Hold that line boys! How precise should you hold depth?

[The Chairman]

OK, I am finally taking things a bit deeper and have one more day in my Advanced NitrOx/Deco Procedures cours (TDI) with Dayo Scuba! The books were easy to master, and the methodology by TJ and Denise has been great... but what's the dealeo with holding to within 1'? I don't believe I got that in my reading, and I guess it's really not that hard to accomplish (except in Blue Spring with a metric butt tonne of current).

What is the acceptable variance here?

FWIW, I dove this plan with vplanner:

Stops|120/13|120/16|120/10
|(13)|(16)|(10)
50|--|(19)|--
40|(16)|1(20)|--
30|1(17)|2(22|(14)
20|5(22)|8(30)|1(15)

does vPlanner say anything about how precise the stops must be? Does it vary from planner to planner?

FWIW #2... tomorrow we hit Hudson Grotto! It should be fun!

 

[Blackwood]

My gut feeling is that 1/2 the resolution of the model is fine (in other words +/-5' for the schedule you posted above).

I've never seen a depth tolerance in any decompression schedule or software.

 

[David P]

I know Im going to kick myself for getting into this but +/- 5' is a BAD dive. Unless there's major swells, I would be upset if I varied 5'. I believe we were held to 1.5 in fundies... One of my dives for adv nitrox/deco/heliox I varied from my 150' bottom and hit 154... I had a talking to....

 

[Blackwood]

Sure, I'm certainly holding a much tighter tolerance than that in real life.

I think the question was "what matters to the decompression as a whole," though, and +/-1.5 seems like it's in the noise considering everything in the schedule is forced into 10 foot or 3m intervals.

I'd speculate that if I have a schedule which calls for stops at 110 through 10 in ten foot intervals, I could deco out at 115 through 15 in ten foot intervals and be perfectly fine. Hell, I'm sure enough about it that I'd actually go out and dive it (say 150' for 30 minutes on 21/35 followed by 115/1, 105/1, 95/1, 85/1, switch to 50% at 75, 75/4. 65/4, 55/2, 45/2, 35/3, 25/10, 15/5). You with me?

I guess what I'm saying is time matters a lot more than where you happen to be provided you're sufficiently shallow relative to your average loading depth such that you are in fact offgassing.

 

[rjack321]

A 154 dip vs. a planned 150 is totally immaterial you can average that out easy. I've done 50min BT dives with a max of 130 and an average of 90, now that's depth averaging.

As far as depth variation goes its not as critical from a deco persepective as it is from a buddy perspective. 5ft up for you and 5ft down for them is ALOT of seperation. We aim for plus minus 2ft.

 

[Meng_Tze]

Training variation was accurate... +/- 1ft. Practical variation usually is no more than 2 ft. Unless there is a nice ledge, then I am willing to go up to 3ft variation to lay down.

 

[JahJahwarrior]

In the overhead, it's easy to hold a stop to a foot. In the ocean, it might be easy...but it takes more work. I don't do much ocean diving, so when I do, I often go up or down by 2-3 feet, and if I really don't watch it for a while on the later stops, I might shift by 5' before I notice, if a teammate also moves up by 2-3 feet...

But I'm not DIR so I can go to sleep happy with my 2-3' variations.

 

[ZenDiver.3D]

In the overhead, it's easy to hold a stop to a foot. In the ocean, it might be easy...but it takes more work. I don't do much ocean diving, so when I do, I often go up or down by 2-3 feet, and if I really don't watch it for a while on the later stops, I might shift by 5' before I notice, if a teammate also moves up by 2-3 feet...

But I'm not DIR so I can go to sleep happy with my 2-3' variations.

In the ocean, it takes constant monitoring. Especially if you have no visual reference.
We go for no more than 3/10 of a meter. About a foot. Hard to do when that whale shark cruises by.....

 

[divingguru]

Talking from my experience and what I teach my students, is that there shouldn't be any more than a 1meter variation during stops. I find that you you tell people 1m is ok, that can soon become 1.5m or 2m, that's when things get out of control. Technical diving takes a huge amount of discipline and deserves an equal amount of respect, if your instructors have said 1 ft then that's what you should try and keep to.

 

[Doc Harry]

+/- 5 feet is okay?

Deco at 20 feet with 100% O2 gives you a pO2 = 1.6 ATA

Drop to 25 feet with 100% O2 gives you a pO2 = 1.75 ATA



Not me. I am not interested in exploring my epileptic potential while underwater.

I think we all recognize that practical depth tolerances vary by gas and by depth. But I'm not smart enough to remember complicated formulas or rules for various depths and gasses. So I'll stick with a tolerance of <2 feet and stick with it regardless of my gas or depth.

My digital gauge doesn't show inches. So I'm okay with seeing a change of +/- one foot. But when I stray 2 feet from my target depth, that's a failure to maintain deco depth.