Unrealistic Deco Time

[WetLens]

I had a recent experience with my Wisdom II that really concerns me. First, let me say that I screwed up. I should have been watching my time/depth more attentively and not gotten myself close to deco. My bad, no question. However, how does one go from just flipping into deco at 45' and wind up with 44 minutes of 10' stop time 5 minutes later - while ascending? This makes no sense.

Situation: 4th dive of the day and started the dive with 5 NIBR. 1:08 into the dive (avg depth 47') went into deco (current depth - 46'; deco obligation - 4 minutes). Notified my buddy of the need to ascend and began ascent at 1:10 (current depth - 44'; deco obligation - 24 minutes!). Hit roughly 10' at 1:13 well within ascent rate deco obligation 43 minutes!!!!!

Again, looking at the profile, I know that I was it was too deep/too long of a dive for a fourth dive of the day. I was not watching my depth/time closely enough to avoid deco. I understand all of that. What I don't understand is how I can incur 43 minutes of deco obligation after just 5 minutes in deco. I can find no basis for that in any modeling that I've done.

I did breath out the tank at 10'; returned to the dive center and got at 32% nitrox tank and returned to depth for well in excess of the noted deco obligation. Needless to say, the computer was locked out until the clock cleared a couple of days later (continued to dive with backup computer).

Attached is spreadsheet with download data and graph of dive. Can you shed any light on what would cause this?

 

[WetLens]

Well, that didn't work... Try again. Added as attachments.

 

[gsk3]

4th dive of the day

Sounds like that may be your problem. What did the other dives of the day look like (depth, time, profile, surface interval)?

Recreational dive computers are known for being conservative on deco, sometimes insanely and arguably unsafely so. That's because you shouldn't be doing deco without deco training. Not trying to beat you up, since you clearly know that and weren't intending this.

 

[WetLens]

Recreational dive computers are known for being conservative on deco, sometimes insanely and arguably unsafely so.

I understand being "conservative," not ridiculous.

 

[WetLens]

1st dive 8:22am. Max depth 72', Avg depth 45', 74 minutes.
2nd dive 11:18am. Max depth 71', Avg depth 40', 77 minutes.
3rd dive 2:37pm, Max depth 50', Avg depth 40', 76 minutes.
4th dive 6:18pm - details already given.

As I said, I know this was a lot of nitrogen loading. I am not questioning that I went into deco. What I am questioning is an algorithym that piles on 44 minutes of obligation in a 5 minute span - while ascending! Not sure I understand the basis for this. Are the deco police adding penalty minutes? If that is truly what the software engineers intended, then I'll know that I need to be looking for another computer. Believe me when I say that I understand that I need to improve my attentiveness, and I need to lose weight, and I need floss, and I need to ...

 

[BabyDuck]

and that's why people who plan on deco diving use computers designed for deco - rec computers aren't generally deco friendly or even deco reasonable and can be deco stupid.

i have a friend who hangs her backup at 20 ft on a line when she & her primary finish a dive. what's the point of a backup then?

 

[a22shady]

1:08 into the dive (avg depth 47') went into deco (current depth - 46'; deco obligation - 4 minutes). Notified my buddy of the need to ascend and began ascent at 1:10 (current depth - 44'; deco obligation - 24 minutes!). Hit roughly 10' at 1:13 well within ascent rate deco obligation 43 minutes!!!!!

What I gather from the way you wrote is you kept passing the your Deco stop depth that the computer wanted you to stop at. Doing this will absolutly make your time keep shooting up as each stop you miss and pass the time it compunds a lot and add it to next stop. And reading what you did It does not seem like you made any stop's other then the one at 10'. Either since you do not do deco diving you did not notice the stop depth in deco mode that the computer showed it made the time climb and this sounds right to me. I do not know what depths you should have stopped in but you go to Vplanner I believe you can plan those dives and times and it will give you what depths you should have stopped at to check.

 

[WetLens]

If you look at the spreadsheet output, it was telling me to go to 10'. I skipped the "safety stop" at 20' and went directly to 10' just what the computer said to do. I understand deco planning. I understand deco theory. Does it make sense to go from 0 to 44 minutes obligation in 5 minutes while ascending - ascending from 45'?

 

[Doc Intrepid]

Yes it makes sense.

When you "miss" a computer-identified deco stop for X minutes at Y depth, the computer defines this as a "skipped deco stop" - and compensates by recalculating for that missed deco on the next stop - which would logically be 10' higher (many models calculate a deco schedule at 10' intervals during ascent).

You stated that you skipped a deco stop identified by your computer at 46 fsw:

"1:08 into the dive (avg depth 47') went into deco (current depth - 46'; deco obligation - 4 minutes). Notified my buddy of the need to ascend and began ascent at 1:10 (current depth - 44'; deco obligation - 24 minutes!)."

Clearly if you "began your ascent" at 1:10, then you failed to hold a deco stop identified at 1:08 at 46' for 4 minutes.

The computer noted that you skipped your stop, missed the identified deco, and accordingly recalculated adding compensatory deco at the next stop. The algorithm will progress geometrically along a steep curve - because the manufacturer does have some liability built in there.

Three minutes later at 1:13 you're at 10'.

Your computer is screaming at you because you missed every deco stop between 45' and 10'. Every deco stop that you missed compounds your required deco time. As you were ascending from 45' to 10' in 3 minutes (according to data you supplied), the computer simply kept recalculating your required deco time adding compensatory deco for each stop that you missed between 45' and 10'.

You confessed to being particularly surprised that your time kept expanding so rapidly 'while you were ascending'.

One critical element of the deco topic that you may not understand completely is that ascending and ascent rate are one of the two primary factors that drive decompression rates - which for this post I'm defining as the rate of N2 molecular diffusion across a semipermiable membrane (alveoli). (The other being which gas you happen to be breathing.) Without getting too far down in the weeds, what drives molecular diffusion rates is the ratio between high N2 density on one side of the membrane (influenced in turn by the rate at which gas releases from solution), and low N2 density on the other. The diver determines molecular density and rate of gas release from solution on the circulatory system side of the membrane in part by controlling ascent rate. Breathing gasses that contain little or no Nitrogen in them will help keep N2 density low on the lung side of the membrane. The comparative variance between the two sides (or 'ratio') is determinative to the rate at which nitrogen diffuses out of the bloodstream via the lungs. Therefore, in this specific case your ascent rate - and ascending from 45' to 10' in 3 minutes is a relatively high ascent rate when you have a deco obligation and missed deco stops - determined in part the rate at which N2 gas released out of solution, and impacted the deco obligation identified by your computer. You went from 2 ATA to 1 ATA in less than 3 minutes. This might not be wise when you have a deco obligation and you've missed stops. Given that your computer identified a 4 minute stop at 46 fsw, which rounded up would be your 50' stop, you would logically have had a deco schedule of X minutes at 40', Y minutes at 30', and Z minutes at 20', in addition to whatever minutes you initially had scheduled at the 10' stop. By going directly to 10' you ommitted decompression scheduled at the 46' (50'), 40', 30', and 20' stops - which is what caused your computer to calculate the lengthy 10' stop. Your computer was trying to compensate for this set of conditions - high ascent rate, compounded exposure to bubbles coming out of solution, missed stops - by adding compensatory deco time to your profile. When you finally quit ascending at 10', you had an impressive amount of hang time waiting for you.

Remember that divers have followed all the rules, paid attention to their deco exposure, and have still been bent. Deco isn't an exact science - deco is imperfectly understood in the medical and scientific communities.

But one thing that is clear is that divers can get lucky one time, and can get bent the next time - so I encourage you to take some deco training courses, learn all you can about seed bubble formation, the different models (such as RGBM and VPM-B, etc.), and keep your deco diving to within conservative limits to avoid any need for a quick trip to a chamber.

Hope this helps. Regards,

Doc

 

[WetLens]

Doc: Thanks for taking the time to respond. I did not put this post in the deco thread as I was not looking to get into an in-depth discussion of decompression theory - I was trying to understand why my gage reacted the way it did - which I do not believe to be based in decompression theory.

I am having trouble following your logic - primarily on the "missed" deco stop.

When you "miss" a computer-identified deco stop for X minutes at Y depth, the computer defines this as a "skipped deco stop" - and compensates by recalculating for that missed deco on the next stop - which would logically be 10' higher (many models calculate a deco schedule at 10' intervals during ascent).

I did not "miss" a deco stop - I cut it short, but that is much later in the dive. That is another topic altogether - one that may belong in the deco section.

You stated that you skipped a deco stop identified by your computer at 46 fsw:

Clearly if you "began your ascent" at 1:10, then you failed to hold a deco stop identified at 1:08 at 46' for 4 minutes.

Let me see if I follow this logic... Ascend from 47' to 46' and stop for 4 minutes. The average depth of the dive to this point was 47'. The maximum depth of the dive was 59'. Why would I "stop" at 46'? This would simply add more time to my deco obligation.

At 1:08 the computer was telling me to go to 10' and spend 4 minutes. [In all honesty, I likely did not see the computer at this point - it was probably at the 1:09 mark, perhaps even 1:10, before I recognized the situation.]

The computer noted that you skipped your stop, missed the identified deco, and accordingly recalculated adding compensatory deco at the next stop. The algorithm will progress geometrically along a steep curve - because the manufacturer does have some liability built in there.

Three minutes later at 1:13 you're at 10'.

Your computer is screaming at you because you missed every deco stop between 45' and 10'. Every deco stop that you missed compounds your required deco time. As you were ascending from 45' to 10' in 3 minutes (according to data you supplied), the computer simply kept recalculating your required deco time adding compensatory deco for each stop that you missed between 45' and 10'.

Again, I fail to see how I "missed" every stop between 45' and 10'. The computer was telling me to go to 10' - and I was, a bit slower than it wanted me to. My ascent rate was about 10 feet per minute due to following the slope of the bottom. When we hit the area of the boat, my direct ascent to the 10' stop was in the 26-30 feet per minute range. I understand that ascent rate is part of the decompression process. As it was slower than the theoretical 30' per minute, it was rightfully counting my ascent time as a continuation of the dive. I can accept that.

What I can't accept is the rate at which it added the required 10' decompression stop time. This is where I cannot find logic.