Q re: N2 loading bar on Peregrine

[melliodora]

(initially posted in the general computer forum before I found this one!).

I bought my first dive computer - a Shearwater Peregrine - last year, after getting a DCS hit (another post about that coming one day!). I immediately read the manual, set it to the high conservatism and over time have been reading into and using GFs.

But, I just realized I am confused about how to interpret the N2 loading bar on the main screen, the one that shows next to NDL that I have circled in the screen grab below (not the one showing the different tissue compartments). The manual says "The nitrogen bar graph is scaled such that it is full once decompression stops will be needed". However, on multiple recent dives, this bar has appeared full, ie with red at the top - sometimes not tooooo long into the dive. I have never dropped below 10 mins NDL and always aim to surface with SurGF <70 (and never exceeding my GFHigh throughout the dive). Also, when I scroll to the tissue compartment N2 loading graph, they're variously in green to yellow, never near red. Only noticing this now as previously was diving on Nitrox and/or shallower profiles so perhaps wasn't an issue with N2 loading.

Could someone please explain how I should interpret that N2 bar? Am I missing something? Is this a ridiculously simple question/misunderstanding from my side?!

 

[scubadada]

Hi @melliodora

The display on the nitrogen loading bar appears to be in error. Works the same with air or nitrox. Contact Shearwater with your concern.

 

[melliodora]

Hi @melliodora

The display on the nitrogen loading bar appears to be in error. Works the same with air or nitrox. Contact Shearwater with your concern.

Thanks, I will do that. My comment about air vs Nitrox was just in reference to the fact that this issue - the N2 bar filling up - has only come to my attention since doing some recent dives on air, ie previously when mostly on Nitrox it wasn't an issue.

 

[inquis]

when I scroll to the tissue compartment N2 loading graph, they're variously in green to yellow, never near red.

Were you looking at this during a dive? That graph is based on the ambient pressure, so not necessarily a good comparison to the home screen bar. As you ascend, the tissue bars will grow. I can't speak to the home screen bar, but if it's really full before NDL=0, you should contact Shearwater.

 

[melliodora]

Were you looking at this during a dive? That graph is based on the ambient pressure, so not necessarily a good comparison to the home screen bar. As you ascend, the tissue bars will grow. I can't speak to the home screen bar, but if it's really full before NDL=0, you should contact Shearwater.

For the tissues bar, no not during the dive, generally immediately on surfacing though (when they are varyingly green to yellow). As I mentioned, the home screen bar gets full (or as full as it looks like it can get to, definitely with red!) during the dive, then clears to empty during SI as expected.

 

[inquis]

At a given NDL, the home screen bar will depend on the depth. For instance, if NDL is 10 min when at 40 ft, that bar will be quite full. If NDL is 10 min at 90 ft, it will be nearly empty. I suspect the bar is working correctly in your case, but your intuition about how those relate may need a slight refinement.

 

[inquis]

Could someone please explain how I should interpret that N2 bar?

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.)

 

[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.)

Okay, this would makes sense... I think it has appeared full when SurGF has been in the 60s (I use the 35/75 high conservatism setting). It appears *full* though, not say 85%-90% full - at least to my eyes underwater - hence the confusion about the first part of the manual (that I quoted). I tend to hang out a bit longer and/or shallower than the 5m safety stop to try and drop SurGF a bit more, as on my dive profiles GF99 usually doesn't rise to anything meaningful until 3-5m, but given I'm in groups it's not always possible.

I guess my take away question is, should I be concerned that I'm seeing this bar full in these contexts? And if so, is it just a matter of keeping my SurGF MUCH more below 75 (eg <50)?

 

[inquis]

should I be concerned that I'm seeing this bar full in these contexts? And if so, is it just a matter of keeping my SurGF MUCH more below 75 (eg <50)?

A GFHigh of 75 is indeed high conservatism. The DSAT algorithm (designed and tested for repetitive NDL diving) is approximated by a GFHigh of 90-95, depending on # of dives. By all means let surfGF drop further before getting out of the water. However, if you keep surfGF < 50 the *entire* dive, that's incredibly conservative.

There may also be a vision thing going on. You'll see red in the bar before it's "full". An example from the manual:

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

 

[melliodora]

Thanks, yep I saw that example from the manual (and also figured that diving with air on current profiles and trying to keep SfGF lower than I am is near impossible, I'm already being a pain for others but explained my past DCS and they understand). Another reason I think I will avoid places that don't offer Nitrox, but hey, I wanted to come to Raja Ampat and dive local

I'll look reaaaaallll closely today when I dive and see if I can see a faint gap at the top!

Edit: and thanks again for your replies and patient explanations! A bit of a new world for me, digging into these details and GFs etc