What's your SurfGF and how does it compare to your (Rec) GFHi?

[dmaziuk]

"surfacing grapefruit" LOLOLOL!!

As distinct from 99th GirlFriend.

 

[tursiops]

That is because, for an NDL dive, the horizontal line that starts in the upper right, at the arrow that says “Ascent starts”, runs in a straight (sloped) line all the way to the left and intersects the vertical axis at or below the value for GFHi. So, no matter what GFLo is, the ascent line would never intersect it. If they did intersect, that would represent a deco stop and it would no longer be an NDL dive.

Disclaimer: The ascent line would not actually be straight. I think it would be curved, reflecting that off-gassing occurs exponentially, not linearly.

Great explanation!

 

[dmaziuk]

Great explanation!

Except it does not really work that way, sorry. As Stuart said, nothing says that on a no-stop dive your ascent profile is a straight line from the intersection of "ascent starts here" and "ambient pressure" lines, to where the red line hits the "1 ATA" in the lower left corner.

You can start up with faster ascent rate until you "hit the line" and then slow down and "ride the line" the rest of the way. And since "the line" is only GFHi on a no-stop dive, you could go over GFLo just fine at that cusp point.

 

[stuartv]

Except it does not really work that way, sorry. As Sturat said, nothing says that on a no-stop dive your ascent profile is a straight line from the intersection of "ascent starts here" and "ambient pressure" lines, to where the red line hits the "1 ATA" in the lower left corner.

You can start up with faster ascent rate until you "hit the line" and then slow down and "ride the line" the rest of the way. And since "the line" is only GFHi on a no-stop dive, you could go over GFLo just fine at that cusp point.

Agreed. I realized that much later, when was thinking back on my post.

If you draw a line from GFHi on the left vertical axis to GFHi on the upper horizontal axis, the NDL ascent would be a curve that never crosses that line and eventually intersects the vertical axis, on the far left, at GFHi. On the right side of the chart, this curve would be close(ish) to horizontal. I.e. almost a straight line heading to the left. It would start to curve down and when you get to the left side of the chart, it would go steeply down to stay under the GFHi line until it intersects it on the left axis.

During that ascent, yes, it could cross the line that was drawn to a chosen GFLo value (at the top horizontal axis).

I wish I was good at doing graphics like the one posted previously, so I could draw a similar one showing an NDL ascent that only pays attention to GFHi.

Personally, I have never seen why it was important to worry about crossing the line to GFLo, as long as you don't cross the line to GFHi. And, based on conversations with at least one very knowledgeable deco theorist, I think there is no real data to support the idea that you need to worry about crossing the line that runs from GFHi (on the far left) to GFLo (on the top) - as long as you don't cross the line that runs from GFHi to GFHi. There is no real data to say that respecting GFLo is any safer than just respecting only GFHi.

 

[stuartv]

That is because, for an NDL dive, the horizontal line that starts in the upper right, at the arrow that says “Ascent starts”, runs in a straight (sloped) line all the way to the left and intersects the vertical axis at or below the value for GFHi. So, no matter what GFLo is, the ascent line would never intersect it. If they did intersect, that would represent a deco stop and it would no longer be an NDL dive.

Disclaimer: The ascent line would not actually be straight. I think it would be curved, reflecting that off-gassing occurs exponentially, not linearly.

My point here was really just to note that it would NOT be a stair step affair like the ascent shown in the graphic. It would be a smooth curve.

 

[dmaziuk]

Personally, I have never seen why it was important to worry about crossing the line to GFLo, as long as you don't cross the line to GFHi. And, based on conversations with at least one very knowledgeable deco theorist, I think there is no real data to support the idea that you need to worry about crossing the line that runs from GFHi (on the far left) to GFLo (on the top) - as long as you don't cross the line that runs from GFHi to GFHi. There is no real data to say that respecting GFLo is any safer than just respecting only GFHi.

Yup. The original M-value lines come from people who actually counted bent divers and exploded goats. And then NEDU people figured those lines seem to be correct when it comes to real divers on real dives. GFLows and deeper stops: not so much.

 

[dmaziuk]

My point here was really just to note that it would NOT be a stair step affair like the ascent shown in the graphic. It would be a smooth curve.

There's probably a bump when the leading TC switches, that's my nitpick of the day

 

[Jay]

Drawings by hand are perfectly acceptable! bumps, curves, kinks too

 

[Fastmarc]

Drawings by hand are perfectly acceptable! bumps, curves, kinks too

Agree. No diagram snobs here.
Would like to see that visualization.

 

[AfterDark]

I never realized how my inability to meet or exceed any NDL at any depth as show on tables or PDCs on any single tank I own uncomplicates my diving until I read this thread. Something to be said for a high SAC rate and one dive per day.