Why 2 gradient factors ?

[inquis]

How are you planning it?

Subsurface is one that does it.

GF Low can't be higher than GF High by definition.

Sure, the "low" and "high" monikers no longer make sense, but the first factor can be higher than the second. Think of them as GFdeep/GFshallow or GFfaster / GFslower.

GF High is the maximum percentage of any M value that is allowed at any point during the dive.

Not on a deco dive, which is the only time that the first gradient factor (GFLow or GFdeep or GFfast) applies.

 

[shark_za]

Am I wrong in my understanding?

GF High - overall % the model will be reduced by.
GF Low - the deco calculation working out the first deco with respect to the newly calculated M value cieling (taking GF High into account)

 

[inquis]

Here's the diagram just before ascent for a 90 minute dive to 100 ft dive for GF=50/80. (Ignore the blue extension to the left of the red circle; that's just showing the direct surfacing pressure.) The diver ascends along the blue line from the * on the right, until crossing the GFlow line (50%). This is the first ceiling (about 44 ft). (I'm drawing the GF line in red, rather than the 10 ft stair-stepped ascent profile.) They then follow the red line to the surface. (Again, in practice, a stair-stepped version of the red line.)

Here it is for GF=100/80. Again, ascend until hitting the first GF percentage (100%, and a ceiling of 28 ft), then follow the red line (will be stair-stepped in practice) until surfacing at 80%. (Here tissue 5 is the controlling tissue, since Tissue 4 sufficiently off-gassed before getting to the shallower first stop.)

 

[inquis]

Am I wrong in my understanding?

GF High - overall % the model will be reduced by.
GF Low - the deco calculation working out the first deco with respect to the newly calculated M value cieling (taking GF High into account)

Close. GF High is the overall % you will surface with. The cushion (or lack thereof) before surfacing depends on GFlow.
GF Low does work out the first deco stop, but that's with respect to the original M-value line, nothing to do with GFHigh.

 

[dmaziuk]

Am I wrong in my understanding?

GF High - overall % the model will be reduced by.
GF Low - the deco calculation working out the first deco with respect to the newly calculated M value cieling (taking GF High into account)

GF High: M-value at the surface.
GF Low: M-value at the first deco stop.

GF actual at stop X is the point on the line from GF Low to GF High, at depth X.

If GF High = GF Low: the actual M-value line is parallel to the model one (i.e. 100/100), but shifted down (GF < 100) or up (GF > 100) by the GF%.

If GF Low < GF High, the M-value line has a steeper slope and the first deco stop is shifted down (deeper).

If GF Low > GF High, the M-value line has a shallower slope and the first deco stop is shifted up (shallower).

On a no-stop dive GF Low doesn't matter unless you overstay your NDL and it's not a no-stop dive anymore.

Common implementations will not allow negative GFs, nor (usually) GF Low > GF High.

 

[inquis]

If GF High = GF Low: the actual M-value line is parallel to the model one (i.e. 100/100), but shifted down (positive GF) or up (negative GF) by the GF%.

Minor correction: the line is shifted up with a GF > 100. (A negative GF would be below the ambient pressure line.)

 

[dmaziuk]

The line is shifted up with a GF > 100. (A negative GF would be below the ambient pressure line.)

Right, bad brain, no cookie. Post fixed.

 

[lowwall]

Close. GF High is the overall % you will surface with. The cushion (or lack thereof) before surfacing depends on GFlow.

Where are you coming up with this definition of GF High and Low? This does not match my reading at all. GF High is the maximum allowable percentage of the M value at any time during the dive. GF Low is only used to determine the ceiling for the first stop (if any).

The theoretical basis of gradient factors is Erik Baker's papers on using percentages of M values to determine conservatism in ascent profiles. GF High corresponds to what he calls the "personal limit" or "fixed margin of safety" over the maximum tolerable overpressure value aka M value.

Here's how DAN puts it at Gradient Factors

This brings us back to gradient factors, which are defined by two values: The first number of the pair (“GF low”) represents the percentage of the M-value that establishes the first stop during ascent; the second number (“GF high”) is the percentage of the M-value that should not be exceeded at any point during surfacing. The dive computer effectively draws a straight line between the two, creating the ascent slope.

Note that GF High is not to be exceeded at any point "during surfacing." It is not only considered at the surface, it's during the entire ascent.

Shearwater had essentially the same wording at Flexible Control of Decompression Stress - Shearwater Research

Gradient factors are typically applied in two steps. The first number (GFlow) represents that percentage of the M-value used to determine the first stop during ascent. The second number (GFhigh) is the percentage of the M-value that cannot be exceeded at any point during surfacing. The dive computer effectively draws a straight line between the two, creating the ascent slope. The alteration of limits is applicable to both technical and non-technical exposures.

And we have already seen that OSTC uses the same definition.

But even if we were to agree that GF High applies only at the surface, setting GF Low greater than GF High would mean that during a deco dive, a substantial portion of your initial ascent and some time around every stop you will have tissue saturations greater than you feel is safe at the surface. Why would you want to do that?

 

[shark_za]

That is why I understand the GF Low to take the GF High into account. The lower the GF Low becomes the deeper you will need to stop and wait a little but you dont ever want to go over the GF High. (the overall conservatism factor)

 

[dmaziuk]

That is why I understand the GF Low to take the GF High into account. The lower the GF Low becomes the deeper you will need to stop and wait a little but you dont ever want to go over the GF High. (the overall conservatism factor)

No. Going "over GF High" could in theory give you the first stop above the last stop, i.e. a nonsense profile, but off the top of my head I'm not even sure what sort of number would be required. Other than that: no.