Is my approach to diving with Nitrox logical?

[BlueTrin]

thanks for sharing but I have to admit I am lost with the graph lol. Might need another cup of coffee to understand this. What is the difference between GFlo and GFHi? so if they are both set to 100% then its very liberal correct since its Bulhman?

RGBM does not have a GF factor correct?

and yes as others have said here to adding in longer stop, not pushing NDL, etc all adds and I agree

I didn’t want to give too many details at once. Thought it would be confusing.

So if you dive Buhlamn without gradient factors you have only the green/m-value and blue/ambiant pressure line.

Buhlman is saying that the bigger the difference of gas saturation between your tissues and the ambiant pressure the faster you will offgas or on gas.

• If you stay on the blue line, you will neither offgas or ongas, because you are at equilibrium.
• When you on-gas you move up on the chart.
• When you off-gas, you move down on the chart.
• When you go deeper you move right on the chart.
• When you go shallower you are moving left on the chart.

If you are on the right of the blue line, it means that you are deeper than your tissues gas saturation and your tissues will on-gas.

If you move to the left of the blue line, your tissues gases saturation is higher than the one from the ambiant pressure and you will off gas.

However Buhlman is saying that you shouldn’t go past the green line or you will off gas too fast and you’ll risk being bent.

So if you follow Buhlman, you will ascend (this means moving to the left on the chart) until you reach the green line, then this will make you offgas: so you will go lower on the chart. When you are low enough you can ascend more (move to the left on that chart) … etc

To add some conservatism people decided to move the green line (see purple arrow), the way that people commonly do it is to use a gradient factor. GFlo and GFHi will define another line you can use instead of the green line. It’s an arbitrary way to define a line between the m-value and ambiant pressure lines.

(All, did I get this right? )

+1 for Deco for Divers: it’s a very good book if you are interested in the topic.

RGBM is a totally different algo, like @Scubada said, it is not related to Buhlman and gradient factors.

 

[scubadada]

Yes, exactly, how astute of you! Indeed it was Jupiter. I am just back from a near month of diving Florida from one end to the other. And darn if I do not have to go back in a week or so to recover my Toyota Tacoma whose engine decided to explode on me. But ever true blue it got me to the Toyota dealer before expiring where the poor things sits awaiting a new engine. So yes, I plan to hit Jupiter again. Captain wants all divers up at 55 minutes, I over stayed 1 minute and 20 seconds because I did a 5 minute (adaptive setting) safety stop.

James

Hi James,

Ha, where else you do your whole dive at 70-85 feet and see turtles, sharks, and Goliath Grouper? Who was your operator? I use JDC, 45 min bottom time, everyone on the surface by 55 min. So you padded your deco stop by 5 min. I see you are using a Peregrine, are you using the SurfGF to help guide your safety stops and post deco stops? I also dive at a GF high of 95 and do some light deco. I generally surface no higher than a GF of 80.

I've been diving Boynton, West Palm, and Jupiter since 2009. I just got back from a great visit, 8 days, 21 dives Trip Report - Back in Florida, 2022 #3

 

[inquis]

@BlueTrin everything sounds good! Nice explanation.

Possibly a simplification... Diver controls left and right (via depth). The up & down is automatic and always moves toward the blue line as time passes. The goal is to exit through the rainbow bar on the far left at your desired level of risk (GFhigh) without crossing the green line (which represents where Buhlmann felt was the "do not exceed" point.)

The GF line limits how far left you can move before temporarily halting ascent. (It's a user-defined conservatism from the green line.)

 

[Nemrod]

Hi James,

Ha, where else you do your whole dive at 70-85 feet and see turtles, sharks, and Goliath Grouper? Who was your operator? I use JDC, 45 min bottom time, everyone on the surface by 55 min. So you padded your deco stop by 5 min. I see you are using a Peregrine, are you using the SurfGF to help guide your safety stops and post deco stops? I also dive at a GF high of 95 and do some light deco. I generally surface no higher than a GF of 80.

I've been diving Boynton, West Palm, and Jupiter since 2009. I just got back from a great visit, 8 days, 21 dives Trip Report - Back in Florida, 2022 #3

I use several ops but prefer JDC. That dive was with JDC, yes, 45m bottom, everyone up at 55m, min 500 psi back on deck, no deco. Okay, so I stretch the rules a little that day, spear me!

Yes, I am using the Peregrine, set at the High setting and yes using the surface GF to guide my surface interval time. After this dive I reset my computer to the Medium setting. There is High, Medium, Low conservatism and Custom. I need to get another Shearwater so both of my computers can be configured the same though I like that Scubapro hose end Matrix, I wonder if it can be adjusted to run similar to the Shearwater computers. I have my current second computer set to try and match my Shearwater but it never went into deco on that dive so obviously this is a problem with two computers not of the same make or algorithm. I reset as I said the Shearwater to Medium and now the two computers seem to mimic each other well enough. I need to follow my own advice and get matching (similar settings and algorithm) computers! But I just got new computers for my wife and now a new engine for my Toyota so I am once again poor ;(.

James

 

[scubadada]

I use several ops but prefer JDC. That dive was with JDC, yes, 45m bottom, everyone up at 55m, min 500 psi back on deck, no deco. Okay, so I stretch the rules a little that day, spear me!

Yes, I am using the Peregrine, set at the High setting and yes using the surface GF to guide my surface interval time. After this dive I reset my computer to the Medium setting. There is High, Medium, Low conservatism and Custom. I need to get another Shearwater so both of my computers can be configured the same though I like that Scubapro hose end Matrix, I wonder if it can be adjusted to run similar to the Shearwater computers. I have my current second computer set to try and match my Shearwater but it never went into deco on that dive so obviously this is a problem with two computers not of the same make or algorithm. I need to follow my own advice! But I just got new computers for my wife and now a new engine for my Toyota so I am once again poor ;(.

James

I dive a 12 year old Oceanic VT3 and a Shearwater Teric. DSAT and a GF high of 95 match pretty well. DSAT is generally a bit more liberal on the 1st dive, Buhlmann often a bit more liberal on repetitive dives. I dive the more conservative for no stop, and clear one or both for light deco.

I have never gone into deco on the Jupiter reefs with the JDC profile and 36%

 

[Jcp2]

I have been an active diver since 1968. I have never been bent and this during a time when we did not do safety stops, used the then liberal Navy Tables and did 60 fpm ascents straight to the surface. Of course I know of reported cases of the bends and we all read of people who visit the chamber whether for real need or precautionary need. I think the OPs concern over being bent is a little exaggerated.

A recent dive of mine that did incur a little deco obligation. Not ideal, but heck, there was turtle after turtle after shark after shark and goliath after goliath, What, like am I supposed to leave all that down there unmolested?



My mix was 36% (aluminum 80 starting psi 3200), my GF was set to High, I was back standing on deck with 800 psi and that after a 10 minute wait for the boat to come get me (still on my regulator/tank) as it worked it's way from group to group to finally me, yes, I was solo. I had my slung pony with bottom mix but of course do not use that but for emergency. Fun dive. No bends.

N

What was the SPG at when you decided to ascend?

 

[Nemrod]

What was the SPG at when you decided to ascend?

I do not recall exactly, I was back on deck with 800 after loitering for 10 minutes on the surface so I was on top with about 1,000 psi. I probably began paying attention to the urge to return to the surface at 1,500 psi or at least I begin watching the dial closely in between photo ops. Just to remind me I do not have gills, yet. If I could quit buying new engines and transmissions I might just ought to get a suicide machine (rebreather). I had a slung 30cf pony for back up, I was technically solo. And I had plenty of air time remaining to handle the 3 minutes of deco and expected 5 minutes of safety stop and get back on the boat with the requested 500 psi on deck.

I showed that dive graph so the OP could see a minor deco dive (High conservatism) as that dive would not have been deco at Medium or Low. All of these settings and algorithms are just models that try to approximate (and for the OPs concern, do a very good job of being safe) human tissue response to Nitrogen loading and off gassing. And then we can use our own judgement.

If I had been buddy diving rather than (planned) solo I would reserve additional gas for their (emergency) need.

James

 

[scubadada]

What was the SPG at when you decided to ascend?

It's a good idea to know how much gas you need to make a normal ascent. If you know your average RMV, it is very easy to make the calculation.

As an example, to ascend from 90 feet and make a 3 min SS, I need about 165 psi from an AL80. For a 5 min SS that increases to a little over 200 psi.

For a dive with the profile that @Nemrod posted above, the operator recommends leaving the bottom at 800-1000 psi, that would work out fine for most divers and allow them to surface with the requested 500 psi or more. I did 9 dives on the Jupiter reefs during my recent visit and dived with AL80s with around 36% nitrox. My average surfacing pressure was just under 1100 psi (916-1334). My average surfacing GF was 63 (57-71).

You can learn a tremendous amount by paying attention to your previous dives. It helps you avoid being surprised.

 

[Nitrox_DiverNY]

Sorry, it's just not that easy.

The RGBM algorithms are generally more conservative, Mares, Cressi, Suunto. They can be made more conservative through adjustment. However, Suunto has several versions of RGBM and one can be made to be quite liberal.

Buhlmann is adjusted with the GFs and can be anywhere between very liberal and very conservative. Only the GF high applies to no stop diving. A GF high of 95 would generally be considered liberal, 85 moderate or middle of the road, and 75 conservative.

There are several other common deco algorithms such as DSAT, liberal and PZ+, moderate. Both of these can be made more conservative with settings.

You seem very inquisitive, I would suggest you consider reading Deco for Divers by Mark Powell, it will address many of your questions.

thank you for taking the time to explain this, I will definitely have to get that book and try to find some other post here on scubaboard that really go into it. If you or anyone here has a link to a good SB post about GF that would be great. thanks!!

 

[Nitrox_DiverNY]

I didn’t want to give too many details at once. Thought it would be confusing.

View attachment 733490

So if you dive Buhlamn without gradient factors you have only the green/m-value and blue/ambiant pressure line.

Buhlman is saying that the bigger the difference of gas saturation between your tissues and the ambiant pressure the faster you will offgas or on gas.

• If you stay on the blue line, you will neither offgas or ongas, because you are at equilibrium.
• When you on-gas you move up on the chart.
• When you off-gas, you move down on the chart.
• When you go deeper you move right on the chart.
• When you go shallower you are moving left on the chart.

If you are on the right of the blue line, it means that you are deeper than your tissues gas saturation and your tissues will on-gas.

If you move to the left of the blue line, your tissues gases saturation is higher than the one from the ambiant pressure and you will off gas.

However Buhlman is saying that you shouldn’t go past the green line or you will off gas too fast and you’ll risk being bent.

So if you follow Buhlman, you will ascend (this means moving to the left on the chart) until you reach the green line, then this will make you offgas: so you will go lower on the chart. When you are low enough you can ascend more (move to the left on that chart) … etc

To add some conservatism people decided to move the green line (see purple arrow), the way that people commonly do it is to use a gradient factor. GFlo and GFHi will define another line you can use instead of the green line. It’s an arbitrary way to define a line between the m-value and ambiant pressure lines.

(All, did I get this right? )

+1 for Deco for Divers: it’s a very good book if you are interested in the topic.

RGBM is a totally different algo, like @Scubada said, it is not related to Buhlman and gradient factors.

thank you for bring so detailed with explaining this to me, as a get the change to really study this a few times it will start to become more clear haha