Q re: N2 loading bar on Peregrine

[melliodora]

ETA: again, "full" means NDL=0. Typically, one would shallow up before hitting 0.

Definitely! Quite a bit before this in my case!

 

[melliodora]

My take is that it's simply a graphical depiction of the ratio surfGF/GFHigh. That's why the manual says the bar "gives a much better indication of decompression stress and the risk of decompression sickness than NDL does." (See my previous post for an example.)

An update from my morning dive. 58 mins, max 20.6m, avg 12.6m, mostly hanging at ~11-13m watching mantas play for most of the dive. At ~35 mins, at that level, SfGF was 54 (72% of my GFHigh) but the N2 bar looked full. Surfaced with SfGF in the 50s, never got below 25mins NDL. This terrible photo is what it looked like a few mins after, back on the boat. Maybe that's a tiny black sliver at the top, but I feel like I was far from a tiny sliver away from deco so I'm still not at all sure what to read from this bar.

 

[inquis]

I agree, that picture does seem contrary to having a maximum surfGF during the dive of 54 and GFHigh of 75. I think an email to Shearwater would be useful at this point. They are usually very responsive and should explain it definitively. Please let us know what you learn!

 

[melliodora]

Please let us know what you learn!

Will do!

 

[MarkA]

Interesting that NDL=0 and surface =0h04m. Is it still in the period before it considers the dive over?

 

[melliodora]

Interesting that NDL=0 and surface =0h04m. Is it still in the period before it considers the dive over?

Nope, that's the default that it shows when not in the water. ie right now, 6hrs + since last dive, it shows 0 NDL and an empty N2 bar. At least on mine!

 

[David Carron]

An update from my morning dive. 58 mins, max 20.6m, avg 12.6m, mostly hanging at ~11-13m watching mantas play for most of the dive. At ~35 mins, at that level, SfGF was 54 (72% of my GFHigh) but the N2 bar looked full. Surfaced with SfGF in the 50s, never got below 25mins NDL. This terrible photo is what it looked like a few mins after, back on the boat. Maybe that's a tiny black sliver at the top, but I feel like I was far from a tiny sliver away from deco so I'm still not at all sure what to read from this bar.

Seems strange that SurfGF and GF99 (which I assume to be the instantaneous diver GF at ambient pressure) would be different when you're at the surface.

 

[melliodora]

Seems strange that SurfGF and GF99 (which I assume to be the instantaneous diver GF at ambient pressure) would be different when you're at the surface.

I am 1) new to this and 2) an ecologist who always struggled with physics, but at a guess, could it be due to a slight difference between actual atmospheric pressure at the surface, and the standard surface pressure assumed as part of the SfGF equation? I'm not even sure if I'm understanding that right, others please jump in as I'm also interested!

 

[J-Vo]

I am 1) new to this and 2) an ecologist who always struggled with physics, but at a guess, could it be due to a slight difference between actual atmospheric pressure at the surface, and the standard surface pressure assumed as part of the SfGF equation? I'm not even sure if I'm understanding that right, others please jump in as I'm also interested!

Not quite, sfgf should be based on what it recorded as the surface pressure just before the last dive, while GF99 is based on the current ambient pressure. I see the behavior often. It's due to barometric pressure changes.

 

[melliodora]

Not quite, sfgf should be based on what it recorded as the surface pressure just before the last dive, while GF99 is based on the current ambient pressure. I see the behavior often. It's due to barometric pressure changes.

Thank you, that's good to know. So I was on the right track (difference between the assumed and actual surface pressure) it seems!