Incident due to battery change on dive computer

[stuartv]

So, here you have an agency that is spending substantial effort (and $$$) in promoting the use of dive computers (and offer a simulator that allows a student to see model-specific dive computers in action during simulated dives) and then you have a dive store / instructor that do not use the tool and do not teach tables either.
So, who is watering down education?

If I had to guess, I imagine that most students in OW class don't have their own computer yet. So, I imagine they, and their instructor, say something like "there's no point in using model-specific training when you will probably end up with a different model of computer when you get your own. And you'll probably have a different model as a rental until then. So, save your card for the online simulator and use it once you get your own computer."

That was the basic explanation, when I took OW class, for why we didn't spend very much time on computers. All we did was look at the type that the shop uses for training and learn how to read NDL, EDT, and depth.

---------- Post added April 13th, 2015 at 08:44 AM ----------

This is what happens when tables are no longer taught. One must be able to trust the computer and you cant do that if you don't know if the computer is lying or not.

So, in the OP's situation, you would tell him "you're going to 93'. Look at the table. That says you have 25 minutes of NDL. If you get down there on the bottom and your computer says you can stay longer than 25 minutes, you should not trust it and you should come up anyway."

Is that correct? If it is, then why even bother having a computer at all?

And if it's not, then what was the point of knowing/looking at the dive table?

You go down to 93', then you come up to 70' and swim around and your computer EDT + NDL is now running more like 45 minutes, instead of 25. You can either trust the table and not the computer and come up, or you can trust the computer and ignore the "correct" use of the table and stay down. Which are you saying a person should do?

In that situation, how could you ever know if the computer is lying or not?

 

[Wookie]

lovis

you are right about the extreems when using the 120 rule, however he was with in the window to use it as a sanity check. From his post I got that he appeared to use a time probably around 30 min for an ndl because of his conservative setting in the past. It seams that when you hit 40 min one should say hey Ive been down here for a long time.
Then to find your computer is now calling for that much deco after 46 min. I also suspect that 70 ft may have been the center of his dive depth window and not time corrected. Even though if not time corrected, being down 10 or nmore minutes olonger than normal should be a flag. If the 70 ft is time corrected then the 120 rule should have been looked at to validate what the computer said. Especially since he knew he had done a battery change and perameter change.

To me, a sanity check gets you out of the water safely when you have no other method of doing so. I've had batteries on my Shearwater fail in the middle of recreational dive before, and my Stinger, configured as a bottom timer, allowed me to know and understand that I wasn't in deco by the rule of 120, that I could safely surface with an extended safety stop (5 min vs 3), but I sure as heck wouldn't use a rule of thumb to make a second dive.

 

[Storker]

So, in the OP's situation, you would tell him "you're going to 93'. Look at the table. That says you have 25 minutes of NDL. If you get down there on the bottom and your computer says you can stay longer than 25 minutes, you should not trust it and you should come up anyway."

Is that correct?

No, of course not. You tell the student "The table can't calculate the same small changes a PDC can, so by using it correctly it will be more conservative and give you a better margin. Both are right, within their limits of knowledge"

If it is, then why even bother having a computer at all?

This isn't a problem if the student learns that neither tables nor PDCs are 100% correct, and that deco theory is "measuring with a micrometer, marking with chalk and cutting with an axe", i.e. a large grey area where you have to make choices as to where to draw the dividing line.

And if it's not, then what was the point of knowing/looking at the dive table?

1. Knowing how to use tables gives you a backup in case your PDC dies and you don't carry a backup PDC
2. Understanding tables gives you a sanity check, a feeling of NDL times at various depths and a planning tool which is quicker and easier than pushing a lot of buttons on your PDC.

You go down to 93', then you come up to 70' and swim around and your computer EDT + NDL is now running more like 45 minutes, instead of 25. You can either trust the table and not the computer and come up, or you can trust the computer and ignore the "correct" use of the table and stay down. Which are you saying a person should do?

In that situation, how could you ever know if the computer is lying or not?

The computer isn't lying. The tables aren't lying. But on multilevel dives, a PDC can give you dramatically increased bottom times since it recalculates your (assumed) nitrogen loading on the fly. The table can't do that, so you err on the side of caution, i.e. using your max depth. That's better than erring on the other side, because the latter may get you bent. On square profile dives, tables and a computer will give very similar NDL times at each depth threshold that the table includes.

If there is a problem understanding this, the student should choose using either a computer or tables, and stick to the instructions that come with each of the tools.

 

[stuartv]

To me, a sanity check gets you out of the water safely when you have no other method of doing so. I've had batteries on my Shearwater fail in the middle of recreational dive before, and my Stinger, configured as a bottom timer, allowed me to know and understand that I wasn't in deco by the rule of 120, that I could safely surface with an extended safety stop (5 min vs 3), but I sure as heck wouldn't use a rule of thumb to make a second dive.

So, no use of tables. Just two "computers" and one very basic arithmetic rule.

And if you were actually using a second DC (versus depth/timer gauge), you wouldn't even have to do the arithmetic. And you could probably still do a normal second dive.

---------- Post added April 13th, 2015 at 09:07 AM ----------

1. Knowing how to use tables gives you a backup in case your PDC dies and you don't carry a backup PDC

We're talking about Recreational diving. If your PDC dies and you don't carry a backup PDC, then the rule is to abort the dive immediately, make a normal ascent with a safety stop and get out. How does a table help with that? In particular, how does a table help if you consulted your table ahead of time and it told you 25 minutes of NDL, but your computer has just died at 70', after 30 minutes EDT, and was showing 15 minutes NDL time left immediately before it died? You looked at the table before the dive and know that it said 25 minutes. How does that information help you at all? Your process - abort the dive - is the same, regardless.

Nothing in your post really refutes what I said. You either trust your computer or you don't. If you trust it, then most dives are going to go way outside of what the tables would allow. And if you don't trust it, then you may as well just put it in gauge mode and leave it there.

 

[rhwestfall]

Stuart:
Here is an example of how I prepare and dive (recreational). Before dive #1, either using the planner on the dive computer or a form of tables, I will put on a wrist slate the NDLs for the three levels relating to deepest point (or hard bottom), planned max, and one above that (at the same time, the 120 rule is a sanity check that something isn't wonky). Along with my PDC, I have something else to track time and depth. That gives a sanity check, or a way out if something goes south. After surfacing from the dive, and recording dive details, I will jot down the 5 or 6 NDL times for dive #2, then go chill. Before suiting up & splashing for dive #2, I'll quickly update the slate with revised NDL's based on the planner on the PDC. Again with auxiliary devices, I have a back-up. Cycle to repeat itself until the day is done....

YMMV.

 

[Wookie]

So, no use of tables. Just two "computers" and one very basic arithmetic rule.

And if you were actually using a second DC (versus depth/timer gauge), you wouldn't even have to do the arithmetic. And you could probably still do a normal second dive.

The rule of 120 is based on Navy no deco tables (table 9.6). In depths in the 100 foot range or shallower, depth plus bottom time is about equal to 120 or thereabouts for no deco limits. But I wouldn't know that if I hadn't figured out for myself. I actually thought I discovered it, turns out lots of way smarter folks than me figured it out when I was a twinkle in Mom's eye. I figured it out by looking at tables as an OW student, something you say you didn't do in your class.

And yes, I wear one computer and one bottom timer for recreational no deco dives because I am familiar enough with the tables to use the bottom timer. When planning a 300 foot multilevel dive, I cut tables on v-planner for the plan, 10 feet deeper, and 10 minutes longer (3 sets of tables), dive with 2 Shearwaters, and still use the Stinger as a bottom timer.

it just doesn't do to have the Captain get bent. Some would consider it rude.

 

[victorzamora]

We're talking about Recreational diving. If your PDC dies and you don't carry a backup PDC, then the rule is to abort the dive immediately, make a normal ascent with a safety stop and get out. How does a table help with that? In particular, how does a table help if you consulted your table ahead of time and it told you 25 minutes of NDL, but your computer has just died at 70', after 30 minutes EDT, and was showing 15 minutes NDL time left immediately before it died? You looked at the table before the dive and know that it said 25 minutes. How does that information help you at all? Your process - abort the dive - is the same, regardless.

According to the "rules" of diving with a computer and no tables, if your computer dies you abort the dive and can't dive again for a while (24 hours??)....which is not what I'm willing to live with. Let's say I fly to Roatan ($700), book a room ($400), pay ahead for twenty dives ($600), and set aside some cash for food ($300) for a decent set of meals whenever I want with reserves.....and I go with my wife. That's $4000, and it's 24 hours before I can get back in the water again??? Nope! I'm on the tables and back in.

 

[stuartv]

According to the "rules" of diving with a computer and no tables, if your computer dies you abort the dive and can't dive again for a while (24 hours??)....which is not what I'm willing to live with. Let's say I fly to Roatan ($700), book a room ($400), pay ahead for twenty dives ($600), and set aside some cash for food ($300) for a decent set of meals whenever I want with reserves.....and I go with my wife. That's $4000, and it's 24 hours before I can get back in the water again??? Nope! I'm on the tables and back in.

If your computer died, how do you know what your max depth and BT were?

If you're going to spend that much money, why not spend $200 to dive with a backup DC?

Once you're there, if your DC really dies (as in, permanently), why not spend a few bucks a day to rent a DC for the rest of the trip? You'll get more BT that way than using tables, right?

---------- Post added April 13th, 2015 at 11:09 AM ----------

Oh, and if you follow the "rules" of diving and the tables say your first dive was into deco and you can't dive again, wouldn't you be a lot happier if you had just taken a backup DC along on the first dive and it said you COULD do your second dive (because it wasn't assuming your first dive was square, like the tables do)?

---------- Post added April 13th, 2015 at 11:22 AM ----------

The rule of 120 is based on Navy no deco tables (table 9.6). In depths in the 100 foot range or shallower, depth plus bottom time is about equal to 120 or thereabouts for no deco limits. But I wouldn't know that if I hadn't figured out for myself. I actually thought I discovered it, turns out lots of way smarter folks than me figured it out when I was a twinkle in Mom's eye. I figured it out by looking at tables as an OW student, something you say you didn't do in your class.

And yes, I wear one computer and one bottom timer for recreational no deco dives because I am familiar enough with the tables to use the bottom timer. When planning a 300 foot multilevel dive, I cut tables on v-planner for the plan, 10 feet deeper, and 10 minutes longer (3 sets of tables), dive with 2 Shearwaters, and still use the Stinger as a bottom timer.

it just doesn't do to have the Captain get bent. Some would consider it rude.

Teaching the 120 rule would be simple and easy to do in OW class and does not require tables, in any way.

You can get a brand new DC for $200, easily (that even supports Nitrox), and often cheaper. How much does a backup depth gauge and bottom timer cost?

If a new OW diver has bought their first DC and they decide they want some way to keep diving, in the event their DC dies, would you tell them to buy a depth gauge, bottom timer, and learn to use tables tables? Or would you tell them to spend $200 or less on a backup DC?

And between the two options, which do you think a new OW diver would get the most safety and most pleasurable diving experience from? Regarding safety, I am stipulating that your response should factor in the likelihood of a new OW diver doing thing correctly, too. No response of "if the diver follows the tables correctly, that will be the safest." No kidding. Following the tables correctly is not the point. ATTEMPTING to follow them correctly is the issue where overall diver safety comes into question. New diver. DC dies mid-dive. Will the diver follow the tables correctly as often as they would follow a backup DC?

I AM a pretty new diver and I know how to use the tables. Even with my knowledge of the tables, I can say with complete confidence that, if my primary DC died during a dive, it would be a much more pleasurable dive experience for me to have a secondary DC than backup gauges and a set of tables. I would be able to continue my (typically) multi-level dive. I would be able to do my second dive without the much-reduced bottom time I would get from using tables to calculate my pressure group from the first dive and then calculate my NDL for the second dive. And it would be WAY better than if I did my first dive without carrying backup gauges and pre-planning my NDL based on tables, as that path would require me to abort the first dive as soon as my DC died. And, as the example of the OP shows, even if I did pre-plan my NDL, when the primary DC dies, I might still have to abort the dive immediately, since, in this example, the tables allowed 25 minutes and the DC allowed something much longer.

 

[redacted]

If your computer died, how do you know what your max depth and BT were?

If you're going to spend that much money, why not spend $200 to dive with a backup DC?

Once you're there, if your DC really dies (as in, permanently), why not spend a few bucks a day to rent a DC for the rest of the trip? You'll get more BT that way than using tables, right?

---------- Post added April 13th, 2015 at 11:09 AM ----------

Oh, and if you follow the "rules" of diving and the tables say your first dive was into deco and you can't dive again, wouldn't you be a lot happier if you had just taken a backup DC along on the first dive and it said you COULD do your second dive (because it wasn't assuming your first dive was square, like the tables do)?

If you are a recreational diver, none of yours dives should incur a deco obligation. So, no ceilings involved. Good situational awareness will mean that you are aware of depth and time, at least within a few feet and minutes. So, as long as you have good SA and are diving fairly conservatively, there should be little problem reverting to tables with minimal disruption of dive plans. If you have little idea what your max depth and dive time was when your computer died, you have a bigger problem than a dead computer.

I always carry a spare computer in my save-a-dive kit; but I rarely wear it. Most of my diving is profiles (even days of profiles) I have done before and I know they are not pushing NDLs. Even for more aggressive dive profiles that might be off the tables (within NDLs) I would probably count myself in PG Z, take an extended SI (like 3 hours) and resume diving on tables while I do a bit of loading on my replacement computer. I usually limit my surfacing N2 load to about 80% for the lead tissue compartment. From experience, if I knew my lead tissue compartments had moved up to one of the slower compartments (you can study this on DivePal) I might back off to a 75% limit for my remaining dives while that new computer takes a couple days to load the slower compartments.

 

[victorzamora]

Once you're there, if your DC really dies (as in, permanently), why not spend a few bucks a day to rent a DC for the rest of the trip? You'll get more BT that way than using tables, right?.

Firstly, MANY locations don't have rental computers. Secondly, and most importantly, you'd still have to stay out of the water until ALL of your tissues offgas fully. Following rules properly, switching to a new computer keeps you out of the water.

Also, if you don't know your max depth and dive time on a dive you a) shouldn't be diving and b) could just ask the divemaster/guide/buddy. Then switch to a rental PDC. Either way, that rental isn't going to know your tissue loadings.