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

[Gilldiver]

Do you know the accuracy of your depth gauge?

I have seen many of the same model have a variance of +-3' or more. So, if you are keeping to +-1' that is fine, but if you think you are +-1' of real depth, you need to do a full Repeatability & Reproducibility (R&R) analysis ANOVA Gauge R&R - Wikipedia, the free encyclopedia
on every depth gauge you have across the deco range. Next, a good swell or chop can have the line in front of you doing a yo-yo, so you may average 10' but real depth can be +-5-10' from that average through the wave period.

In other words, keeping +-1' is a good goal, and can/should be done in nice calm water but I will not obsess over it. Open water conditions and other factors can/will all affect your ability to keep station.

 

[Doc Harry]

Do you know the accuracy of your depth gauge?

My digital depth gauge is accurate to +/- 1 foot per the manufacturer.

Good enough for me to obsess over a 2 foot depth change.

 

[Meng_Tze]

So let me throw this out there then:

How accurate is diving overall? Let's say the depth gauge has a tolerance of +/- 3% on display and on depth sensor, the O2 cell's that analyzed the gas at the filling station and your verification have a tolerance of +/- 1%.

When manufacturers say: accurate within 1ft. Is that 1 ft of full scale, mid scale, or something else?

 

[Diver0001]

Pete,

What you *need* to do and what you should be able to do aren't really the same.

The smallest step (resolution) between stops on the IANTD tables is 1.5 metres (6 to 4.5). From what I've seen most models use a 3m resolution for stops shallower than 21 metres. (21, 18, 15, 12, 9, 6 and then 4.5 in the case of IANTD). Obviously zero is best and 3 metres is too much. Take 1/2 of the 150cm from teh IANTD tables is 75cm, which in terms of the model is pushing the envelope of skipping a stop depth in the IANTD system. 1/2 again and you're on target, which is about 38cm or a little over a foot. That's acceptable, in terms of the model, I would think.

Personally, I don't beat myself up mentally if I'm within 50cm. I feel good if the deviation is less than 30cm (1 ft). I feel uberdiverish about my stops if I get any closer than that.

A typical computer is accurate to within about 10 cm and if you're not task loaded then there's no reason an experienced diver can't hold that depth too.

Good luck with the rest of the course, and welcome to the dark side..

R..

 

[Diver0001]

So let me throw this out there then:

How accurate is diving overall? Let's say the depth gauge has a tolerance of +/- 3% on display and on depth sensor, the O2 cell's that analyzed the gas at the filling station and your verification have a tolerance of +/- 1%.

When manufacturers say: accurate within 1ft. Is that 1 ft of full scale, mid scale, or something else?

Those are good questions. I check my depth gauges from time to time and my bottom timer, computer and watch all show within 10cm of each other on the whole. The biggest variance I've seen when comparing what I thought was a normally functioning computer was 30cm (1ft). I *have* seen computers off by as much as 3-4 metres, though. During one dive where my buddy and I were planning on taking on about 1/2 hour of deco, my buddy kept blowing through the agreed upon depth (not something he would normally do). I brought him back twice and on the third time I showed him my depth .... His computer was showing 28 metres and my was showing 32. He kept going to what he thought was 30, which for me looked like 34.

We decided to use my depth measurement as leading (when in doubt go conservative) and finished the dive like that and after the fact he discovered that his computer was malfunctioning. Good enough reason for each diver to have two depth sensors on any dive (and expecially a deco dive). Problems with instruments show up like big white lights when you have something to compare to....

R..

 

[Diver0001]

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

So on a slightly different topic, how did they tell you to sort it out if your ascent rate wasn't spot on....?

R..

 

[Rick Inman]

Like my sig line says, "Decompression algorithms are akin to measuring with a micrometer, marking with chalk and cutting with an ax."

If you have a 3 min obligation at 50', you can clear that stop on a HammerHead computer by staying at 60' a few mins longer than the 3 mins. You can also blow past stops (ala bend-and-mend, US Navy-ish) and head to 20' and hang there forever and clear. Because it's not a function of hitting exactly this stop or that. It's about the different tissue models and the slow-to-fast tissues compartments being able to off gas verses bubbling up at the surface and giving you a big oh-oh.

Following the plan is GOOD (I like 1' myself). But getting out clean is the goal, and everybody is different and every dive is different.

Personally I feel that the key is to move with control. Better to do slow, easy controlled moves and be 3' under your stop than to bounce around from depth to depth.

Of course, the ability you hold your stop is a part of minimum requirements to do any kind of decompression diving, and it should be. You don't want to be dropping a bunch below 1.6 on O2, for example. And you should be able to deal with issues, be tasked and still maintain your depth, so just deco-ing at a set depth should be easy. But will you get bent because you were inaccurate within 5' in stop depth? All things being equal, I doubt it.

 

[Blackwood]

+/- 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

Note in my sample schedule I wasn't using Oxygen, nor is the thread about oxtox. It is (I believe) about how close you have to stay to the depths (which were arbitrarily listed in even intervals) in a decompression schedule to consider yourself as diving that schedule (from a decompression standpoint).

Am I on track, Pete?

Personally, I shoot for with the team first and precision on the schedule second. If my deco captain is hanging where my gauge says 31 for our 30 foot stop, I'll hang at (my) 31. Same with 32. At 33 I'd probably signal him to look at his gauge. If he moved accordingly, I'd follow, but if he was fine with the depth, I'd stay there.

When I'm deco captain, I shoot for dead nuts per my gauge, however my gut feeling is that within reason it doesn't matter a whole heck of a lot. I'll call 'reason' +/-5 (oxygen considerations notwithstanding). Of course, many of us have moved stops around on the fly due to the environment. We've pushed 10 foot stops down to 20 (which is outside of my +/-5) because of swell. Didn't matter to the deco.

 

[Diver0001]

Personally, I shoot for with the team first and precision on the schedule second. If my deco captain is hanging where my gauge says 31 for our 30 foot stop, I'll hang at (my) 31. Same with 32. At 33 I'd probably signal him to look at his gauge. If he moved accordingly, I'd follow, but if he was fine with the depth, I'd stay there.

Having a depth jockey is a good idea. We do that too. One depth jockey, one nav jockey, and remaining team members are apostles of one or the other, watching time or doing the overview.

Obviously, the depth jockey's job is to keep their depth within 10cm and they are relieved from all other team tasks. IN some ways, it's best to have your most anal-retentive diver play this roll...

BTW, Blackwood, I loved your report of your wreck course. I read it once and I'll read it again before commenting. It was really good.

R..

 

[TSandM]

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.