GF's on Shearwater and Vision

Ben Field

Just back from a cracking weekend diving the lovely wrecks off Newquay. Did dives with buddies diving another Shearwater and the Inspiration Vision.

In both cases we chose matching GF's for the dive, Shearwater's reassuringly gave almost identical profiles.

Diving against the Vision stops started the same but then change to a creeping ceiling rather than defined stops as per Shearwater (was diving 15:85)

Also noticed Vision does not give a stop time? Is this correct or is it in another menu perhaps, much helium fueled chatter on the shotline about this to the amusement of passing OC divers However TTS seemed to be pretty much the same all the time, again rather reassuring.

Anyone got a plain english explanation of the visions deco behaviour? I'd like to understand it better so can be abit more sympathetic to my Vision buddies, couldn't understand why he had to stop every meter!


On the other hand it was nice to be diving two very different units and coming out on about the same time frames, handy as our group has two visions and two shearwater (plus one boris but we don't like to talk about that )

BEN

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decoweenie

I hope this is simple enough...

Vision computes a "moving" ceiling which is the current allowable depth of the slowest tissue (without overpressuring).

Shearwater (and most tables) set a discreet step of 3m for each deco stop, and wait for all tissues to clear before the next 3m stop.

So, in theory, you could move up continuously with the Vision as long as you move slow enough (at depth) and really slow (shallow). Whereas you can't violate the 3m/10ft step of the Shearwater.

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dave t

Phi

ceiling is dictated by the leading tissue isnt it? fast or slow its the one that is closest to the overpressure limit??

at least thats how I understand it

decoweenie

Dave,

I believe we are talking about the same basic thing in the grand scheme, I was thinking along the line of slowest to off-gas.

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Ben Field

If I read that right the vision diver could stop when he/she first meet the ceiling and pull a stop for a set period of time (rather than follow the ceiling.) Then move up to where the ceilings got to while they where stopped and stop there next, thus converting a moving ceiling to stops? If they needed to match a table generated GF profile.

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dave t

Phi

Ah ok, yes we mean the same thing

Ben

The beauty of this moving ceiling is that you can deco more or less where you like (within reason) which makes the ascent much more flexible but its still best to stay just under the ceiling and follow it up.
An example, I dived with a guy using a vr3, as we started to ascend my ceiling moved shallower. Stopping with my buddy while he did his pyle stops as per the vr3 actually caused my ceiling to begin going deeper again. But following my shearwater with GF's set the same as my vision and doing the shearwater stops more or less kept my ceiling reducing in depth. When it comes to the shallow stops and the shot is crowded with divers you can sit a little above or below the crowd as long as its under your ceiling, some fixed stop computers would shout at you if you did that

At the end its all theory anyway but I like the ceiling over the fixed stops

Dave

PhilSiswick

Hi Dave,

Thanks for the clarification (I was one of the divers Ben was experiencing this with last weekend).

I have one more question - to what degree is one penalised for staying deeper as the ceiling creeps up (e.g doing 3 minutes at 6, rather than following the ceiling up to 3 as it lifts?). I was trying to work this out empirically, but didn't want to prolong my buddy's deco unnecessarily. Is it like Suunto 'minutes' where a minute that should have been done at 3m takes 1.5ish minutes at 6m?

This is my first real experience of GF deco, other than in theory and I like it, but obviously want to understand it better.

Cheers,

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dave t

Phil

I my experience it doesnt seem to make much difference once you are at 6m. I have tried it both ways and came to the conclusion there was maybe only a minute or two difference. I feel more comfortable staying at 6m as I guess thats what I have always done but its nice to have the flexibility, for instance when decoing in the red sea it means I can mooch around on the reef looking at the beasties and not worry too much about my depth as long as Im under my ceiling.

I have to say there are some critics for this type of deco but it works for me even from fairly deep dives

Dave

Ben Field

I've seen critisim of most systems but not GF's, what specifically does anyone have against Gradient Factors?

I come from a background of Recreational diving with Sunnto's and OC Trimix diving with traditional tables (perhaps with Pyle stops thrown in) to me GF seems like an obvious and intelligent blend of the two.

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dave t

I meant the moving ceiling. GF's are pretty much accepted as good stuff

To my knowledge the only other computer I have seen with a moving ceiling was the "skinnydipper" made by a company called orca, (actually I still have one) was in my opinion well ahead of its time. was based on a bubble model as far as I can remenber

Dave

ps I think GF's are the dogs danglies!! but you need to understand them and how they work