old Data 100 was termed 'less cautious' than its competition in 1997.

http://www.divernet.com/gear/dtest497.htm#data

This fits with our personal experience with the Oceanic Data Plus units (our first dive computers) vs. Dive Rite NiTek3 and TUSA IQ-700 units as well. We've passed our Data Plus units on to fairly new divers who are old friends in St. Croix, since they don't do any planned decompression stop required diving.

And, you might note the dive computers recommended (and not recommended) by the folks who run the dive operation on Bikini Atoll (repetitive diving and planned decompression stop required diving).

http://www.bikiniatoll.com/fabioletter.html

 

[Charlie99]

I just found your post contrary to published info is all. Nothing secret nor especially new - this is why the Pelagic dive computers are often charachterized as 'liberal' versus most of the competing models.

I'm not sure what you are talking about since I was not comparing computers to computers, but instead comparing how computers calculate vs. how tables calculate.

I definitely did not intend to convey the impression that the Oceanic/Pelagic computers are conservative.

 

[WarmWaterDiver]

Just what I put in bold in the quote on "Other computers" back in your post #51, copied into my post #53.

 

[Charlie99]

Just what I put in bold in the quote on "Other computers" back in your post #51, copied into my post #53.

I see what's unclear. Back up a little bit more and read the sentence before the one you quoted. "Other computers" are ones that do not do surface credit control, not every non-Oceanic computer.

"Some computers, Oceanic for example, do what is called "surface control credit" where the faster than 60 minute halftime compartments are treated as 60 minute halftime during the SI. Other computers don't do this and therefore will be more liberal than tables for repetitive dives, even for square profile dives (as long as they are deeper than 40' or so -- for very shallow dives the 60 min compartment controls). "

Just reading the section you bolded also makes it appear that I claim Oceanic computers are more conservative than tables. Obviously, that isn't my intent either.

 

[WarmWaterDiver]

My point is if other computers didn't do this, why are they (generally - Citizen Aqualand / Cyberland notwithstanding) consistently more conservative than Oceanic, particularly by the third and fourth repetitive dive for the day?

Perhaps you can list the makes / models of dive computers that don't do this?

Hence my confusion. I haven't done the best job of clarifying.

 

[nauidiver2004]

Why not use both. Plan using tables then compare with your computer. Follow your plan with the tables and use the computer for ascent rates, bottom time, temp, depth, and if need be deco.

I don't agree. if your planning off the table then the computer offers no benefit, it would be a waste of money.

AMBERJACK, I would say buy the computer and learn how to use it. It has a planning function, know how it works. Yes, I still review my tables and will often compare my dives to the tables after a day of diving. It's entertaining for me and helps to keep my table skills sharp. But if your spending money on a computer, learn how to use it. Too many people fail to plan dives when using a computer, they just jump in the water and stay until they run out of breathing gas or the computer tells them they have to acend. I don't dive like that and I don't reccommend anyone does.