Going into deco

[Drewski]

Curiously enough, I've had the recent experience of diving tables vs. a computer on two trips and got to see the difference between each.

I use a DiveRite Nitek Duo with the UWATEC BT as backup. While dressing, I dropped the Nitek to the deck and it went blank. Attempts at resuscitation were unsuccessful. My buddy and I had planned a 40 min BT at 110 FT max depth using 34% and 50/50 mix in a slung 40 for the safety stop. Generally, using the computer, this is a no-deco profile.

However, according to the IANTD Nitrox tables (always carried in my pocket), it IS a deco dive. So, (going from memory here) I "planned" a 13 min stop. We did the BT between 80 and 103 FT, then a slow ascent. When we reached the hang bar, my buddy's computer was WAY clear of deco, but we did the hang anyhow just for practice. Surface waves were 4 FT, so it was good experience (URRRP!). Switching stages and keeping yourself together when that bar is going up and down can be a little challenging. That ALONE, can create "issues" for the inexperienced.

After a 1 HR and 45 min SIT, we did a second dive with a 30 min BT, same results with computer vs. table. The following weekend we ended up re-visiting the same wreck (we got blown out down off Hatteras) with 32% and a 50/50 slung 40 following the same profile . My NEW computer was shipped by DiveRite to the Hatteras shop where we had the charter (oops...) so, again, I used tables and the BT. Same results, same deco “obligation.”

It really is interesting seeing the difference between the computer calculated profiles and the "credit" given for multi-level vs. the "deepest depth" approach on the tables. I'm planning on picking up some software soon to give it a try...

 

[matt_unique]

Just strikes me as curious here. Lot's of responses like "Never, not once" as though there is something horrible about going into deco. I've gone into deco a few times diving in Cozumel. I'm blessed with very good air consumption and I'm stretching tanks out pretty far. I knew I was getting close because my computer was sitting in the yellow for quite a while. Towards the end of the dive it went into the red and required a stop on the way up. So I did. And it was happy. And all was well. And I didn't really need any special training to do it.

Really all dives are deco dives, aren't they? The only question becomes whether or not a stop on the way up is required to allow for some off-gassing under something more than 1 atmosphere of pressure.

Once I started dropping into deco regularly I got my Nitrox certification and started diving that whenever I could. I don't think my computer has even been in the yellow since then.

-Charles

I thought this way before I was trained for deco diving. Most recreational computers are not designed for proper deco diving. My last "deco" dive before my first class was used as a great case study of why recreational computers are not designed for deco diving. I was diving a single tank with pony on the U853 off RI. Long story short, my Suunto computer called for a short stop at 10'. The most commonly used deco programs (V-Planner and GUE's Deco Planner) BOTH called for MUCH MORE deco time and deeper stops than my recreational computer.

As an additional anecdote, the Dive Master recalled a time, ironically also on the U853, when he became bent after clearing the deco obligations on two recreational dive computers (two different brands).

There lot's of models and theories but day 1 of this course was spent dispelling the myth that recreational computers are designed for deco diving. The software programs I mentioned also present an easy way to calculate your gas requirements, alternative profiles in the event that you loose deco gas, etc., etc.

--Matt

 

[redacted]

Another question . . . For those who are comfortable going into deco on a "recreational" dive, at what point do you assess whether you are going to have the gas to do the deco, and how do you do it?

As I said, I have never done recreational deco. But I'm not uncomfortable with the idea and have thought about it. I figure that for "light" deco, I'd need about 100psi in my Al80 for each 2 minutes. Should be a little on the safe side for on-the-fly estimates.

 

[Ryan96]

Until i'd read this thread i had no idea there was an unofficial signal for "decompression".

Here in Beirut our sign for deco is the thumb, index and middle finger together and pointing up. Imagine the Italians saying "What your f.... problem?". In this region, the gesture means "wait please" and so makes a nice sign for deco. It's a command sign which warrants a number response in minutes.

Another question . . . For those who are comfortable going into deco on a "recreational" dive, at what point do you assess whether you are going to have the gas to do the deco, and how do you do it?

Typically we don't do it without a prior agreement about how much deco obligation we'll acquire and with knowledge of what our gas requirements are for the expected plan. I wouldn't mind making the calculation/decision to take on a little more deco than planned while on the bottom but only with another competent diver who I have dived with and who knows his breathing rate. We also plan for a little more safety margin and so to abort the dive plan if any diver hits the 80bar to be sure we have enough gas for a slow ascent from 36m-40m.

 

[Charlie99]

Another question . . . For those who are comfortable going into deco on a "recreational" dive, at what point do you assess whether you are going to have the gas to do the deco, and how do you do it?

If I have a deco obligation showing because of dallying too long at 120', it's pretty much a non-issue, because after 15 minutes at depth I'll still have well over half a tank left. For deco times of 5 minutes or so, my previously determined (and relatively agressive) rockbottom of 1200psi is the controlling item.

Where gas can become an issue is longer medium depth (60-90' range) dives, particularly on repetitive dives. NDL can be a problem for me on repetitive dives as shallow as 50-60', even when using AL80. The other thing about going into deco on my Oceanic computers in these medium depths is that I prefer to get a bit of margin by doing stops such that I get back close to the green on the N2 bargraph. If running profiles on a decompression program, this would be equivalent to using an upper gradient factor in the 0.85 or 85% range. Since multilevel dives and repetitive dives to 60' range tend to have controlling compartments around 30 minute halftimes, a total ascent time of 10 to 15 minutes may be needed to get N2 loading to where I want it. This is pretty much the situation Peter was in on the two profiles he posted.

Since my SAC is pretty low, the controlling factor is how much gas my buddy, if any, has. My normal ascent is about 7 minutes from 40' to the surface and uses the same amount of gas as would about 12 minutes on the surface. Normally, I use 0.4cfm or about 15psi/min from an AL80 and rarely more than 0.5cfm or 20psi/min; so this normal ascent takes only about 250psi. .

If I'm near NDL (which would be a ways into deco on a Suunto), then I'll probably be taking about 10 -12 minutes in ascent from 40' ---- something like 2 at 40', 4 at 25-30', 6 minutes at 20' sliding up to 10'. This ascent takes the same gas as would 20 minutes on the surface --- or about 400psi.

Even if I needed to extend stops quite a bit more, to something like 3 at 40', 5 at 25', 10 at 15' it's only around 600psi used, and each additional minute at the 15' stop is only another 30psi or so. (If I'm starting the ascent phase from 80' then the stop depth are around 45', 30', and then 20' to 10' glide. If a shallower dive, or one where I've multileveled up to the 60' range the the stops are more like 40', 25', and 15' to 8' glide.)

My willingness to spend time hanging is generally much more the limiting factor on how far I will push things rather than how much gas is left in my tank.

 

[DandyDon]

I don't know if it does either. But my guess is that it requires you to dive within your training/certification level. So if one is INTENTIONALLY going into deco w/o being tech trained, I would guess it could impact your coverage.

You can access Dan dive insurance coverages easily on the net - I don't think there are any limits on the Preferred and Master plans. Hits are rare even for sloppy divers, so they don't have any guards against that, I guess. Besides, proving it would be very difficult, even if someone was posting under their own legal name - not court quality evidence, and would give Dan a bad name. I just do not see a risk there.

 

[TSandM]

Charlie, your post is a beautiful argument for understanding one's gas consumption rates and having a good feeling for what you should be using at any given depth.

I know that, at home, I use about 100 psi out of my doubles every five minutes at 60 feet. It's easy to do quick ratios and adjust that for depth. When I used the 130s in SoCal, I got the tank factor and made the appropriate conversions, so I knew what to expect. So I can sit at 90 feet and think, "If I want to do this ascent, how much gas do I need?" and do the calculations in my head.

The limiting factor becomes rock bottom, really. You have to know your buddy's consumption, and make sure you're hanging on to enough gas to get THEM to the surface -- with the deco obligation you're incurring -- not just you.

 

[Charlie99]

So how do you guys let your dive buddy know you have a deco obligation ... or how much of one you have? How do you signal when you've cleared your obligation?

I just move my hand horizontally whether I'm indicating a stop showing on my computer or just a stop I feel like making. Then I'll follow by showing a bunch of fingers. Sometimes I'll remind my buddy that I'll be doing a some deep stops on the way up by doing the horizontal wave of my hand at 3 different ascending levels, and then showing my intended total ascent time.

An upwards thumb with the other hand horizontal above it is pretty clear as "move up and stop again", but most of the time we just start drifting up and since when hanging we stay within a couple feet in depth, the more conservative diver automatically sets the ascent with no signals being exchanged.

From the shallow stop or a safety stop, a thumb up followed by "OK?" either gets a response of "OK" or an indication of how much longer the other person wants to hang. As with the other signals exchanged, it doesn't really matter whether the hang time is to clear a stop showing on a computer, or if a diver just wants to extend a stop for whatever reason.

People that I dive with frequently catch on that I do a sliding final stop that starts around 20' and gradually goes to 8-10' level, and know that I'm not ready to ascend until I'm up around the 10' level. They only signal "Up" "OK?" if they are getting impatient.

 

[miketsp]

So how do you guys let your dive buddy know you have a deco obligation ... or how much of one you have? How do you signal when you've cleared your obligation?

... Bob (Grateful Diver)

Point at computer to indicate where data came from.
Make an active T sign holding left hand flat and bringing right hand flat vertically up against the palm to form the stem of the T to indicate a ceiling.
Then no of fingers for ceiling depth (m)
Then point at watch with index finger sliding around bezel to indicate time
Then no of fingers for time (minutes).

Even someone that never saw this sequence before should understand what you want to convey.

 

[MXGratefulDiver]

Even someone that never saw this sequence before should understand what you want to convey.

Perhaps ... but I'd prefer not to make that assumption.

This is a good demonstration of why it's important to go through hand signals and sequences when you're diving with people other than your usual dive buddies ... lots of different ways to communicate the same information.

Joe Talavera wrote a good article on the subject once ...

... Bob (Grateful Diver)