Gradient Factors For a Rookie

[inquis]

Manipulating gf lo gives you your first stops, the hi value ultimately determines their length as you can’t surface till that has dropped low enough.

You just gave the case where one could surface DIRECTLY with tissue loading less than GFHigh. Why would you increase the loading by making various stops... in order to exit with a tension that is NOT less than GFHigh?

 

[PfcAJ]

You just gave the case where one could surface DIRECTLY with tissue loading less than GFHigh. Why would you increase the loading by making various stops... in order to exit with a tension that is NOT less than GFHigh?

I guess that comes down to why are we doin anything with a gf lo.

Isn’t the idea that we need the gf low to stop deeper to prevent an adverse event? There is of course a trade off, but otherwise, we’d just rock the same lo and hi values.

Not an expert on the math here, genuinely trying to get a hold of this stuff.

 

[inquis]

Isn’t the idea that we need the gf low to stop deeper to prevent an adverse event?

That's certainly a mystery, but a different issue. Pretty much by definition, an NDL dive *can* make a direct ascent. The trajectory in the tissue loading vs pressure graph seen in the GF explanations is nearly horizontal, moving left (lower depth). Crossing the GF-line at any point above GFHigh means you would have crossed it earlier when you had less tissue loading. The "limit" is that tissue loading where you would just cross the line at surface pressure.

A shorter time starts lower on the graph, and shooting left (ascending in the real world) reaches the surface with loading under GFHigh. Staying longer starts higher, and shooting left hits the line before reaching the surface. Therefore, you have to stop.

 

[rongoodman]

Shearwater Teric and Perdix both default to OC recreational mode. On the Teric, the default conservatism for OC rec is 40/85 (pg 27). I don't own a Perdix, but the manual says the default is 30/70, as of 2019 (pg 39). Not sure why there is a difference, or if that is still true today, somebody who owns a Perdix can say for sure.

I think the default has always been 30/70 in tech mode, no matter which model.

 

[100days-a-year]

Can't you chose to set custom gfs like 77/84? And for NDL I can't see it making much difference. Maybe a minute here or there but more an anorak task than practical.

 

[Blackcrusader]

I think the default has always been 30/70 in tech mode, no matter which model.

I dive in OC Tec mode with 45/95 on my Perdix AI. Rarely doing any light back gas deco mainly recreational NDL dives.

 

[Blackcrusader]

Can't you chose to set custom gfs like 77/84? And for NDL I can't see it making much difference. Maybe a minute here or there but more an anorak task than practical.

Over say 40 dives in 12 days it will make a difference.

 

[EFX]

I guess that comes down to why are we doin anything with a gf lo.

Isn’t the idea that we need the gf low to stop deeper to prevent an adverse event? There is of course a trade off, but otherwise, we’d just rock the same lo and hi values.

Not an expert on the math here, genuinely trying to get a hold of this stuff.

Erik Baker's original idea when he introduced gradient factors was to make the entire dive profile more conservative and not just at surfacing which the GFHi controls. Introducing a GFLo made the first stop deeper and added additional decompression stops. The deeper stop had a negative effect in that it allowed the slower non-controlling tissues to on-gas longer than they would for a GFHi only dive. As the ascent progressed some of those slower tissues could become the controlling tissue. The trade off is a reduced rate of off-gassing at the shallower stops at the expense of longer shallower stops. The lower the GFLo the deeper the stop and the longer the shallower stops become for the same GFHi.

While the decrease of GFLo may suggest safer dives, the increase in gas to complete deco stops and exposure to the environment will tend to offset whatever gains in safety are achieved by lowering the off-gas rate. This suggests a sweet spot for a GFLo value.

 

[KenGordon]

1) check if I can ascend directly to the surface (that's based on M0 aka GF Hi),

Which bits of Baker’s writing or code supports this?

 

[inquis]

Why wouldn't one check the easy case first?

Edit: the whole concept is about not crossing the GF line. By definition, that's the ceiling. Not crossing it while ascending means no ceiling and therefore no stops.