What to do when you no longer trust your computer ....

[Rick Inman]

Rick:

I am curious what you base the comment "that most recreations divers use their computer to set profile and limits". So far I have not seen that behaviour with anyone I have been diving with.

Darnold,

Of course, I'm not talking about anyone on this board.

And you're right. My conclusions are based on experiential data only. I've seen/dived with only a few hundred divers, and most of them have been computer-trusting folks, giving their profiles and limitations over to their computers. I rarely hear pre-dive discussions of "rock bottom", or turn pressure", or stop depths (except the always-computer-generated "safety stop") in their planning.

Even on charters, I've seen DMs encourage computer profiling, with phrases such as, "Anyone who exceeds their computer's NDL will sit out the next dive." One DM even us gave us a "low NDL" signal, and said he would send the divers with the lowest NDLs up first. Another DM said to ignore our NDLs, because we would be diving his computer (he said his was the most conservative and he dived it for hundreds of dives everyday, and he trusted it).

At my LDS, I've heard the instructors hocking their computers with the promise of thinking-free diving.

Locally, when the new divers start joining our weekly dive group, they have no clue how to plan a dive, and totally give themselves over to either their computer or your dive plan (just like I used to). Most of the more experienced divers are the same way.

I have been blessed with a few divers (mostly 300 miles away in the Seattle area) who have helped me to move toward lack of dependence on computers for dive planning. But in my limited diving experience, they have been, by far, the minority.

Darnold, if most of the people you dive with are using their brains for dive planning and implementation, you are truly an enviable diver.

 

[Argos]

Well I've heard all this talk of computers and I must admit I bought one couple months a go ........... Its too easy. Takes all the fun outta diving.......... My God when i first started we used to tie a piece of red ribbon around the mask and when it turned black then we knew roughly how deep we were. We also had a little poem / ryhme that we would say over and over again and if we couldn't remember the words then we knew ........aha time to get to the 30mtr mark and start the deco LOL ! All this high tech tut tut !!

 

[DandyDon]

Well I've heard all this talk of computers and I must admit I bought one couple months a go ........... Its too easy. Takes all the fun outta diving.......... My God when i first started we used to tie a piece of red ribbon around the mask and when it turned black then we knew roughly how deep we were. We also had a little poem / ryhme that we would say over and over again and if we couldn't remember the words then we knew ........aha time to get to the 30mtr mark and start the deco LOL ! All this high tech tut tut !!

Love it. Got to try those.

 

[Darnold9999]

So what people are telling me (call the dive rather than dive the buddy's computer) is that they rely on their computers to plan their dives.

So if you were diving without a computer and your depth guage or watch failed you would call the dive rather than rely on your buddy's. Not trying to be argumentative here, just trying to work it through.

I think if I were close to the edge on no deco time I would agree - call the dive or at least change the planned profile, but on a purely recreational dive nowhere near deco limits e.g. we are diving to our plan and the plan is conservative, I see this as being overly reliant on the technology to start with. Many people dive without a computer, safely, and if I were doing that and my depth guage failed I would keep close(er) to my buddy and continue the dive. Perhaps adjust it to be more conservative than the initial plan.

Maybe this comes from learning with only a depth guage and pressure guage - not even a watch. You dive your air, not a clock. Leave this depth with so much air - count off 180 seconds at this depth - make sure you arrive at the surface with this much air. However doing this kind of diving you always err on the conservative side and multiple dives are not encouraged. I still find that I really only use the computer as a depth guage while in the water. Once I am out I look at the time but I still dive air and depth and keep a mental clock re time.

Not bashing the computer, I own one and having these new tools is a good thing, I really like the ability to review the dive, but the device between your ears is still the best computer built and I would rather rely on it first and use the computer as a backup, with my buddy's computer as a further backup. However almost always I am using the computer as a depth guage only during the dive and bottom timer when I am out of the water.

Rick - I do seem to be fortunate re buddies lately. Perhaps it comes from diving in a harsher environment or maybe just luck of the draw. (I see having written this that you are just down the road so diving in the same environment so must be luck of the draw)

I think on reflection I will stick to my plan. If the computer fails, shift to buddy's computer(remember I am only using it as a depth guage) unless: near no deco limits or the vis is soo poor that it is impossible to safely complete the dive on an emergency basis with no depth guage i.e. can't see the surface from 15' or so. In both cases or some other condition tells me that this plan would be unsafe I will call the dive. I would almost certainly change the dive plan to be a more conservative profile at the point of failure, but I don't think I will call it. With luck I won't ever need to find out!

 

[H2Andy]

So what people are telling me (call the dive rather than dive the buddy's computer) is that they rely on their computers to plan their dives.

yes... i rely on my computer to plan and carry my dive

just like i rely on my scuba tanks to plan and carry my dive

and i also rely on my regulators

i also rely on my fins to do it faster and not have to crawl on the bottom

i rely on my mask so i can see the computer that i'm relying on in the first place

i rely on my drysuit so i don't freeze my butt off

so yea.... you could say i rely on a lot of equipment to carry
out a dive



and if any of those pieces of equipment went south on me,
i'd seriously consider ending the dive.... depending, of course,
on the circumstances and details, etc.

 

[Darnold9999]

H2Andy

hmmmm... think I will rely on my brain to plan and use the computer, tanks, fins, mask etc. as tools to to carry out the plan.

Totally agree - if something fails you rethink and decide if you are safe to continue. If you are relying on the computer to tell you how long to stay at depth, when to turn around, surface etc. then call the dive, you are no longer safe and your buddy's computer doesn't have the information you need to continue. I don't rely on it to tell me how long to stay at depth or if I am in deco or not so believe I am diving safely and might not call the dive. Depends.

 

[lars2005]

I carry two computers that operate on the same algorithms, plus an analog SPG. Since I'm a conserative type, being not being 20 something any more, I use a Mares M-1 RBGM and a Nemo.

 

[bradshsi]

So if you were diving without a computer and your depth guage or watch failed you would call the dive rather than rely on your buddy's. Not trying to be argumentative here, just trying to work it through.

I think on reflection I will stick to my plan. If the computer fails, shift to buddy's computer(remember I am only using it as a depth guage)

My apologies if I misunderstood your previous post. I'd agree that if you are using the computer only as a depth gauge, then you don't necessarily need to call the dive if it fails.

The distinction for me is that a depth gauge is a primary instrument (as is a watch and a spg). A computer (when used to determine NDL's) is a rather opaque calculator, based on inputs from primary instruments.

For the record I use tables (and my computer as a depth gauge/bottom timer). This is mainly for conservatism. I've seen too many "excursions from the normal statistical performance" in my professional engineering career. I therefore resolved to make my diving fun and not an exercise in testing the accuracy of the computer makers algorithm.

 

[Darnold9999]

No apologies required, this whole thread just makes me, and hopefully others, think about "what if". I of course knew that I only use the computer as a depth guage in the water, but that is not at all clear from my initial post.

I would rather think all this through in a nice comfy chair then at depth. Then if something happens you can just apply the thinking as opposed to making a bad decision in a hurry. I would also rather those of you out there with more experience tell me I was all wet for some reason then do it wrong and create more business for my local estate lawyer. I will however want to hear the reason - I like that about this board. People are willing to give their reasons and take the flak from others who disagree.

One of the things that I do is write software. Do I trust that the algorithm in my computer was implemented in the particular hardware that I have completely bug free. Not a chance - fall on the floor laughing - no way. Any program of any degree of complexity cannot be proven to be bug free - just is not possible and no matter how much time you take finding the bugs, there is at least one more you haven't found yet. So be prepared for your computer to fail, or worse give you a completely wrong result at any time. ergo in the water it is a depth guage out of the water use it to check my plan.

 

[WarmWaterDiver]

www.scubaboard.com/showthread.php?t=124299

In this thread at least, the evidence suggests that the diver in question did exactly that. I'm not aware of any study that gives the absolute % of divers exhibiting this behaviour, so we're left with anecdotes which may or may not be representative of average diver behaviour.





With all due respect, you didn't answer Rick's question which was what do you do if your most conservative computer fails ? To me the answer would be to call the dive since your plan would require following the most conservative computer. In which case, having 2 computers isn't really helpful since it doesn't improve your overall system reliability.

Another point is that just because the 2 computers have the same algorithm, does not mean they will agree particularly well. A lot depends on how the computer extrapolates (and how often).





This goes against PADI's recomendations regarding computer use. Quoting from my copy of their OW manual:

"You and your buddy should not attempt to share a single computer....If your computer malfunctions while diving, ascend immediately according to the instructions of the computer manufacturer....Always back up your computer by calculating with tables".

I would submit that this is quite sensible advice. Say for example your computer failed with 10 minutes of NDL remaining. But your buddies computer showed 25 minutes. It would seem very foolish to share a computer in that situation.

With respect, I follow my backup computer - if I wasn't comfortable with the backup computer at all, I wouldn't be diving with it - period. My backup computer used to be my primary computer. I struggle with the concept of using a secondary computer you wouldn't be comfortable to alternately use as your primary computer - why dive with it at all if that's the case?

The TUSA IQ-700 and the Dive Rite Nitek3 both use a Buhlmann algorithm. The primary difference is the TUSA algorithm uses 12 theoretical tissue compartments while the DR NT3 uses 9 theoretical tissue compartments. The half-times of the fastest and slowest theoretical compartment are the same for both models, the TUSA has 3 more between slowest & fastest than the DR NT3.

Having used both side by side on recreational dive profiles, the TUSA is slightly more conservative - not my much. Say I've done a two tank day dive and a night dive within the last 24 hours. The TUSA might show 22 hours and 30 minutes to completely desaturate per the algorithm, the DR NT 3 might say 22 hours and 15 minutes for the same parameter. In terms of NDL time remaining, I've never seen them differ by more than 2 minutes on any profile I've executed while wearing both.

These graphs can be used to see the similarity - the Sapience (IQ-800) is advertised as using the same algorithm as the IQ-700. Note how the Sapience lies either on the same line or a very, very close line to the Dive Rite models (Duo and Plus) plotted.

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

Thus the value in similar algorithm dive computers if using redundant dive computers. If two identical model computers were used, neither one is less conservative or more conservative than the other - unless you're using some fairly unique and imaginitive mounting so that they're at significantly different depths throughout the dive, but that would argue to use similar mounting I would think (for my case anyway - I certainly believe to each his or her own).

The fact that my dive buddy / spouse also uses the same two models of dive computer as I do while diving with me increases the team level redundancy.

Otherwise, we're again back to the issue of if I wear 2 different timepieces which run at 2 different speeds, which (if either) is telling the right time for me?

I've checked again on battery consumption - the TUSA IQ-700 uses about one sixth of the power drain for a lot more functionality than the Oceanic Data Plus looking at the owners' manuals corroborating what I've experienced thus far. That's 83% reduction - if I were to find aa newer house that used 17% of the energy of the one I already had, that's the same size, with lots more features, it would be pretty significant.