Question GF Low

[MyBuddy]

I’m talking about the crossing of the vertical line through the GF Low/High points with the horizontal ambient pressure line (X-axis). Look at the vertical arrow at the right side please.

The graph above posted by @Wibble should be read from right (Where the “Accent Starts” arrow is.) to left (At the surface).

The GF Low is the percentage, in this case 30%, of the difference between the M value and the ambient pressure of the given depth that the diver doesn’t want to exceed at the beginning of the accent. The GF Low ONLY APPLIES TO DECO DIVES. That maximum percentage of the M value gradually increases from the desired GF Low at the start of the accent to the desired GF High at the end of the accent at the surface. There is not an any one pressure associated with a GF Low or GF High.

I would recommend reading this in-depth Shearwater article on Understanding M-Values.

 

[Wibble]

The graph below shows the:

• Ambient pressure (X axis) -- the pressure acting upon the diver's body and all air spaces
• Tissue tension ambient pressure (Y axis) -- over time, the pressure squeezes the inert gases (nitrogen, helium) into the diver's tissues (gas diffusion), call it compressing.

If a diver does a dive down to 40m/132ft/5ATA (ATA = Atmospheres Absolute) the gases would diffuse from the breathing gas in their lungs, through their blood and absorbed into their other tissues. The speed that the inert gases are absorbed depends on the type of tissues, some absorb gas quickly (e.g. blood), some slowly (e.g. bone), and other tissues in between (muscle, brain). These are called "tissue compartments" for the decompression models.

Over time the tissues will reach an equilibrium where no more inert gas can be absorbed by those tissues; the tissues are saturated, fast tissues quickly saturate, slow tissues very slowly.

As the diver ascends, the ambient pressure reduces, and the inert gases inside their tissues will diffuse OUT of the tissues. This needs to be done very slowly to allow the gases to dissolve back into the blood and out though the lungs to be exhaled. If the diver was to bolt to the surface after a long time on the bottom, the gases would turn into bubbles and the diver would suffer from Decompression Sickness (DCS) -- the "bends".

If a diver did a "bounce" dive meaning they descended quickly, then came up immediately, not much inert gas would get into their tissues as it takes a while for it to diffuse. There would be very little or no decompression required (obviously they'd do a safety stop!).

However, if the diver stayed down for a long time (e.g. 90 mins at 40m/132ft/5ATA), then they would have a considerable decompression obligation as many of the tissue compartments will become saturated and much time (probably over an hour) would be required before the diver could surface as there's a lot of dissolved inert gas that takes a long time to come out without forming bubbles.

The timings for this depends upon the decompression algorithm chosen and any "fiddle factors" to allow for greater or lesser safety.

Gradient Factors are the Buhllmann algorithm's method of adjusting the time taken to decompress. This creates a "ceiling" where NO tissue can be allowed to exceed a certain "tension", i.e. the amount of pressure the inert gas inside that tissue experiences as the ascent progresses. In the diagram, there's the stepped line where the diver ascends until the "ceiling line" is met, then stops (at their decompression stop) to allow the inert gas to slowly diffuse out of the tissue until it's safe for them to ascend further (typically 3m/10ft) and wait again.

As the ambient pressure reduces, the time taken for the inert gas to diffuse out of the tissues slows, hence deeper decompression stops are much shorter than the shallow stops (the rule of thumb is the 6m/20ft stop is about half of your total decompression time).

Am sure that Mark Powell's presentation would explain this better than me!

 

[BlueTrin]

Still not at the same frequency… let me put it this way: the more you move the GF Low to the left, the flatter (more horizontal) the ceiling line becomes. The more you move it to the right the steeper. So, the place where you put GF Low influences the ceiling line.

it does not have a fixed value, it is set at the first stop (on the X axis of the graphs above)

It is set so that you would not break 30% between the two lines at your first stop (m-value and ambiant lines)

Also you got it wrong, GFLo is on the right, so if you move it to the left it would become more steep, I think.

Another way to say it is that it is the pressure at which you’ll do your first stop such as you never cross the value below under a certain assumption of an ascent rate.

[ambiant(stop_depth) + GFLo * (mvalue(stop_depth) - ambiant(stop_depth)]

stop_depth is a positive multiple of 3 and >= 6 usually.

(Oh @dmaziuk , Buhlmann god, I invoke your powers to correct me above if needed)

 

[JMarc]

If I understand your question correctly, read Bühlmann – The Theoretical Diver and search for "Looking at the graphics that is supposed to define gradient factors" to get the opinion of the author of the relevant code used by Subsurface.

 

[L13]

Still not at the same frequency… let me put it this way: the more you move the GF Low to the left, the flatter (more horizontal) the ceiling line becomes. The more you move it to the right the steeper. So, the place where you put GF Low influences the ceiling line.

Not really.

Using your concepts(not standard terminology):

GF-Low is defined at the ambient pressure of your first stop. It isn't at a fixed ambient pressure. It is determined procedurally, and is always the ambient pressure at the stop depth immediately below where your GF would exceed GF-Low.​

​

Also, GF-High is not defined just at at 1 ATA. GF-High and GF-Low are defined at all ambient pressures. But procedurally, we only use the GF-High value at 1 ATA, and the GF-Low values only used at the first stop. For all stops in between, we use a GF that is linearly interpolated between the two.

 

[BlueTrin]

Out of curiosity, does any know when a Shearwater will decide you reached your first stop and will ‘anchor’ GFLo?

 

[Wibble]

Out of curiosity, does any know when a Shearwater will decide you reached your first stop and will ‘anchor’ GFLo?

Never easy…

It’s when any (of the 16) tissue compartments has an inert gas loading that ‘touches’ the Gradient Factor ceiling line. It’ll stop you at the lower 3m/10ft depth.

 

[BlueTrin]

Never easy…

It’s when any (of the 16) tissue compartments has an inert gas loading that ‘touches’ the Gradient Factor ceiling line. It’ll stop you at the lower 3m/10ft depth.

But they must add some tolerance?

I don’t touch the line (or maybe I did but there are no witnesses)

 

[L13]

But they must add some tolerance?

Why? The algorithm is already so fuzzy.

Measure with a micrometer, cut with a chainsaw...

 

[BlueTrin]

Why? The algorithm is already so fuzzy.

Measure with a micrometer, cut with a chainsaw...

Because you may never touch the line and then you’d never set the GF lo.

I imagine they set some arbitrary tolerance where if your distance to this stop is less than tolerance then you started your ascent for the purpose of GF factors.

Or maybe it’s based on your max ceiling?