Gradient Factor for Dummies

01RMB

As mentioned, looking at the gragh shows that if you have a low of 85 then you will be running close to straight Buhlmann for the deeper stops (or typically not much in the way of deep stops).

When you get shallower because you are only using a high GF of 15 you are very close to the ambient pressure line which will mean that you will be in the water for a long time since you are going to only be slightly shallower than ambient pressure and thus decompressing very slowly.

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Cheers
Rod

UWSojourner

Hi Peter,

I can think of a few reasons:

1. Deep stops - there isn't anything magical about the GF parameters. If you use a low GF that is close to Buhlmann's M-value (e.g. 85 ) then you sort of negate the reason for the GF method in the first place ... to generate deep stops and slower ascents into pressure.

2. Bubbles - the idea of deeper stops was to reflect a growing awareness of bubbles developing in decompression Buhlmann-type profiles. Some would say your hypothetical strategy would be an extreme form of bubble development / bubble treatment ... your high GF allows substantial bubble formation, and then your extreme shallow stop (very low surfacing GF) treats the bubbles just generated.

3. Time - the shallow stops required to get a diver down to a GF of, say, 15 would be very very long.

Ant Slegg

GFs is one of those topics I understand and then have a devil of a time explaining to anyone - thanks for the clarity in this article.

One question - Am I right that the graph shown in Post No.7 shows only one of the 16 TCs and that the trick that s/w does so well is to track all 16 TCs to make sure that none are crossing the line that would take their tissue pressure above that allowed by the GF at that depth?

Ant S

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Ant' S
Learning to dive since 1990...

dantheman

Yes, sort of.

It shows the leading compartment, which controls what your ceiling is at any point in time.
So, it's one compartment, but not necessarily the same one from stop to stop ..

--dan

UWSojourner

Just for clarity, the leading compartment isn't the one winning the deco race .

At any point in your ascent one of Buhlmann's 16 compartments will be the violating compartment, that is, the compartment requiring the stop because its GF is too high relative to the MaxGF line. The violating compartment is commonly called the leading compartment as Dan mentioned.

Ant Slegg

So the graph shows the combined profile of all 16 compartments reaching the GF line and the "violating/leading compartment" is the one driving the deco stop depth. The "Buhlmann" line shown is therefore a line connecting the M-Value lines for the 16 compartments rather than a single M-Value line (using as a reference Erik.C.Baker 'Clearing Up The Confusion About "Deep Stops"'.

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Ant' S
Learning to dive since 1990...

01RMB

The graph was intended to show a simple representation of Gradient Factors and what drives the deco profile but without showing the 16 compartment lines as shown in Erik.C.Baker 'Clearing Up The Confusion About "Deep Stops"'. As such you would probably best see it as a one compartment model but you could also see it as showing the leading compartment.

Rather than a copy of the figures shown in Up The Confusion About "Deep Stops"' I find a simple picture explains things well for me and thought it may also be helpful for others.

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Cheers
Rod

decoweenie

... and it does a perfect job of that. Don't worry about it.

As you said, think of it as a simple way to explain the concept of GF without having to bog down about multi-compartment, etc.

Ant, don't be offended but IMHO, if you can't explain something, you don't understand it (well enough). For example, sometimes I've asked students (whom I had doubt that they understood the issue) to explain back to me. If they repeat verbose what I said, then I know... Those who did understand will explain the way they understood in their mind.

Ant Slegg

Ant, don't be offended but ... Those who did understand will explain the way they understood in their mind.[/quote]

Not offended - the assumption I think I was missing was that the "leading" compartment remains the same one throughout the example.

Ant S

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Ant' S
Learning to dive since 1990...

svendv

Kevin,
Dear Forum members,

I was lucky to receive a copy of your article. It is really a good initiative to write such articles, helping us all understand basic decompression principles.

I have a question, so I registered on the Forum to post it.

Why is the Gradient considered T-p.abs (as also indicated on the pressure T/P.abs graph by E. Baker) and not T-pp.N2.alv. ?

In fact it is the partial pressure difference who makes the Gradient, not the absolute pressure.

I don't understand why the pressure graphs show an ambient pressure line, giving for P.abs=1 --> T=1, normally T should be P.N2.alv=(P.abs-0,063)x0,79 for air according Bühlmann. This line gives you the equilibrium situation, above you desaturate, unther you saturate.

Best regards,

Sven.