Question Dive planning exercise, please check my work

[crofrog]

If you want to do technical diving. Get technical training, and use equipment suitable for technical diving.

Cold, dark, water with a silty bottom and fragile wrecks really isn’t the place to do a tech dive with recreational equipment and skills

 

[johndiver999]

sorry. i mean no offense to the op. but it is clear they have no advanced training whatsoever. and although i do support peoples desire to advance their knowledge in the sport, these questions seem a bit too specific for my taste.
it is one thing to ask a general question about how tables work, or what the pros and cons are when using nitrox for example, in order to gain some understanding before taking a proper class.
but it is another ball game when a newer diver starts asking specific questions regarding specific dive sites and about the allowable bottom time, which gas to use, how much deco is acceptable etc.
do you want to be the one who provides all the answers and then read about a diving accident 6 months from now?
i am not suggesting this diver is planning to do that, but they might. and so might someone else who reads this thread.

I have ZERO concerns about that. If I provide reasonable information, how a person uses that is up to them.

Personally, I think I have learned a good bit from reading on-line information and opinions and much of it was outside of my training, which is strictly recreational.

 

[rhwestfall]

If you want to do technical diving. Get technical training, and use equipment suitable for technical diving.

Cold, dark, water with a silty bottom and fragile wrecks really isn’t the place to do a tech dive with recreational equipment and skills

Plausible statement.

Each of us have familiarity with our own geographical area. I'm modestly competent in Great Lakes diving, yet have no idea of New Jersey wreck diving, Nor-Cal surf, etc... or for the matter, what was explained for the Washington State diving...

 

[rick00001967]

I have ZERO concerns about that. If I provide reasonable information, how a person uses that is up to them.

as instructors (former) we do not have that luxury.

as much as i do understand your point in general (people should take responsibility for their own actions etc). sometimes the people receiving the info do not have any idea what they are getting themselves into. sometimes it is up to the ones that are able to see the possible risks, to protect others that cannot.

 

[rx7diver]

I recently heard about a bunch of wrecks at around 120' FFW in Lake Washington. It will probably be another year or two before I am ready to dive them, but I figured I would take a stab at the planning process for this sort of dive. I was also curious about the utility of nitrox at depths of 100-130', and so I am self teaching tables.

Before someone asks, no, I am not nitrox certified. I am trying to teach myself the process of this because I will have a better understanding from figuring it out on my own before I take the class.

So, working from the NOAA air tables a 120' dive on air would give me a NDL bottom time of 15 minutes, and no possibility of a repetitive dive within a reasonable surface interval. If I stayed to 20 minutes, I would have a 2 minute deco penalty at 20'.

On the other hand, if I used EAN28, that would give me a NDL bottom time of 20 minutes at a PPO2 of 1.3. If I stayed to 25 minutes, that would incur a 3 minute deco penalty.

EAN 30 would increase my PPO2 to 1.39 without changing NDL times.

Questions: 1, do I have this right? And 2, does the fact that this is fresh, rather than saltwater make any difference at these depths?

@Cthippo,

1. Using my old Navy Air tables, EAN28 at 120 ffw has EAD = 110 ffw (rounding up from 106.4 ffw), which has a NDL = 20 min.

Solve 0.79 * (x/34 + 1) = 0.72 * (120/34 + 1) for x in ffw.

IMPORTANT: My old Navy Air tables require an ascent rate of 60 fpm.

2. For a BT of 25 min, instead, there is a single deco stop, a 3 min deco stop at 10 ffw.

3. And for a BT of 30 min, instead, there is a single deco stop, a 7 min deco stop at 10 ffw.

4. Yes, EAN28 has a PO2 = 0.28 * (120/34 + 1) = 1.27 ATA at 120 ffw.

5. Determine the minimum amount of gas (including emergency reserve) you "need" to take to make this recreational dive. (You'll need to know your working RMV.)

6. Is there a hard bottom no deeper than 120 ffw? If not, then I would plan also for the next deeper depth, namely, 130 ffw.

ETA: My Recreational Nitrox course (IANTD, in 1993) specified that there are only two recreational Nitrox mixes: NOAA Nitrox I (EAN32) and NOAA Nitrox II (EAN36).

rx7diver

 

[tursiops]

@Cthippo,

1. Using my old Navy Air tables, EAN28 at 120 ffw has EAD = 110 ffw (rounding up from 106.4 ffw), which has a NDL = 20 min.

Solve 0.79 * (x/34 + 1) = 0.72 * (120/34 + 1) for x in ffw.

IMPORTANT: My old Navy Air tables require an ascent rate of 60 fpm.

2. For a BT of 25 min, instead, there is a single deco stop, a 3 min deco stop at 10 ffw.

3. And for a BT of 30 min, instead, there is a single deco stop, a 7 min deco stop at 10 ffw.

4. Yes EAN28 has a PO2 = 0.28 * (120/34 + 1) = 1.27 ATA at 120 ffw.

5. Determine the minimum amount of gas (including emergency reserve) you "need" to take to make this recreational dive. (You'll need to know your working RMV.)

6. Is there a hard bottom no deeper than 120 ffw? If not, then I would plan also for the next deeper depth, namely, 130 ffw.

rx7diver

Why plan a dive with the old Navy tables? Even the Navy doesn't use its old tables.
The new tables are readily available at https://www.navsea.navy.mil/Portals/103/Documents/SUPSALV/Diving/Dive Manual Rev 7 Change A.pdf. Table 9-7 is the NDL table; Table 9-8 is the repetivie dive table, and Table 9-9 is the decompression table.

 

[boulderjohn]

IMPORTANT: My old Navy Air tables require an ascent rate of 60 fpm.

Do they actually require that ascent rate, or is that just the rate on which they were created?

I used to think the PADI tables required a 60 fpm ascent rate as well. If you want to do a mind-numbing ScubaBoard search from about 15 years or so ago, you will find posts in which I mistakenly said that if you were doing it slower than the 60 fpm used in creating the tables, the results would not be accurate because you were actually getting more bottom time than planned.

I was flat out wrong. From their inception, the tables said to ascend "no faster than 60 fpm." What I eventually learned was that in their research to create those tables, PADI learned that their was no harm in going slower, even much slower. That is why they used the "no faster than" language. The tables actually do not have a required ascent rate at all.

 

[tursiops]

Do they actually require that ascent rate, or is that just the rate on which they were created?

I used to think the PADI tables required a 60 fpm ascent rate as well. If you want to do a mind-numbing ScubaBoard search from about 15 years or so ago, you will find posts in which I mistakenly said that if you were doing it slower than the 60 fpm used in creating the tables, the results would not be accurate because you were actually getting more bottom time than planned.

I was flat out wrong. From their inception, the tables said to ascend "no faster than 60 fpm." What I eventually learned was that in their research to create those tables, PADI learned that their was no harm in going slower, even much slower. That is why they used the "no faster than" language. The tables actually do not have a required ascent rate at all.

The current Navy tables (Rev 7A) say nothing about ascent rates for NDL dives. The only statement is:

9-6.3 ​

Ascent Rate. The ascent rate from the bottom to the first decompression stop,​

between decompression stops, and from the last decompression stop to the surface​

is 30 fsw/min (20 seconds per 10 fsw). Minor variations in the rate of ascent​

between 20 and 40 fsw/min are acceptable.​

Older versions of the Navy tables:
Rev 4 (1999): 30 ft/min
Rev 1 (1981): 60 ft/min

EDITED: 1981 was Rev 1, not Rev 2. See U.S. Navy Diving Manual - Wikipedia.

 

[rx7diver]

Do they actually require that ascent rate, or is that just the rate on which they were created? ...

[HIJACK]
I'm not certain. My understanding is that the ascent, itself, was taken into account during the derivation of the decompression tables, and that (my old) Navy Air Tables "require" a 60 fpm ascent rate.

For me, personally, this is moot, though, since I always endeavor to dive conservatively. So, for recreational (NDL) diving, I make sure I am back at the surface (or at the 10 ffw safety stop) no later than my planned BT, and I make sure to not exceed a 30 [sic] fpm ascent rate at any point during my ascent.

For example, for a (air) dive planned to 70 ffw for a BT = 50 min (NDL per my old Navy Tables), I make sure that I am back at the surface (or at the 10 ffw safety stop) no later than RT = 50 min, having not exceeded a 30 fpm ascent rate.

rx7diver
[/HIJACK]

 

[JMarc]

Do they actually require that ascent rate, or is that just the rate on which they were created?

I just checked my old LIFRAS (french speaking Belgian CMAS branch) manual on the use if US navy tables. It says that the time of entry is from the immersion to the start of the ascent at the specified rate with an explicit mention that if the ascent was slower that meant the time when reaching the first of the deepest stop or 9m. I don't know if the origin of that rule is from the US Navy or from the LIFRAS manual authors.