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

[34109411]

i was not saying that they teach deco theory i am saying it s a great tool to learn the basic of recreational scuba diving it s a good way to open and understand other stuff.

cheers

 

[Belzelbub]

Knowledge of the diving tables provides the basis of knowing safety limits [as pertains to nitrogen] while diving. A noobs reliance upon computers without first having learned the laws of physics and physiology of nitrogen absorption is a disservice to the student. Teach the tables.

Teaching the tables is not the same thing as teaching decompression theory. You can teach the exact same things with a computer as you can with tables. If your computer has a planning function (I’ve never used one that doesn’t), you can do exactly what the table will tell you. The difference is that you won’t get a Pressure Group letter designator. The pressure group is only there to keep the table to a manageable size.

And the value of the tables is gone the moment the diver tries to use it to on multi-level repetitive dives. Reliance on a computer is the same as relying on the tables. With a key difference. The table is probably up top in a bag. The computer is along for the dive and can give real time advice in the event the dive differs from the plan. Better to find out underwater that you should make a stop than to find out afterwards.

 

[steinbil]

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.

Here's a simulation of the profile Boulderjohn suggested:
Subsurface (5.0.10.0) dive plan created on 4/7/24
Runtime: 70min
Descend to 100 ft in 2:00 min - runtime 2:00 on air
Stay at 100 ft for 3:00 min - runtime 5:00 on air Open circuit
Ascend to 60 ft in 2:00 min - runtime 7:00 on air
Stay at 60 ft for 23:00 min - runtime 30:00 on air Open circuit
Ascend to 30 ft in 1:00 min - runtime 31:00 on air
Stay at 30 ft for 34:00 min - runtime 65:00 on air Open circuit
Ascend to 15 ft in 1:00 min - runtime 66:00 on air
Stay at 15 ft for 3:00 min - runtime 69:00 on air Open circuit
Ascend to 0 ft in 0:34 min - runtime 69:34 on air
End SurfGF: 58%
According to Subsurface the mean depth is 45.2ft

Here's a simulation of a square profile to 46ft:
Subsurface (5.0.10.0) dive plan created on 4/7/24
Runtime: 70min
Descend to 46 ft in 1:00 min - runtime 1:00 on air
Stay at 46 ft for 64:00 min - runtime 65:00 on air Open circuit
Ascend to 15 ft in 1:00 min - runtime 66:00 on air
Stay at 15 ft for 3:00 min - runtime 69:00 on air Open circuit
Ascend to 0 ft in 0:34 min - runtime 69:34 on air
End SurfGF: 69%

Using the ZHL16-C algorithm, the N2 loading of the leading tissue is less on the multilevel dive, than on a square profile to the average depth. So, if you use depth averaging on this type of profile it will be more conservative than a square profile – in other words, unless there is some other concern, diving tables using depth averaging on these types of profiles is more conservative than doing square profiles on the same tables. And following that same logic, on your next dive, if you use the pressure group from the table, you would be more conservative than after a square profile.

PS.
I'm not talking about simulating repeated dives underwater with pressure group letters. Just keeping track of the average depth and using the tables as if you were diving a square profile to the average depth. Yes, this only works with profiles of decreasing depth.

 

[USdiver1]

Spending classroom time going over the time/depth = pressure group [X,Y,Z], surface interval reducing that residual nitrogen level to pressure group [A,B,C] helps the student to visualize and learn the potential harm that may occur to the body if NDL time is exceeded. There is really no need to go into deco theory nor different tissue absorption / off gassing rates, just teach the basics. Learning of a potential danger of our sport to those joining it is the responsibility of both an instructor, an agency and a buddy who is going to dive with that person.

While almost all recreational diving is done at multi-levels, knowledge of pressure grouping is not "old school" that doesn't need to be taught. It is a knowledge set that may just keep you from getting bent, which can be life changing [and not in a positive way].

 

[inquis]

Knowledge of the diving tables provides the basis of knowing safety limits [as pertains to nitrogen]

A computer provides that same basis. Either way you're just looking at a number, either on a piece of plastic or on an electronic screen. The concept of residual nitrogen decreasing NDL on a second dive is hardly exclusive to tables -- it's just common sense.

For a multi-level profile (what is being discussed here), the proper application of tables (see the procedure @tursiops attached above) is quite error prone.

Honestly, even the depth averaging approach that @steinbil mentioned is quite error prone as well. The time-weighted average of those depths is not something I could do in my head, especially while underwater. (Note, I'm not disputing that it can be a conservative guide on a strictly ascending profile IF computations are accurate.)

The bottom line for me is that it is common to eliminate dangerous equipment or practices (e.g., danglies/entanglement hazards, single source of gas when buddy separation is likely, etc.). In my view, such error-prone calculations are no different.

 

[USdiver1]

The bottom line for me is that it is common to eliminate dangerous equipment or practices (e.g., danglies/entanglement hazards, single source of gas when buddy separation is likely, etc.). In my view, such error-prone calculations are no different.

And here I would differ. Equating knowledge of diving tables is in no way "dangerous equipment or practice". It is a fundamental block of understanding our sport. Granted the tables are based upon fewer tissues, and gas absorption/ diffusion rates than are computer algorithms [and there are several algorithms utilized by different manufacturers of dive computers]. But what they all have in common is that there are N2 diffussion levels that must be adhered to to not get hit by DCS. Teaching the tables provides the introduction for students, and the earlier a student is exposed to understanding such levels the better.

 

[inquis]

Equating knowledge of diving tables is in no way "dangerous equipment or practice".

I didn't say that. I said the error-prone mental calculations required to apply tables to a multi-level profile were dangerous. High likelihood of a mistake and high severity of consequence makes for high risk.

 

[steinbil]

Honestly, even the depth averaging approach that @steinbil mentioned is quite error prone as well. The time-weighted average of those depths is not something I could do in my head, especially while underwater. (Note, I'm not disputing that it can be a conservative guide on a strictly ascending profile IF computations are accurate.)

Yes, keeping a running average in your head can be tricky. But in any case you need a bottom timer and depth gauge to keep track of your exposure, so I would just use a bottom timer that shows me the average depth, and then it's not so error prone, since I'm not doing any calculations.

 

[Belzelbub]

Spending classroom time going over the time/depth = pressure group [X,Y,Z], surface interval reducing that residual nitrogen level to pressure group [A,B,C] helps the student to visualize and learn the potential harm that may occur to the body if NDL time is exceeded.

May help some students. Not everyone learns like that. For me, I understand the tables, but knowing the pressure grouping did not help me one bit in understanding the important part. The pressure letter was just an arbitrary designator to be able to use the other side of the table for dive 2.

Another way to do this is with computers. The instructor can do it during a dive. By noting NDL at a point in the dive, then ascending a bit. The students could see the effect in real time.

There is really no need to go into deco theory nor different tissue absorption / off gassing rates, just teach the basics. Learning of a potential danger of our sport to those joining it is the responsibility of both an instructor, an agency and a buddy who is going to dive with that person.

Deco theory is the basics. Agree that different tissue compartment rates are not needed at OW level.

While almost all recreational diving is done at multi-levels, knowledge of pressure grouping is not "old school" that doesn't need to be taught. It is a knowledge set that may just keep you from getting bent, which can be life changing [and not in a positive way].

Which is why tables are really not necessary. They simply don’t work for the diving most do.

 

[inquis]

I would just use a bottom timer that shows me the average depth, and then it's not so error prone, since I'm not doing any calculations.

A rose by any other name...