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

[stuartv]

But what system should I have used to do twenty-five 80-100' dives in a row, 4-5 a day (counting night dives), as I drank in the sights of the second dropoff on Bonaire last September?
It wasn't DSAT, and it wasn't 95/95.

I did 27 dives in TCI in 5.5 days in August. Just like you said. Every one of them had the option to go to 100' (over the wall).

I dived with my computers set for 95/95, so as to be able to honestly report that I did not go into deco. And then I hung at my safety stop every time, until my SurfGF dropped below 70. Worked fine and felt great all week.

 

[rsingler]

Yeah, I think we may eventually find out that reverse profiles (like your 95/70) might even have advantages for some dives. As long as we can agree that ("wink-wink") you might have been in deco using another profile.

 

[stuartv]

Yeah, I think we may eventually find out that reverse profiles (like your 95/70) might even have advantages for some dives. As long as we can agree that ("wink-wink") you might have been in deco using another profile.

Ha! No wink-wink needed. I would most assuredly have been in deco often, had I been using lower numbers for my GF. I really don't think any dive charter operators care. They just want to know that I was not in mandatory deco, according to my computer. Thus, if they sound a Recall and I have to surface immediately, liability for any negative consequences that may occur does not fall on them. They can always point to my dive computer and say "it said he was not in deco. Sometimes people get bent. We did not do anything unsafe or unreasonable and we have no responsibility for what happened to him."

Besides, what negative consequences are likely to occur? GF95/95 is basically the same (very nearly) to diving another computer that runs DSAT. It's not like 95/95 is aggressive beyond what many other dive computers do by default.

Personally, my SUSPICION is that a reverse profile like 95/70 may have advantages on ALL dives. But, without sufficient data, that is all it is - a suspicion. I do note, after looking at some dives in my log, that when I stop at 15 - 20 feet, my actual GF (GF99) is not anywhere close to 95. Generally, more in the 50-70 range. Then I hang there until my SurfGF drops to 70. So, my actual load gradient never goes over about 70% of the M-value (in the leading compartment).

 

[dmaziuk]

I think that's a mischaracterization of what Subsurface does. The difference is only whether he arrives at 10m with a teensy-tiny deco obligation (Subsurface rec planner says go to the surface after clearing the seconds or whatever of deco) or with no deco obligation (Subsurface rec planner says you can stay at 10m until gas runs out, because the NDL there is really long).



That seems worrying and not how I understand the algorithms to work. Can you give an example of how that would happen?

If you arrive at SS with no deco obligation, then by definition your SurfGF is under 100. Arrive with a few seconds of deco and it is, again: by definition, over 100. Exactly how long you need to hang there for it to drop to, say, 70, depends on the half-time of the leading tissue compartment: is it 18.5 minutes? Or 12.5, or 27? And conversely, how much your SurfGF drops after you spend 5 minutes at SS, depends solely on your leading compartment: the 12.5 one would've lost almost half of its gas loading by then, but the 18.5 one: not even a third. Do you really believe this is what's actually happening in your meat and bones?

 

[scubadada]

As this thread gets longer, we seem to have lost the bubble. When @Jay first started this conversation over two years ago, I think he was reacting to the surprising finding once SurGF was introduced as an available datum.
Whether you dive 30/85, 50/70 or 95/95, it is surprising that in my experience, the vast majority of regular recreational dives end up with GF99 upon surfacing between 40 and 60. In other words, you could dive 100/100 and there would be no effect on your safety.
It's only as recreational diving has begun to push the envelope that these gradient factor discussions have merit. It's only when you start doing repetitive dives that are long (CCR) or deep or maybe on trimix that the argument has much possibility of being worth pursuing.

For me, it's cumulative risk that I'm trying to reduce. When I'm on Helium with my CCR, I understand that I need to be conservative. But it's the other half of my diving that I'm really worried about. Cold water California diving is beautiful, but a lot of prep and driving and cleanup to do every weekend. So when I have the chance, I pack 25 dives into a week in some tropical paradise. And it's those dives where I want an approach that will keep me out of trouble. Yeah 95/95 will (statistically) work for our 40 min bounce dive to 120 feet and the 40 foot second dive that afternoon, with a pair of 60 footers tomorrow before I go home.
But what system should I have used to do twenty-five 80-100' dives in a row, 4-5 a day (counting night dives), as I drank in the sights of the second dropoff on Bonaire last September?
It wasn't DSAT, and it wasn't 95/95.

I have dived DSAT since 2002, over 2000 dives, with about 5% light deco, many close to the NDL. I have dived Buhlmann since 2016, over 900 dives. I've been diving a Shearwater Teric, along with my trusty Oceanic VT3, since May 2019. I started using the Shearwater cloud in February of 2020 and have my surfacing GF for my last 222 dives.

Well before I started diving the Teric, I adopted an adaptive safety stop strategy and did a 5 min stop if I was close to the NDL. I also padded my last deco stop by 3-5 min. I dive my Teric at 80/95. For the last 222 dives, my average surfacing GF has been 55, with a range of 13-80. I recently dived in Bonaire and did 45 dives over 13 days. My average surfacing GF was 37, with a range of 13-70. After seeing the rapid increase in GF99 during the final ascent, I have made sure that my final ascent is over a minute or more.

So, it turns out that my diving is more conservative than I would have thought, I'm happy with that.

 

[rsingler]

It's great to see this data, @scubadada !

What I hope folks take away from this and @stuartv 's post above is NOT that "diving 95/95 is just fine". Rather, my takeaway is that off-gassing is a complex phenomenon, and our newer computers are giving us a window into what might be happening. And should a diver choose to do a dive that fills several compartments to the brim (to use the theoretical construct), they shouldn't (IMO) presume that just because they've been successfully using the same settings for years, that those settings will get them out of the water without DCS. Rather, they may have just been doing more conservative dives than they thought, and their settings were effectively irrelevant.

Thank you for sharing this!

 

[100days-a-year]

Never found the surfgf on my EDGE but I have done 40+ dives in a week past rec limits from NDL to light deco. As I hit the surface about one to two minutes after deco or direct to surface on NDL I'd assume it was 70ish or better. Did that til I was early 50s. Dove lean nitrox to give myself a slight cushion.
The deciding factor in the guys doing the same that did get bent seemed to be total exposure on a single dive vs. total exposure in a day or a trip.

 

[EFX]

Actually, I think this whole thing is ignoring a fundamental point. The actual M value. GF60 means you are 60% of the way to the M value. Not 60% greater than surface ambient. If you surface with tissue tension at 1.6 ATA (i.e. saturated at 20'), and the M value for that compartment is 3.2 (I think! Not sure off the top of my head), then you're only at GF50.

rsingler: @EFX ?

I was going through this thread starting from the beginning and came across these responses from stuartv and rsingler. I did not see these posts when I originally read this thread. Sometimes when I read a thread I don't come back to it for days and rather than start where I left off, I go to the last page and read those posts. If there is something interesting I'll read further back to get some context. This is probably what happened with these posts I'm finally responding to although rather late. As they say, "better late than never".

Stuartv describes GF60 as "60% of the way to the M value. Not 60% greater than surface ambient." Stuartv doesn't mention whether he's talking about GF99 or SurfGF, but judging from the context of the preceeding posts it appears the topic is SurfGF. I agree with this although I can see how some readers new to decompression theory and GF's might still be confused.

There are two equations that are used to calculate GF99 and SurfGF. Just so we are on the same page GF99 is the GF (gradient factor) at the current time and depth of the dive. It is measured from ambient pressure which is essentially the inert gas pressure the diver is breathing at the current depth. SurfGF is the resulting GF if you could instantaneously surface. It is measured from the surface pressure. For GF99, a GF of 0% would correspond to an inert tissue pressure loading at the existing ambient pressure. For SurfGF, a GF of 0% would correspond to a tissue pressure equal to the surface pressure. In both cases a GF of 100% corrsponds to a leading tissue compartment (LTC) loading equal to the M value. These are the equations I use in my Excel spreadsheet (I call GF99 CurGF):

GF99 = (p_gas - p_amb) / (M - p_amb) x 100
SurfGF = (p_gas - Surf_press) / (M - Surf_press) x 100

p_gas is the total LTC inert gas pressure.
p_amb is the inert gas pressure at the current depth (also called the inspired inert gas pressure).
M is the m-value and is the maximum tolerated inert gas pressure in the TC.
Surf_press is the inert gas pressure at the surface.

In both equations the value in the first set of parenthesis must be positive if GF99 and SurfGF are to be positive numbers. This means that the gas pressure in the tissues must be higher than the ambient and surface pressure respectively. In other words the LTC must be off gassing. If you have a Perdix as you descend on your dive GF99 will show "on gas" because p_amb is greater than p_gas. In both equations the quantity in the second set of parenthesis forms the basis of the calculation and is equal to 100% of the range. Multiplying by 100 converts the calculation to a percentage.

Divers suffer from DCS when the blood/lung system cannot remove the flow of inert gas coming out of the tissues at a safe rate. The result is bubbles and if you excuse the pun, could result in a boat load of pain. In order to get flow there must be a difference in pressure between the tissues and the lungs. In diving literature this pressure difference is oftem called a gradient. If the LTC becomes saturated at ambient pressure then p_gas equals p_amb and GF99 becomes 0, hence p_amb is the baseline. If p_gas equals M then the resulting calculation produces a GF of 100%, hence the m-value is at the upper limit for the pressure in the LTC.

Notice in the equations p_amb is going to be changing throughout the dive but Surf_press remains the same. This suggests that SurfGF is the highest just as the ascent starts and decreases as the diver ascends because the LTC is off gassing and therefore reducing its pressure relative to the surface pressure. However, the value can increase again as the LTC switches to a higher (slower) TC. Because the tissues cannot change as fast as the ambient pressure is decreasing as the diver ascends, GF99 will generally increase throughout the ascent to the surface. The NDL calculation takes into account the GFHi and as long as the diver does not reduce the NDL to zero a direct ascent to the surface should result in a GF coming in at or below GFHi. As the diver reaches the surface p_amb becomes equal to the surface pressure (Surf_press) so that GF99 and SurfGF converge to the same value.

 

[dmaziuk]

...
This suggests that SurfGF is the highest just as the ascent starts and decreases as the diver ascends because the LTC is off gassing and therefore reducing its pressure relative to the surface pressure. However, the value can increase again as the LTC switches to a higher (slower) TC.

The way around it is to add more compartments with finer "spacing" of course. We can do that for "A" set and retrofit the fudge factors from B and C by simple curve fit. The resulting model would of course have no practical trials, but it should fit perfectly within the 16-comparetment model that was validated by Herr Doktor et al, so that could be argued back and forth until the cows come home.

 

[EFX]

GF99 = (p_gas - p_amb) / (M - p_amb) x 100
SurfGF = (p_gas - Surf_press) / (M - Surf_press) x 100

In my previous post I forgot to mention that in the GF99 and SurfGF formulas repeated here above, the M is equivalent to a GFHi of 100%. It is NOT equal to the GFHi you set on your dive computer unless you're setting the GFHi equal to 100!

The Shearwater Perdix manual mentions this. The GF99 on a dive computer gives you a real-time indication of how much inert gas your leading tissue compartment has absorbed. SurfGF uses the same m value (100%) but uses the surface as the basis as explained in the preceding post.

You might be asking well OK but where does the GFLo and GFHi come into use. GFLo is used only for deco dives that require an ascent schedule that includes deco stops, usually at equal increments of 10 ft or 3 m. GFLo determines the depth of the first stop. Here is the equation I use in my spreadsheet:

first_stop = ((p_gas - gf_lo * a_n2he) / (gf_lo / b_n2he - gf_lo + 1)) - sp

p_gas is the total inert gas pressure in the leading tissue compartment (LTC).
a_n2he and b_n2he are the combined a and b coefficients for nitrogen and helium.
sp is the surface pressure.
gf_lo is the GFLo value entered pre-dive by the diver.

GFHi is the percentage of Buhlmann's m-value for that LTC and becomes the limiting value upon surfacing. So, the first stop is set by GFLo and the surfacing segment is controlled by GFHi. As an example if the GF was 70/90 and the first stop was 50 ft. The GF progresses from 70 to 90 beginning at 50 ft. The first stop would use a GF of 70. The next stop at 40 ft would use a GF of 74. The next stop at 30 ft would use a GF of 78. 20 ft would use 82. 10 ft would use 86 and the surface would use a GF of 90. You can see this progression in GF on the chart reproduced below from page 14, with the m-value line (orange) running from GFLo at the top right to GFHi at the bottom left. The green line is Buhlmann's m-value line equal to a GF of 100%. The blue line is ambient pressure. Off gassing only occurs above the blue line. The green and orange lines shown below are only shown for one tissue compartment. The dive progresses from top to bottom and right to left along the black line. Where the black line hits the orange line marks the beginning of a stop. As time progresses at the stop the LTC off gasses which is shown by moving down the vertical black line. When the time at the stop is complete, the diver moves shallower to the next stop shown by the black line moving slightly down and to the left until it intersects with the orange line, which is the stop at the next shallower depth and the process repeats until the surface is reached.

Both types of dives (NDL and deco) calculate the ceiling. For deco dives a time is calculated for the current stop using the new GF until the ceiling equals the next shallower stop. NDL (non-deco) dives only use GFHi. There is no first stop so GFLo is not used. For NDL dives a time is calculated to remain at the current depth using GFHi until the ceiling drops from some negative depth (above the water) to zero depth (at the surface). Here's the formula I use from the spreadsheet:

Pamb = (p_gas - common(GF) * a_n2he) / (common(GF) / b_n2he - common(GF) + 1)
new_ceiling = Pamb - sp

For NDL dives common(GF) is equal to GFHi. For deco dives common(GF) is the GF that progresses from GFLo to GFHi as the stops progress from the first stop to the surface. The pressures calculated in the spreadsheet are absolute so subtracting the surface pressure converts absolute pressure to gauge pressure which is essentially the depth (negative if NDL, 0 at the surface, positive for deco dives).

If you're wondering about that last sentence, i.e., how a pressure can be a depth read the next section which was taken right out of the dive spreadsheet help page.

UNITS OF PRESSURE
All pressure values on the ss are displayed as absolute pressures in feet or meters of salt or fresh water rather than the typical units of psi (imperial) or Kpa (metric). This seems odd at first because a distance unit (feet or meters) is used to define a pressure unit. What seems even stranger is that the gauge pressure is equal to the depth in feet or meters. To understand why this is true consider this relationship:
33 fsw/33 ft (or 10 msw/10m). We can describe it this way: there is a pressure of 33 fsw which is equivalent to a pressure exerted by 33 ft (depth) of water. The spreadsheet converts depth to a pressure in order to calculate insP, the inspired inert gas pressure. For example, to convert 80 ft of depth to its equivalent gauge pressure (P) in fsw we could write: P = 80 ft x 33 fsw/33 ft. As a sanity check on the math the ft divide out leaving fsw which is what we want. The really interesting part is that the 33 divides out giving us an answer that is actually the depth but in pressure units of fsw. This is convenient because it eliminates the need to use cumbersome unit conversions in the formulas. Absolute pressures are obtained by adding the pressure of the atmosphere at the surface to the pressure exerted by the water (which is gauge pressure).

I know this is a lot to absorb but I thought I would give you a more complete picture of GF's and how they are used in dive programs (at least mine anyway).