Gradient Factors and recreational diving

[KenGordon]

Can somebody enlighten me?

From what I understand, it's about the partial pressure difference per gas between tissue pressure and ambient pressure.
If the tissue pressure is lower, you're on-gassing. If the tissue pressure is higher, you're off-gassing.
The m-value is the point where bubble forming might start, above the m-value it's critical supersaturation.

To make the ascent across the m-value line less aggressive, you apply a gradient factor. First value is a percentage of the supersaturation at the deepest part, second value is a percentage of the supersaturation at the surface. These two points give you a new ascent line, which is more conservative. I've tried to illustrate this in two diagrams:

Please correct me if my reasoning is incorrect.

View attachment 410570
I've used 50/80 as an example only.

What happens to that purple line if the computer ignores a GF high or low factor?

What is the new ascent line gonna look like?

The statement 'first value is a percentage of the supersaturation at the deepest part' is wrong. It is at the first stop. The purple line doesn't exist below the first stop (when it is 50) That bit is sometimes wrong in diagrams in texts describing GF. You will see a 30/70 GF line continue and pass off the page at 10 or 20 ish.

Actual computers will assume the first stop is at the deepest ceiling. If they do not do this some other ridiculous number related stuff may result in much longer shallow stops if the dive stays below the ceiling nearly all the way up.

 

[KenGordon]

So your description results in the red line for the dive (which will probably be the fastest of all compartments)
View attachment 410576
I know there's no scale present, but in this case you reach the NDL at the end of the bottom time.
I get it.

This diagram is wrong too.
First you will be on gassing still once you leave the bottom. The ascent line needs to be increasing to the left until you at least reach the ambient pressure line. Also it is likely to be a curve.

The dotted purple line does not exist. When the diver crosses 50% between ambient and mvalue lines he hits a stop. Now the dotted line can exist. It happens that if you just touch that 50% line shallow then because the next stop is at 65% or 80% they are not limiting, so the dive feels like it had no stops.

 

[Miyaru]

This diagram is wrong too.
First you will be on gassing still once you leave the bottom. The ascent line needs to be increasing to the left until you at least reach the ambient pressure line. Also it is likely to be a curve.

I understand that if I keep a steady ascent rate of 30ft/min, the pressure-difference doesn't drop linear. But please improve my graph.

The dotted purple line does not exist. When the diver crosses 50% between ambient and mvalue lines he hits a stop. Now the dotted line can exist. It happens that if you just touch that 50% line shallow then because the next stop is at 65% or 80% they are not limiting, so the dive feels like it had no stops.

Get it....just wiped it out partially to show that you surface at the GF low value.

 

[dmaziuk]

Well... would this be a good time to mention that if your perfusion (or diffusion) is the bottleneck, then the pressure difference may well drop linear. Or sufficiently close.

 

[atdotde]

There are small things that are up do discussion in how to implement them. I have written about them here: Why is Bühlmann not like Bühlmann

 

[Blackcrusader]

There are small things that are up do discussion in how to implement them. I have written about them here: Why is Bühlmann not like Bühlmann

Bumping this thread as it came up in a discussion on an FB Scuba Uncensored forum in two separate threads. One was asking wht GF settings people are using on deep tec dives and others on general discussions on what GF setting to use.

I am mainly doing non deco recreational diving. However did Deco diving with bsac sports diving courses and dives. My Perdix I use in OC Tec mode with a defined 6m 5 minute safety stop and GF of 45/95 which gives me longer times to NDL on a 12 day dive vacation where I do 40 or more dives.
My Suunto would be giving me far less time to NDL by the end of the third day.