Have I understood the basics of decompression theory, GF99 and SurfGF?

[boulderjohn]

0% - normal nitrogen in body

GF99 is a measure of how much excess nitrogen is in your body as a percentage of the maximum allowable supersaturation for the most loaded tissue at your present depth .

Lowwall's correct response suggests to me that there might be a more fundamental misunderstanding at play here. Forgive me if I misunderstood and am explaining something you already knew.

There is always nitrogen in your body. Right now you are breathing in air, which is about 78% nitrogen, and that nitrogen enters your blood and goes through the different tissues of your body. Those tissues absorb nitrogen at different rates, depending upon factors such as density and the degree of perfusion (blood flow). At the same time that nitrogen is entering your tissues, it is also leaving them, at the same rate through the same process. If you have been at your current pressure for a while, then the amount of nitrogen in your tissues is (through pure randomness) equal to the amount you are inhaling. You are at equilibrium.

When you descend, you begin to inhale more nitrogen than on the surface, so the amount in your tissues increases, because more is coming in than going out. Eventually some tissues will reach equilibrium at your depth. When you ascend and the amount of nitrogen you are breathing decreases, those tissues that reached equilibrium will have more nitrogen leaving than coming in. As you ascend, more tissues will reach equilibrium and then begin to lose more nitrogen than they are getting. You are off-gassing.

 

[Joemdown]

This explains a lot. Almost everything in your initial post is oversimplified or at least partially incorrect. Don't ever rely on AI for any technical explanations. AI doesn't know anything, it's just feeding out a bunch of text that is statistically likely to appear together (and follows standard grammar rules). It may be acceptable for an overview, but always follow through by reading original sources.

Agreed - I actually got most of the info from the second link posted which was by a user talking about shearwater computers.

Some of the errors are addressed above. But I'll make a few notes.



GF99 is a measure of how much excess nitrogen is in your body as a percentage of the maximum allowable supersaturation for the most loaded tissue at your present depth .

SurfGF is a measure of how much excess nitrogen is in your body as a percentage of the maximum allowable supersaturation for the most loaded tissue if you were to make an immediate ascent to the surface.

If you want to visualize "percentage of the maximum allowable supersaturation for the most loaded tissue" as little bubbles versus big bubbles, I guess it's OK. But that's not really what's going on. There's always some tiny bubbles, but if you stay under 100% there is only a minimal likelihood that those bubbles will coalesce and/or grow into something that interferes with your tissues resulting in the symptoms of clinical DCS.

Makes sense, I guess as you say it's over simplified, it doesn't go from zero bubble to a small/big, it just forms and could increase in size.

In its simplest terms then, it seems GF99 and SurfGF are measuring the same thing, just GF99 is at X atmospheres and SurfGF is always at 1 atmosphere (i.e. the surface)?

Now that GF Hi controls the "maximum allowable supersaturation" part of that definition. If you set it (or accept a present) of anything less than 100, you are limiting your maximum allowable supersaturation. As you noted, this will limit your time at depth in exchange for further decreasing the likelihood of experiencing DCS symptoms.

Back to GF99 - Because GF99 is based on pressure at your current depth, it is only useful during the ascent. It will actually be a negative number during the descent and bottom phase of your dive. It displays "On Gas" instead of the negative number to avoid confusion.

Got it.

Meanwhile SurfGF will constantly be increasing during the descent and bottom phase. This is what you want to watch during this portion of the dive as it is what is controlling your remaining no deco time. Or you could just watch the NDL number like us poor schmucks who can't justify the cost of a Shearwater or Garmin DC. Where SurfGF really pays off is after you leave the bottom and are stopping or exploring at lesser depths as it becomes a reasonable measure of the decompression stress you are putting on your body when you surface. Lower is better.

IMO, GF99 is not a particularly useful measure during NDL dives except perhaps if you've really pushed either NDL or your remaining gas. You could then use it to maximize your initial ascent rate while still staying relatively safe by stopping when it got to something less than 100%. Another possible use would be as an additional monitor of your ascent rate if you have a habit of ascending faster than the DC recommends.

I'll second @tursiops recommendation of this article: Shearwater Dive Computer Settings: Become A GF Power User! . It's based on the Shearwater display, but if Garmin is using GF99 and SurfGF the same way, it should be equally valid for their computers.

Agree with this, it seems the main takeaways to me, who's not doing deco diving (so in argument most of this is to in-depth anyway, but like to be prepared) is:

GF99 - ignore it going down, keep it relatively low on the way up to limit large bubble formation and maintain proper ascent rates.

SurfGF - keep an eye on it, arguably the computer NDL will be based on GFHi and if followed exactly will lead me to the surface with an ok SurfGF (based on the settings input), but really the main purpose for me at looking at this, is if I accidentally go into deco, or have extra gas and people are milling around, I can watch the number decrease down, which will only reduce my risk of DCS.

 

[Joemdown]

Lowwall's correct response suggests to me that there might be a more fundamental misunderstanding at play here. Forgive me if I misunderstood and am explaining something you already knew.

There is always nitrogen in your body. Right now you are breathing in air, which is about 78% nitrogen, and that nitrogen enters your blood and goes through the different tissues of your body. Those tissues absorb nitrogen at different rates, depending upon factors such as density and the degree of perfusion (blood flow). At the same time that nitrogen is entering your tissues, it is also leaving them, at the same rate through the same process. If you have been at your current pressure for a while, then the amount of nitrogen in your tissues is (through pure randomness) equal to the amount you are inhaling. You are at equilibrium.

When you descend, you begin to inhale more nitrogen than on the surface, so the amount in your tissues increases, because more is coming in than going out. Eventually some tissues will reach equilibrium at your depth. When you ascend and the amount of nitrogen you are breathing decreases, those tissues that reached equilibrium will have more nitrogen leaving than coming in. As you ascend, more tissues will reach equilibrium and then begin to lose more nitrogen than they are getting. You are off-gassing.

Yep, did understand that and have done some reading into the 16 compartments, but nonetheless useful to see it written in another way. Thanks!

For the purposes of simplification I just took it as at 1ATM 0% nitrogen because for the purpose of starting the dive that's essentially what you calculate from, nil residual nitrogen (in excess of the normal) - i.e. a nil pressure group (using the old tables).

 

[lowwall]

SurfGF - keep an eye on it, arguably the computer NDL will be based on GFHi and if followed exactly will lead me to the surface with an ok SurfGF (based on the settings input), but really the main purpose for me at looking at this, is if I accidentally go into deco, or have extra gas and people are milling around, I can watch the number decrease down, which will only reduce my risk of DCS.

NDL shows how many minutes you can stay at your current depth before you will incur a deco stop obligation on your ascent. So yes, NDL will hit 0 when SurfGF hits 100% (if both calculations use the same assumptions about ascent rate), but other than that they aren't really showing the same thing and have different uses.

Let's start with NDL. It is very useful at depth since time is what matters to you. For example, it helps you decide if you can take another lap around the bottom of the wreck or should go up and look at the fish gathered around the deck railings. Or you can use it to adjust your depth relative to your buddy so you both end up with similar bottom times.

But once you start your ascent, NDL is no longer particularly useful. Unless your ascent is interrupted by another descent or a long stop at depths > 35', it's going to start going up at a rate that will increase rapidly as you get shallower. You will very soon be at a point where you will run out of gas long before you run out of NDL time.

This is where checking SurfGF becomes useful. Once you start ascending, it's basically a deco stress monitor. If it's decreasing, you know it's fine or even beneficial to hang out where you are for a little while longer which is a nice thing to know on multi-level dives. Also, although there aren't any definitive studies on this, it is reasonable to assume that lower tissue supersaturation rates and thus a lower SurfGF number at the end of a dive will result in fewer of the common subclinical DCS issues such as excessive fatigue and the effects of inflammation such as general soreness.

 

[34109411]

For the level you are at the end dont overthink this about gf99 and surf gf and so on. Have a basic understanding of you dive computer respecting your limit and enjoy diving.


Using my sherwater TX i need basic things depth, dive time, GTR and off course PSI reading and NDL. That's what i have on my screen and it does the job for the diving i do. Respect that 3 min safety stop and you be ok.

Be safe.

 

[inquis]

So yes, NDL will hit 0 when SurfGF hits 100%

Minor correction... when SurfGF hits GFHigh.

 

[tursiops]

SurfGF - keep an eye on it, arguably the computer NDL will be based on GFHi and if followed exactly will lead me to the surface with an ok SurfGF (based on the settings input), but really the main purpose for me at looking at this, is if I accidentally go into deco, or have extra gas and people are milling around, I can watch the number decrease down, which will only reduce my risk of DCS.

You can also decide how long a safety stop to take, or whether even to take one at all.

 

[inquis]

really the main purpose for me at looking at this, is if I accidentally go into deco, or have extra gas and people are milling around, I can watch the number decrease down, which will only reduce my risk of DCS.

If you accidentally go into deco, surfGF is the last thing you should care about. Principally, the computer will tell you the depth you should not venture above, and you should obey that unless there is a more serious risk than DCS (say, due to serious injury or lack of gas). It also tells you the expected time until that condition is lifted, because who doesn't like a countdown.

 

[Joemdown]

If you accidentally go into deco, surfGF is the last thing you should care about. Principally, the computer will tell you the depth you should not venture above, and you should obey that unless there is a more serious risk than DCS (say, due to serious injury or lack of gas). It also tells you the expected time until that condition is lifted, because who doesn't like a countdown.

Ahh ok.

I presumed if I overran my NDL and therefore has a deco requirement, I should ascend slowly and prolong my stops which would reduce surfGF.

As context for anyone reading/answering, the reason I am asking is because I recently did 3 dives with an instructor. I don't think he was too safe as when checking over things on dive 1 he hadn't even turned the gas on, so I insisted setting up my own kit for the next 2 dives (note I don't always set up my own kit, but I do always check it over fully to take responsibility).

Anyway, after dive 2 the computer he has given me to wear entered gauge mode, he said it'd be fine and we dived again, I'd never seen this so wasn't sure what it meant. It appears with some googling thereafter this is basically the computer locking me out and telling me not to dive anymore so it won't be helpful in giving me calculations and instead resorts to a basic depth/time counter - i.e. seriously discouraging you from getting back in.

Perhaps I got lucky, perhaps I'm young and in good shape, perhaps both. But I figured from that experience I really should get a bit more knowledgable in the theory stuff behind some of what the calculations and computers are telling so I can make an informed decision next time.

The computer I was using didn't have syncing or detailed functionality, but for anyone wondering it was:

Dive 1 - max depth 22m, total time 38mins, 3min 5m safety stop
Dive 2 - surface interval 1hr 1min, max depth 23m, total time 50mins, 3min 5m safety stop
Dive 3 - surface interval 1hr 10mins, max depth 24m, total time 37mins, 3min 5m safety stop

The dive profiles were pretty much a straight descent to max depth, and a little shallowing as the dive progressed, but only to follow the bull sharks. So wouldn't really call them multilevel, nor a shallowing as the dive went on, more a straight down, yo-yo within a few metres and then back up.

 

[dmaziuk]

Ahh ok.

I presumed if I overran my NDL and therefore has a deco requirement, I should ascend slowly and prolong my stops which would reduce surfGF.

That has nothing to do with deco. The longer you spend at, say, 3 msw, the lower your surfacing GF will be(*). Whether you were "in" or "out of" deco when you got to 3 msw makes no difference.

*) Assuming you don't have enough gas to fully off-gas to 1.3 ATM there.