ceiling/GF

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Got it, that makes sense to me.

Rephrasing to make sure, the GFLow point becomes defined what you get to a point that a compartment is at X% of the M-value. That point can be predicted when you begin you ascent, but can change based of your ascent rate. Once that point is defined it creates the ceiling.

I wouldn't put it that way. The GFLo is a number picked ahead of time, arbitrarily selected, but used (along with GFHi) to generate an ascent plan. The other three variables are depth, mix and bottom time. GFLo is not something that is generated or changed during a dive.

Your ceiling at any give time is defined by a specific overpressure limit. That limit rises in a linear fashion from GFLo to GFHi, over the course of the ascent.
 
I wouldn't put it that way. The GFLo is a number picked ahead of time, arbitrarily selected, but used (along with GFHi) to generate an ascent plan. The other three variables are depth, mix and bottom time. GFLo is not something that is generated or changed during a dive.

Your ceiling at any give time is defined by a specific overpressure limit. That limit rises in a linear fashion from GFLo to GFHi, over the course of the ascent.

I think I’m trying to say the same thing. If you were able to add time to the graph by animating it, the point that is the right end of the ceiling, which is what I was referring to (not clearly or correctly) as GFLow, would change based of those variables - time, mix, and depth.
 
I think I’m trying to say the same thing. If you were able to add time to the graph by animating it, the point that is the right end of the ceiling, which is what I was referring to (not clearly or correctly) as GFLow, would change based of those variables - time, mix, and depth.

Hmmm... not quite sure I get it.

Nitrogen loading is dependent on time, mix and depth. To simplify, consider a square profile with only backgas (no deco gas). Knowing time, mix and depth, you then pick GFs and generate an ascent plan. If you don't have enough gas to complete that ascent with a contingency cushion, then you need to change your time, mix and/or depth. You can also manipulate your plan and gas needs by changing the GFs, but that's not a good idea.

So I think what you are referring to as the right end of the ceiling [line] is just your bottom depth, right? Or am I misunderstanding...?
 
Hmmm... not quite sure I get it.

Nitrogen loading is dependent on time, mix and depth. To simplify, consider a square profile with only backgas (no deco gas). Knowing time, mix and depth, you then pick GFs and generate an ascent plan. If you don't have enough gas to complete that ascent with a contingency cushion, then you need to change your time, mix and/or depth. You can also manipulate your plan and gas needs by changing the GFs, but that's not a good idea.

So I think what you are referring to as the right end of the ceiling [line] is just your bottom depth, right? Or am I misunderstanding...?

I’m not talking about changing the GFLow. I’m saying the depth at which you would reach reach the GFLow if you were to ascend at that moment changes throughout the dive base on time, depth and mix.

The point I was referring to as the right end of the ceiling Is the first stop.
 
I’m not talking about changing the GFLow. I’m saying the depth at which you would reach reach the GFLow if you were to ascend at that moment changes throughout the dive base on time, depth and mix.

The point I was referring to as the right end of the ceiling Is the first stop.

Oh, I see what you are saying...

Yeah, of course! That's the advantage of a dive computer for tech diving. Some tech divers think that's terrible - you plan the dive and dive the plan and that's it. But while I always have enough gas for the maximum depth and time that I plan (with the ascent backed up on a slate), I will often dive the computer for relatively shallow multilevel deco dives.

I take advantage of the fact that the computer is continually is tracking my N2 loading in all of the compartments, and that it will calculate a first stop on on the fly based on what I actually accumulated in my tissues. The computer knows GFLo is 30%. So at any given point in the dive, it will put up a ceiling where my leading compartment would be 30% supersaturated with respect to the M-line.
 
But why does the ceiling line exist before the first stop? What does it mean? At every point on the ascent, the ceiling line represents the overpressure limit, somewhere between GFLo and GFHi, and it represents what you as a diver have decided ahead of time will be the maximum supersaturation you will allow for your limiting component at that depth.

Going back to the graph, I agree that the ceiling line doesn’t need to exist before(to the right of) the first stop.

I think it would make the graph to cluttered to add it, but theoretically there could be a line at X% of the M-value (GFLow). The point at which your ascent intersects that line becomes your first stop and the right end of the ceiling.

Is that making sense now?
 
theoretically there could be a line at X% of the M-value (GFLow). The point at which your ascent intersects that line becomes your first stop and the right end of the ceiling.


Isn't that what this is?

junk.jpg
 
Isn't that what this is?

View attachment 482589
The red dotted line is wrong in this diagram if it claimed to represent the ceiling GF. It should have a kink at the point of the arrow, ie the first stop, and stay at 30% prior to the first stop as that is where the ceiling is until the first stop.

I see people talking about maximum depth in recent posts. Maximum depth does not come into this except indirectly as contributing to gas loading. These diagrams, especially with the confusion about this red dotted line, contribute to people thinking that the 30% applies at max depth and then the interpolation works between there and the surface. To be clear the interpolation starts at the first stop depth (but only in planners, dive computers can’t do that).
 
To be clear the interpolation starts at the first stop depth (but only in planners, dive computers can’t do that).
@Shearwater , I'd be interested in your comments. It was my impression that having set a GF of 30/80, the computer runs a continuous determination of the GF Lo ceiling, giving you a first stop depth at the closest rounded depth below a GF of 30 (in this example) and interpolating that on the fly to generate your 80% GF Hi surfacing overpressure. In other words, the Perdix will compute a real-time 30/80 plan based upon your max depth/time theoretical tissue loading. Why can't they do that?

As for the "kink", your point is valid, but only in portraying where a GF Lo 30 ceiling lies, if you dive deeper than planned. But without units, that is not useful information. Once you make an initial ascent (from a deeper max depth) that intersects the ceiling at a place other than depicted in the chart, the GF 30/80 interpolation is now a slightly different line. And the new stops based upon that new line is something that I thought the Shearwater computers specifically computed (as @doctormike implied above).
 
https://www.shearwater.com/products/perdix-ai/

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