Question Dive planning exercise, please check my work

[Cthippo]

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?

 

[rongoodman]

They sound like good dives to do after taking a recreational trimix course.

 

[boulderjohn]

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.

Are you planning to take the course 15 years ago?

 

[VikingDives]

1. I don't have NOAA tables, and it's way too early to do math, but to use air tables, you need to determine the EAD for the percentage of nitrox used, then use that equivalent depth for the tables.

2. A little bit, yes. Saltwater is denser than freshwater, so 1 ATA is 33ft/10m of Saltwater. For fresh use 34ft/10.1.

 

[inquis]

You are running into one of the issues that motivated the development of the DSAT RDP -- Repetitive dives on the NOAA/Navy tables are based on a longer halftime than is necessary for NDL diving. A reasonable SI of at least 2.25 hrs will allow 81% of the clean NDL time for DSAT (EAN28).

Correctly handling the fresh/salt water difference doesn't matter in this case because the tables aren't terribly granular. 120 ffw = 116.5 fsw (EAN21) or 103.2 fsw (EAN28). You are still in the same depth row on the tables if you pretend it's seawater.

You seem to have a handle on all the above, but you didn't mention what may be the limiting factor: gas consumption. Gas disappears quickly at these depths, especially when considering adequate reserves in the case of a stressed ascent if your buddy's reg hose bursts.

 

[inquis]

Do yourself a favor and plan with modern tools.

 

[scubadada]

Hi @Cthippo

If I were planning a dive to 120 ft, I would choose nitrox 28 to allow me a little buffer if I were to go deeper. The NDL for this dive would be around 16 min. Using my planning gas consumption rate, I would need about 42 cu ft of gas for this dive with a normal descent and ascent and a 3 min safety stop.

Consider getting dive planning software, I use MultiDeco, makes it very easy to run variables in the dive(s)

 

[ofg-1]

You know, if you never have a plan, nothing can go wrong.

 

[DiveLikeAMuppet]

At that depth, you should plan it as a deco dive with doubles and bring a deco stage (50 % or perhaps 80 %). You could easily get 40-60 minutes on the bottom with much nicer safety margins.

 

[johndiver999]

I would use 32%, come up when I hit 1000 lbs or got into a little deco. LOL. but you are smart to plan the dive carefully if you are unfamiliar with the time and gas supply constraints.

As others have mentioned, gas supply might be a limiting factor, if you use nitrox, especially. Also you might want to gain some experience at 100 ft before you go much deeper. You don't need doubles, you don't need to do a bunch of deco. You don't need helium or a bunch of training.