Question Only Instructor wears dive computer (extra charge if I want one)

[ADVRider808]

This was OP's first post and while on vacation and probably in a slight panic about their vacation dive situation.

My guess is that b they're blowing bubbles or sipping cocktails by this point and have forgotten their drive-by post on ScubaBoard.

 

[boulderjohn]

I'd like to point out the obvious... I dove for a decade without a computer. Believe it or not, it is possible to dive without a computer.

Yes, it can be done.

Take, for example, a multi-level dive to 100 feet in which the diver goes to that depth (taking 2 minutes) to look at some garden eels, stays 3 minutes, ascends to 60 feet, stays there for 25 minutes, ascends to 30 feet, stays there for 35 minutes, and does an ascent with a 3 minute safety stop. That is a nice dive with a total dive time of 69 minutes.

The diver could have done the same multi-level dive with the PADI tables. Of course, some of the times would need to be changed, because the maximum bottom time is 20 minutes. That means the total dive time, assuming the final ascent and safety stop from 30 feet, would be 24 minutes. So, yes, it can be done. No real difference, right?

On the other hand, if you are following a dive guide in a place like Cozumel, you can't use tables, because the multi-level dive you do with them will violate the table NDL about half way through the dive.

 

[tursiops]

Yes, it can be done.

Take, for example, a multi-level dive to 100 feet in which the diver goes to that depth (taking 2 minutes) to look at some garden eels, stays 3 minutes, ascends to 60 feet, stays there for 25 minutes, ascends to 30 feet, stays there for 35 minutes, and does an ascent with a 3 minute safety stop. That is a nice dive with a total dive time of 69 minutes.

The diver could have done the same multi-level dive with the PADI tables. Of course, some of the times would need to be changed, because the maximum bottom time is 20 minutes. That means the total dive time, assuming the final ascent and safety stop from 30 feet, would be 24 minutes. So, yes, it can be done. No real difference, right?

On the other hand, if you are following a dive guide in a place like Cozumel, you can't use tables, because the multi-level dive you do with them will violate the table NDL about half way through the dive.

That dive can easily be done using the eRDPML, or The Wheel. The problem is not "tables" or "RDP" per se, but rather using the wrong version for the dive.

 

[tursiops]

That dive can easily be done using the eRDPML, or The Wheel. The problem is not "tables" or "RDP" per se, but rather using the wrong version for the dive.

The dive can also be done with the simple table RDP using the procedures in Duis (1991), attached.

 

[L13]

I'd like to point out the obvious... I dove for a decade without a computer. Believe it or not, it is possible to dive without a computer.

But to do it safely, you need a depth gauge and timer (and the appropriate tables). The total cost for those is a significant fraction of the cost of a basic computer, and the result will be much less convenient diving that can be much easier not to do safely.

 

[BabyLitigator]

FWIW, my last 3 "vacation" dive ops (Nassau, Cabo, Playa Potrero) didn't automatically supply folks doing the standard double dip with computers. I had mine (on a conservative setting) and never came anywhere near NDL limits (and I was diving a smidge deeper than the DM). I got the sense on all three that these were standard dive sites where getting into NDL trouble would be kind of difficult on a single AL80 in reasonable amount of time. Not the way I choose to dive, but for a "one every year" diver, YMMV.

 

[steinbil]

Take, for example, a multi-level dive to 100 feet in which the diver goes to that depth (taking 2 minutes) to look at some garden eels, stays 3 minutes, ascends to 60 feet, stays there for 25 minutes, ascends to 30 feet, stays there for 35 minutes, and does an ascent with a 3 minute safety stop. That is a nice dive with a total dive time of 69 minutes.

But then again, this is a perfect example of a dive where averaging the depth would work for diving on tables. For profiles like these (continuously decreasing depth), depth averaging on tables does not lead to a higher N2 load than a square profile to that average depth. So if you trust your computer to do that dive, you might as well trust the tables to do the same thing.

Disclaimer: if you do depth averaging WRONG with tables, you might get bent or die, so don't, unless you know how

 

[tursiops]

But then again, this is a perfect example of a dive where averaging the depth would work for diving on tables. For profiles like these (continuously decreasing depth), depth averaging on tables does not lead to a higher N2 load than a square profile to that average depth. So if you trust your computer to do that dive, you might as well trust the tables to do the same thing.

Disclaimer: if you do depth averaging WRONG with tables, you might get bent or die, so don't, unless you know how

What is the "average depth" that you would use to give the same N2 loading, and thus allow the calculation of a repetitive dive? The mathematical average depth of that dive is about 43 feet. Is that the depth you would use? If you use the RDP with a 65 minute dive to 43 feet, you get less N2 loading than the ML dive gives. Apparently you do not use the mathematical average depth; what do you mean by average?

 

[steinbil]

What is the "average depth" that you would use to give the same N2 loading, and thus allow the calculation of a repetitive dive? The mathematical average depth of that dive is about 43 feet. Is that the depth you would use? If you use the RDP with a 65 minute dive to 43 feet, you get less N2 loading than the ML dive gives. Apparently you do not use the mathematical average depth; what do you mean by average?

Unless I made an error, the mathematical time-weighted average depth is just over 45ft, but technically using tables you would use the 50ft table in either case. In any case, I've simulated a lot of different dive profiles to test the N2 loading according to ZHL-16C and compared them to square profiles to the average depth. As long as the multilevel profile is decreasing in depth, the SurfGF at the end of the dive would not be higher in the ML profile, on the contrary it would usually be lower.

EDIT:
But if you think I'm wrong, please give me an example of a profile that proves the point. I haven't been able to...

 

[tursiops]

Boulderjohn's profile gives me the following:

Using the eRDPML: to calculate a multilevel; dive, using the three levels of 100 feet for 5 mins, 60 ft for 25 mins, and 30 ft for 35 mins, the ending pressure group is T. A 3-min Safety Stop is mandatory on this dive.​

The average depth (ignoring ascent and descent time) is 46.9 ft. Again using the RDP, a 50 foot dive for 65 mins ends in Group U...i.e., more N2 loading.​

My point is not that the dive using average depth is unsafe, but rather than the N2 loading is not correct (which affects repetitive dives).

In contrast, some argue that you can use simple tables to do this dive by looking at the Pressure Group at the end of 5 mins at 100 ft (B), and then use the next level to add time and end in PG L, then the next level to end in PG R. The method does not give the right answer for this profile, and errs on the side of danger.