Incident due to battery change on dive computer

[Wookie]

Chicken and egg. I say that his complacency came from a unthorough understanding of his pdc and what it was telling him. I contend that that lack of understanding came from no one instilling the importance of education and understanding in him. Our point of contention is so minor it doesn't really matter.

StuartV, I did not take your post to think you impugned my teaching abilities. I can do that on my own. I think in the first page or so I stated that this is why I no longer teach introductory courses, because 10 years ago we spent most of a 2 hour session teaching deco theory. Now, we don't teach it at all, or as BoulderJohn states, it is self taught and the instructor is there to answer questions and evaluate skills.

 

[dberry]

Kudos to the OP for sparking this useful discussion (and taking the inevitable abuse.)

My PADI OW class assumed computer use, so tables were mentioned very briefly and there was maybe one exam question requiring a table. OTOH, none of us in the class actually owned a PDC at that time, but we did get a card for an on-line simulator (which was never used in the class.) I have taught myself how to use the tables (not exactly rocket science, btw) and do plan dives with them, even though I have a decent PDC. In retrospect I'm appalled that something so fundamental wasn't stressed more in the class. First, there's the "stick-shift vs. automatic" argument (i.e. knowing you can dive safely even if you left the PDC at home by mistake), but more important, IMO, is that the time spent learning the tables, and then re-reading the PDC manual to understand HOW it comes up with it's numbers, gives you a better sense of what's really involved.

OBTW, it's possible the OP's idea to use the average depth fsw MAY have come remembering the use of average depth for a quick-n-dirty SAC calculation. The big difference is that unlike SAC, gas loading seems to be very non-linear with depth (i.e. a little time at greater depths makes a huge difference.)

I haven't used nitrox, but probably will do the cert in the future, and have read a lot about it and have tried programming my PDC for nitrox and two different gases, just to see how it (the PDC) works. Seemed straightforward enough, and I'm looking forward to doing the nitrox cert before (or during) a trip to Cozumel in June.

A novice question for the crowd (please be gentle):
Wookie mentioned the one PDC setting I didn't understand: Default 50% O2. The only setting for this is on/off. As long as I'm only diving on air I probably don't NEED to know what this is for, but I'm a curious monkey... can anyone enlighten me?

FWIW, it's an Amphos PDC (Sherwood). Not many around SB seem to have this type, but I got a decent deal (with transmitter) and it has worked well for me thus far. The manual says it keeps tracking of loading in 12 different compartments, but only shows an N2 bar graph for the compartment controlling the dive. It also says the O2 graph only shows when you enter a nitrox F02 value, but that accumulated O2 is tracked during and after dives for 24 hours. Is this O2 value that Wookie said is only very approximate on most recreational PDCs?

Don

 

[Ayisha]

...I have seen people change computers or remove the battery to reset the computer so they can get back to diving.

I have seen a diver, who was a new DM about 10 years ago, hang the beeping computer on a line over the side of a boat during the surface interval as if a diver was completing a deco obligation, in order to not be locked out of her computer, also a Suunto. She did not regard the algorithm of her computer as useful and ignored the deco warnings it gave her during the dive and blew off the deco obligation she incurred. She had been significantly deeper longer than her buddy in order to take photos of a wreck in a current, and didn't have the gas required to complete the deco, but her buddy had enough gas for both of them. She simply refused to do the deco her computer calculated and ascended after a 5 minute safety stop.

When she said that her computer was too conservative and hung it over the side of the boat to fake deco, I asked her, "What's the point of having a computer that you disregard?" She said that she couldn't buy another one, and I suggested she could sell it and get something she might actually want, or just go back to tables. She became an instructor by the following summer...

 

[Wookie]

A novice question for the crowd (please be gentle):
Wookie mentioned the one PDC setting I didn't understand: Default 50% O2. The only setting for this is on/off. As long as I'm only diving on air I probably don't NEED to know what this is for, but I'm a curious monkey... can anyone enlighten me?

FWIW, it's an Amphos PDC (Sherwood). Not many around SB seem to have this type, but I got a decent deal (with transmitter) and it has worked well for me thus far. The manual says it keeps tracking of loading in 12 different compartments, but only shows an N2 bar graph for the compartment controlling the dive. It also says the O2 graph only shows when you enter a nitrox F02 value, but that accumulated O2 is tracked during and after dives for 24 hours. Is this O2 value that Wookie said is only very approximate on most recreational PDCs?

Don

Look up in the owner's manual and see if it resets it's nitrox values at midnight (in nitrox mode only). As far as I know, only Dive Rite computers do this. At midnight a Suunto (in nitrox mode) will reset itself to 79% N2 and 99% O2 (the worst of each gas for recreational divers). I don't know why they do this, but I expect it is to force you to analyze your tank and set your nitrox for the next days diving.

As to your second question. Yes, it only shows a N2 bar graph for nitrogen loading. It is showing you the controlling tissue compartment for your individual dive, which may be different depending on your depth. I can't tell you about the Sherwood Atmos, but I suspect that it is counting O2, and that it doesn't use an algorithm to actually calculate your O2 exposure, because for recreational diving, you are so unlikely to exceed your OTU or CNS O2 that it is really not worth worrying about. So what is the problem you say? The problem is, after multiple days of nitrox diving, your computer MAY limit your bottom times because it isn't doing a real calculation, it's just counting FO2 and multiplying by time.

This is a decent place to start without getting all techie on yourself. Instruction: CNS Oxygen Toxicity | Scuba Diving I don't know that I would draw the authors exact same conclusions, but in this (recreational diving) case, it's close enough.

I have seen a diver, who was a new DM about 10 years ago, hang the beeping computer on a line over the side of a boat during the surface interval as if a diver was completing a deco obligation, in order to not be locked out of her computer, also a Suunto. She did not regard the algorithm of her computer as useful and ignored the deco warnings it gave her during the dive and blew off the deco obligation she incurred. She had been significantly deeper longer than her buddy in order to take photos of a wreck in a current, and didn't have the gas required to complete the deco, but her buddy had enough gas for both of them. She simply refused to do the deco her computer calculated and ascended after a 5 minute safety stop.

When she said that her computer was too conservative and hung it over the side of the boat to fake deco, I asked her, "What's the point of having a computer that you disregard?" She said that she couldn't buy another one, and I suggested she could sell it and get something she might actually want, or just go back to tables. She became an instructor by the following summer...

Was she from the UK by any chance?

 

[uncfnp]

Look up in the owner's manual and see if it resets it's nitrox values at midnight (in nitrox mode only). As far as I know, only Suunto computers do this, but Suuntos are very popular, or at least they were. At midnight a Suunto (in nitrox mode) will reset itself to 79% N2 and 50% O2 (the worst of each gas for recreational divers). I don't know why they do this, but I expect it is to force you to analyze your tank and set your nitrox for the next days diving. Just so you know, I dive a Suunto Stinger, I leave it in air mode always. I never dive air, since nitrox is free for me.

Oceanic's have the same feature but resets when the computer cycles off. I set the default to off. That is one of the nice features of my Cressi. Set the FO2 once and it stays unless you reset to air.

 

[Redshift]

At midnight a Suunto (in nitrox mode) will reset itself to 79% N2 and 50% O2 (the worst of each gas for recreational divers).

79% N2 and 50% O2? Cannot be "and".

Maybe old ones did that? Or those sent to the States? Never seen that happening over here. My gf has a Suunto D4 and it doesn't reset the nitrox mix.

 

[Wookie]

79% N2 and 50% O2? Cannot be "and".

Maybe old ones did that? Or those sent to the States? Never seen that happening over here. My gf has a Suunto D4 and it doesn't reset the nitrox mix.

Sorry, it was my NiTek that did that. It defaulted to 99% O2 and 79% N2 at midnight. My Stinger resets itself to 21% after 2 hours if I don't dive, or at midnight.

 

[RJP]

What tables would an OW diver have that would have helped him do his second dive? According to the PADI RDP (I believe), he went into deco on his first dive.

For me, the point is that the tables would have helped him decide to NOT do the second dive.

---------- Post added April 12th, 2015 at 04:53 PM ----------

At midnight a Suunto (in nitrox mode) will reset itself to 79% N2 and 50% O2 (the worst of each gas for recreational divers).

None of the Suunto's I've owned do this. I last dove my Vytec in Truk three weeks ago. I just turned it on now and it's still set for 28%.

 

[Charlie59]

Computer - Life Support???

Yes, I think I am seeing the scuba instruction problem.

Ouch, dude. I meant it in the sense of a component of the dive set up that is used to make decisions about length of time underwater and used to keep one out of DCS. For some it is used as their pressure gauge and so in that sense a part of the system that benefits surviving a dive. Sorry for offending your pure form....

Party on.

 

[uncfnp]

79% N2 and 50% O2? Cannot be "and".

Maybe old ones did that? Or those sent to the States? Never seen that happening over here. My gf has a Suunto D4 and it doesn't reset the nitrox mix.

Yep, it can. At least for Oceanic. From the Oceanic Veo Owners Manual.