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

[MXGratefulDiver]

First off, when you ask what variance is OK you should also be asking yourself "at what depth"?

You all know how decompression works ... so you understand that what matters is the pressure differential between the gas in your lungs and the dissolved gas in your body. Variations in your stop depth are going to be more critical at shallower depths and less critical at deeper depths.

I seriously doubt your body will react much to a 3-foot variation on your 70-foot stop ... but that same 3-foot difference at your 20-foot stop will matter more. Still, I doubt 3 feet will make that much difference on a typical decompression schedule. If it does, you're cutting your schedules too close to begin with.

Being able to hold a solid stop is a foundational skill in tech diving ... we should all strive for it. But it's kinda dumb to obsess over a minor variation ... especially for very short intervals. Like it or not, deco is always a crap shoot ... there's simply too many variables involved to make it a precise science. If you're paying attention, you'll notice and correct that variation long before your body has a chance to react to it.

... Bob (Grateful Diver)

 

[Diver0001]

I think there are a couple of different questions here.

One is whether it is important to do the stops precisely at the depths that the schedule prescribes, and I think almost everybody will agree that that is not true. For one thing, if you run the same dive through several different programs, you'll come up with somewhat different depths and times, yet people following those different programs are quite likely to execute safe and successful dives (including the pure Buhlmann, bend and mend profiles!) People doing cave deco often "fudge" stop depths, if the cave topography is simply unfriendly to doing a stop precisely where it is called for. Close is good enough in hand grenades, horseshoes, and decompression . . .

The second is whether it is important for a technical diver to be ABLE to hold stops reliably within a certain tolerance, and I think THAT tolerance is and should be tight, for a reason that someone has already mentioned: If you can't hold a stop within a foot or so, you're going up and down in the water column, and it makes it very difficult to maintain communication and coordination within the team. Further, if you can't hold the stop precisely when things are going well, then the yo-yoing is likely to get much worse if something goes wrong and you have to cope with more task-loading than you are used to. If you practice to hold stops within one foot, then when conditions aren't ideal or you have to manage something, your excursions are likely to be in the +/- 3 foot range, rather than +/- 10, which makes team cohesion essentially impossible.

And I say this as someone who had to work for literally years to achieve that degree of precision. It didn't come easily at all, so I really had to be convinced that it was needed.

Damn Lynne, I wish I could put it in black-and-white as clearly as you can. Your ability to keep the big picture in focus makes you the guru.

R..

 

[Blackwood]

+/- 10, which makes team cohesion essentially impossible.

+/-3.5 it is. That way the long hose will always reach

 

[Peter Guy]

Also the notion of a "stop" may vary substantially depending on where one is doing the "stop." For example, having just returned from the Red Sea where we did several staged deco dives, our "stops" were not -- they were "let's swim here" along the wall.

And our "final" stop, at "20 feet" was, let's just say it was often an extended stop -- as we swam along the wall for 20-30 minutes until we decided to surface. Did any of stay at 20 feet +/- one foot? No way -- we just looked around, swam and had fun.

The concept of a "stop" may not really apply to all decompression profiles.

(But what do I know -- I didn't even stay in a Holiday Inn Express last night!)

 

[Diver0001]

Also the notion of a "stop" may vary substantially depending on where one is doing the "stop."

Ehhhhhghhhh....... no....

Depth is depth no matter where you are.....

R..

 

[IDS_Bill]

As far as variance in stops, I teach and dive within 2 feet during air/nitrox backgas diving. When we get to helium, we shoot for 1 foot. I know that is very precise but that works better for us. In reality, you are diving your plan but you are hopefully backing up your plan with a computer so if everything goes to hell in a handbasket, you have your computer as a backup brain as you are re-figuring your deco obligation. A technical diver (especially a trimix diver) should be able to hold those tolerances.

I have seen tec divers 'hanging' on their reels or uplines for deco. I set my reel at the stop depths and let it hang for a visual reference but let it bob up and down with the waves as I stay stationary. This works well for me and keeps my stops consistent. If you are on an anchor line in a current however, this can get tricky fast (especially if you are dragging the anchor like we were in Subic Bay).

 

[ucfdiver]

Ehhhhhghhhh....... no....

Depth is depth no matter where you are.....

R..

I think he was implying that from a decompression perspective, we need to look at the percentage we're varying the ambient pressure by instead of the depth we're varying.

For instance, 5ft at 20ft varies the pressure by nearly 10%, while 5ft at 70ft varies the pressure by < 5%.

I might be wrong, but that's how I took it.

 

[TSandM]

No, what Peter's talking about is the difference between wreck diving where you are ascending on an upline in green water with nothing to see, and you spend your ascent moving up ten feet and just sitting there, and doing deco up a life-covered wall, where you aren't going to move up ten feet and just sit in one place and stare at one another. In the latter situation, "stops" becoming swimming periods, and if you spend a minute or so two feet below your stop depth, you can make it up with a minute or so 2 feet above your stop depth . . . deco just isn't that precise.

For example, for calculating your deco in the 70 to 30 foot range, agencies I know teach "linear pragmatic" (take total time in this segment and divide it by five and do that many minutes at each stop) or "shaping the curve" by spending more time at the gas switch and at the shallowest stops, and shortening the intermediate stops. Both ways work. If you do your gas switch at 70' and there's a gorgeous nudibranch to take a picture of right there, you might add a minute or two there and shorten the subsequent stops. There's a limit to how much you can mess with the schedule, but no schedule is written in stone.

 

[Peter Guy]

The spousal unit is correct (of course). The "where" isn't referring to depth but to geography and time. A nice leisurely swim up a wall, or scootering along a slowly rising bottom, will not involve a "stop" in the same way crawling up a line will.

 

[Diver0001]

I think he was implying that from a decompression perspective, we need to look at the percentage we're varying the ambient pressure by instead of the depth we're varying.

For instance, 5ft at 20ft varies the pressure by nearly 10%, while 5ft at 70ft varies the pressure by < 5%.

I might be wrong, but that's how I took it.

Maybe, but it doesn't matter.

If you can't hold your depth at 100ft, you can't hold it at 10ft.

I understood what he was saying, but in *real* terms a depth is a depth. If you have to count on how deep the stop is in order to determine if your buoyancy control was good enough then something's wrong.... right?

R..