Wisdom of trusting one's dive computer?

[JohnN]

I dive 45/85 and routinely dive small deco amount dives - something along the lines of allowing myself to get 10-15 minutes of deco (TTS 15min max). These are at rec depths with a source of some deco gas. My first stop has never been deeper than 20' per the computer.

With that being said and those type dive profiles - diving 85/85 or 45/85 is no different.

The dives I do are recreational dives, I'm right there with everyone else in the group other than I may stay a bit deeper longer than the others - it would be exactly like someone that accidently allowed themselves to slip past the NDL.

FWIW, here are a couple of dives (asuming you had enough gas to have that much bottom time). One with 40/85, one with 85/85. All depths are in meters. Since this is purely a recreational dive gone bad, back gas is your deco gas.

To me, this is not an insignificant difference

 

[rjack321]

To me, this is not an insignificant difference

Double negative?
I think you meant to say 45/85 vs 85/85 is an insignificant 1 min difference.

 

[ChuckP]

FWIW, here are a couple of dives (asuming you had enough gas to have that much bottom time). One with 40/85, one with 85/85. All depths are in meters. Since this is purely a recreational dive gone bad, back gas is your deco gas.

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To me, this is not an insignificant difference

There's no real difference there at all. One minute and in reality, a short three minute safety stop covers that completely.

I miss spoke earlier, I dive 45/80 for recreational and light deco dives - depending on the dive schedule, how I feel etc.. I typically hang out till SGF drops to 75ish.

On a typical recreational dive profile like I do, a profile that might be typical of any recreational diver not paying attention and slipping into deco accidentally - there is for all intents and purposes no difference between 45/80 and 80/80. You can see attached screen shots of two dives - you could look at it as a diver that didn't pay attention at all and was into deco, the first stop is 20' (I have my computer and do all my planning as the last stop being 20' - personal preference).

Now - if you're a believer in deep stops and dive like 30/70 or something, maybe it'd ask you to stop earlier - my decisions are based on my diving and my experience and I'm talking recreational depths only. I do not want to imply that it's ok to ignore your NDL - it's not - nor do I want to imply that this or that GF is ok for you.

 

[TMHeimer]

Been away too long to read them all, but I'll of course through in my 2 cents.
Agree with drrich2 way back in saying it depends on what kind of diving you do.
My usual 30' shore diving in sites I know thoroughly, I probably need nothing but the old SPG.
When I have done typical rec. boat dives to 80-100' or so I use computer and watch/tables.
I am informed of the depth and usually spend most of the dive(s) on the bottom, meaning
planning with tables (or eRDPml if you like--which I have never used other than for the DM tests) makes some sense. Then I follow the watch and the computer, since the latter tracks my real bottom time. Then I always mention that the computer's "remaining bottom time" doesn't have the little bit of conservatism using tables and analog stuff has. I never hear this mentioned.

 

[wstorms]

FWIW, here are a couple of dives (asuming you had enough gas to have that much bottom time). One with 40/85, one with 85/85. All depths are in meters. Since this is purely a recreational dive gone bad, back gas is your deco gas.

View attachment 550103

View attachment 550104

To me, this is not an insignificant difference

This is an interesting example. For me, I would not consider 60 minutes at 30 meter a recreational dive. If I run this profile, even with GF100, I end up with 20 minutes deco (17 with ZH-L16-B).
Although it is not a long hang if you are doing actual planned deco dives, this is (IMHO) way beyond any accidental deco obligation one might get by accidentally going a bit too deep / stay too long.

Most recreational tables (examples: PADI, GUE, NOAA) for 32% give you about 30 minutes bottom time within NDL /MDL limits, and in my case seeing 60 minutes at 30 meter combined with the table knowledge (and combined with experience) triggered a bit of a concern: "is that actually correct and in line with my personal expectations?"

Now if I would be diving with JohnN, I would ask to go over the plan together to be sure we are on the same page. During that discussion we would probably conclude that the plan(ning software) he used works as dropping from 0-30 in 60 minutes, therefor having a different average depth. This instead of spending the full 60 minutes at 30 meter. This understanding of the dive plan would allow us to come up with a plan we could both be happy with.
That illustrates that you really should plan dives in advance. Over time, you will get the experience to judge whether or not a proposed dive fits your definition of doable. If there is any doubt about that, you can discuss it with your team and adjust if needed.

 

[kelemvor]

Don't let the atypical scubaboard users fool you. Other than in a scuba class I can't recall ever having seen another diver do it any other way than just jump in. End dive on time limit (some boats say you have to be back in xx minutes) or end dive on low gas or end dive when DC tells them that they're coming up on the NDL limit - whichever comes first.

This only applies to "regular" recreational dives mind you. If you're diving at Ginnie Springs or someplace where tech diving and rec diving are both common.. then you'll see people doing stuff other than just following their computer.

 

[JohnN]

This is an interesting example. For me, I would not consider 60 minutes at 30 meter a recreational dive. If I run this profile, even with GF100, I end up with 20 minutes deco (17 with ZH-L16-B).
Although it is not a long hang if you are doing actual planned deco dives, this is (IMHO) way beyond any accidental deco obligation one might get by accidentally going a bit too deep / stay too long.

Most recreational tables (examples: PADI, GUE, NOAA) for 32% give you about 30 minutes bottom time within NDL /MDL limits, and in my case seeing 60 minutes at 30 meter combined with the table knowledge (and combined with experience) triggered a bit of a concern: "is that actually correct and in line with my personal expectations?"

Now if I would be diving with JohnN, I would ask to go over the plan together to be sure we are on the same page. During that discussion we would probably conclude that the plan(ning software) he used works as dropping from 0-30 in 60 minutes, therefor having a different average depth. This instead of spending the full 60 minutes at 30 meter. This understanding of the dive plan would allow us to come up with a plan we could both be happy with.
That illustrates that you really should plan dives in advance. Over time, you will get the experience to judge whether or not a proposed dive fits your definition of doable. If there is any doubt about that, you can discuss it with your team and adjust if needed.

It's unfortunate you didn't pick up on the intent of the post.

My point was (and remains) that for recreational NDL diving, in the rare case you encountered a deco obligation that required a stop, a 85/85 GF setting is unwise.

I'm not proposing a 60 minute bottom time @ 30 m, even if you had gills, that would almost certainly require doubles, at which point we could debate if this really a recreational dive. If I had a ~5 minute deco obligation (what the planning software came up with), I'd much rather take that first stop at 6m, rather than at 3m

 

[Fastmarc]

So @JohnN , what would you suggest to someone who owns a Mares Genius that defaults rec is 85/85 GF?

 

[wstorms]

Yes, I did miss that but do agree that 85/85 is not wise. If you are in an emergency situation (the dive didn't go as planned, you overstayed, went to deep) then being aggressive doesn't help.
Having said that, i still feel that this is the perfect example for the OP's question: should you trust the computer or preplan the dive (in a recreational setting)? My opinion is that the best way to do it, is to do both. If you end up getting 5 minutes of deco, the computer will bring you up safely (and probably more conservative than 85/85), but the preplanning really should show something interesting is going to happen if you overstay / go to deep. This in contrast to say 60 minutes at 15 meter which would be absolutely fine. And if you know that doing that particular dive might become more sporty (ie you plan to push the limits), you could / should opt to add some conservatism to the dive plan ("and after looking at the wreck, let's look at the reef for 10 minutes") or set the computer more conservative.

 

[NAM001]

FWIW, here are a couple of dives (asuming you had enough gas to have that much bottom time). One with 40/85, one with 85/85. All depths are in meters. Since this is purely a recreational dive gone bad, back gas is your deco gas.

View attachment 550103

View attachment 550104

To me, this is not an insignificant difference

Im not sure i see anything significant. I do see the sifference regarding 6m vs 3m. and again the difference if any probably only counts if you go into deco which rec divers do not do. Please help me on what I missed.