Using GF99 and SurGF

[JohnN]

I think that using SurfGF to extend safety stop sounds bit backwards. Personally, I don't see any reason to surface with a GF higher than 70 for the dives I do when things go according to plan, so I set GFHigh to 70. I can then use SurfGF, or a second computer set to less conservative settings, to check that I can go directly to the surface in case the dive has to be aborted.

By doing this the computer will tell me how long I'll have to stay at 3 m (or 6 m or 4.5 m depending on last stop depth setting) to get down to 70 and I can ensure that the length of that stop is something I'm comfortable with, have enough gas for, and the boat operator won't be mad because I went past their max dive time. If I'm on a guided dive I can adjust my dive profile so I end up with a stop time of 3 min to match the safety stop of the rest of the group.

When using GFHigh of 85 it's pretty easy to get a nitrogen load high enough in the middle compartments to have to stay 10 min or more at 3 m to get down to 70, especially on the 2nd dive with a 1 hour surface interval. If it's hard to stay at 3 m for the safety stop then that can be quite a bit longer at 5 m or 6 m.

Even diving EAN32, I find GFHigh 70 limits my bottom time excessively

 

[harvalen]

Even diving EAN32, I find GFHigh 70 limits my bottom time excessively

Just looking at a square profile to 30 m, I would get 25 minutes of bottom time if start ascending when SurfGF reaches 95, which would be close to where I would limit it if I might have to abort the dive and go directly to the surface in an emergency. That would give me a stop time of about 4 minutes at 3 m with GFHigh 70, which isn't unreasonable and likely would be cleared on the ascent if the ascent is slow.

With redundancy and a reliable dive buddy I would be fine doing the full 30 min that my old tables tells me I can dive without decompression, that would give me a SurfGF of 105 when starting to ascend. The important thing is that I want my dive computer to tell me that my TTS is 14 min at that point, so, instead of me having an unknown waiting time at 3 m while the SurfGF slowly goes down, I know I have enough gas and dive time left to finish the dive with a SurfGF of 70.

On shallower dives, or most multi-level profiles, I rarely find myself above a SurfGF of 60 at the end of the dive when diving EAN32.

 

[boulderjohn]

I have been using SurfGF to guide my safety stops since they came out with it. Here are some observations/experiences.

• I leave my computer in tech mode for NDL dives, but I have learned that I need to change the GF high from its customary 70 because it gives too short an NDL dive. Now, I am smart enough to know that I am not really in deco on those occasions, but I have learned I have to keep other people who are not that smart happy. (Yes, I had some experiences.) I usually set it at 85, but I have used 95, too.
• On dives like that, I usually wait until the SurfGF drops below 70. It does not take long. In fact, it often happens before others in the group are done with their 3-minute countdown.
• I tend to use high levels of nitrox on NDL dives (when available), and so when diving with groups, I often reach safety stop depth with the SurfGF already below 70. Since everyone in the group needs to stare at their computers while the final seconds of their safety stops count down, I stick with them, often watching as my SurfGF drops below 50. (The other divers would also be below 50, but they have no idea about that.)
• I did a week of diving with Cocoview in Roatan a while ago, and all the DM-led dives spent most of their time in shallow water. I rarely did a dive where my SurfGF got above 50 at any point in the dive. On one dive, I paid careful attention to it because I was frankly annoyed by the restrictions on the dive. Diving 32% nitrox, I never got my SurfGF above 19, and I assume everyone else on the boat was about the same. When the DM signaled the end of the dive, everyone ascended the few feet necessary to reach safety stop depth, and they dutifully stayed there for the required 3 minutes, staring at their computers as they counted down those magical 3 minutes.

 

[EFX]

I would like to offer some information that may help our understanding of GF99 and SurGF. This may be old information to those of you who have been using GF's. Here are the two equations used to calculate GF99 and SurGF (they may not be exactly the equations Shearwater uses but they serve here to illustrate the concept):

GF99 = (p_ctc - p_amb) / (p_mv - p_amb) x 100
SurGF = (p_ctc - sp) / (p_mv - sp) x 100

where:
GF99 = % of the m_value at the current depth as measured from ambient pressure.
SurGF = % of the m_value at the current depth as measured from the surface pressure.
p_ctc = pressure in the controlling tissue compartment.
p_amb = ambient pressure.
p_mv = m_value pressure at the current depth.
sp = surface pressure.

The definition as given by Shearwater and mentioned in the article sited at the beginning of this thread is that SurGF is the value of GF if you could instantaneously surface from your current depth. The key word here is instantaneously. Obviously, you can't surface instantaneously so the value you read at depth is not the GF you'll see when surfacing. As you ascend your tissues will be off gassing which will reduce your SurGF when you eventually get to the surface. So, it represents a worst case value of the controlling tissue compartment (CTC) at the depth and time you look at it. As you ascend it will slowly decrease because the pressure in the CTC is off gassing and therefore the numerator in the equation above is decreasing faster than the base (p_mv - sp). SurGF should always be higher than GF99 because the sp is always lower than the ambient pressure during the dive.

The opposite is happening with GF99. It is increasing as you go shallower. Because of the half-time of the CTC p_ctc is not decreasing as fast as the ambient pressure. This suggests that GF99 and SurGF are converging as the diver ascends from depth. In fact, this is exactly what happens because when the diver surfaces, p_amb in the equation for GF99 becomes the sp and both equations must yield the same result.

Another thing I would like to point out is that in the equation for GF99 in order for the numerator to be positive, the p_ctc must be higher than p_amb. This implies that the CTC is off gassing. When the p_ctc is less than ambient pressure, the numerator being negative, the CTC is on gassing.

Since SurGF is always measured against the surface pressure it becomes an indication of how much inert gas the CTC contains. This is why you'll see a value for SurGF while GF99 reads on gas. GF99 is measured against ambient pressure and is indicative of the rate of off gassing. The higher the GF99 the higher amount of gas in the CTC and the higher the rate.

Edit 1: added definition for sp and cleaned up the last sentence.
Edit 2: added last paragraph.

 

[harvalen]

• I leave my computer in tech mode for NDL dives, but I have learned that I need to change the GF high from its customary 70 because it gives too short an NDL dive. Now, I am smart enough to know that I am not really in deco on those occasions, but I have learned I have to keep other people who are not that smart happy. (Yes, I had some experiences.) I usually set it at 85, but I have used 95, too.
• On dives like that, I usually wait until the SurfGF drops below 70. It does not take long. In fact, it often happens before others in the group are done with their 3-minute countdown.

I always dive with multiple computers, so I actually have one of them set to low conservatism for NDL times instead of watching SurfGF, while I have my GFHigh 70 computer telling me if TTS starts getting into territory I'm not comfortable with for the dive. With a single computer I would likely still use GFHigh 70 and watch SurfGF to know that I can abort the dive if needed, but I've never had to prove my innocence to the dreaded dive police.

With EAN32 and GFHigh 85 I guess it's pretty hard to get into situations where the stop at the end to get down to 70 would be too long. With GFHigh 95 and EAN32 you could get a stop over 10 min at 3 m on a wreck where you max out time from 30 m to 15 m. With a single AL80 gas consumption will likely be a limit though, especially in a guided group dive. Would be far from impossible with 15l tanks in a more experienced group though. Calculated a test profile and while the SurfGF went down from 95 to 92 at 15 m the stop time at 3 m went up from 10 min to 11 min (GFHigh 95 NDL time went from 60 min to 54 min), so I prefer to have one computer with GFHigh set to 70 just to be sure, since the SurfGF for the safety stop is a pretty blunt tool.

 

[Hodg]

I would like to offer some information that may help our understanding of GF99 and SurGF. This may be old information to those of you who have been using GF's. Here are the two equations used to calculate GF99 and SurGF (they may not be exactly the equations Shearwater uses but they serve here to illustrate the concept):

GF99 = (p_ctc - p_amb) / (p_mv - p_amb) x 100
SurGF = (p_ctc - sp) / (p_mv - sp) x 100

where:
GF99 = % of the m_value at the current depth as measured from ambient pressure.
SurGF = % of the m_value at the current depth as measured from the surface pressure.
p_ctc = pressure in the controlling tissue compartment.
p_amb = ambient pressure.
p_mv = m_value pressure at the current depth.
sp = surface pressure.

The definition as given by Shearwater and mentioned in the article sited at the beginning of this thread is that SurGF is the value of GF if you could instantaneously surface from your current depth. The key word here is instantaneously. Obviously, you can't surface instantaneously so the value you read at depth is not the GF you'll see when surfacing. As you ascend your tissues will be off gassing which will reduce your SurGF when you eventually get to the surface. So, it represents a worst case value of the controlling tissue compartment (CTC) at the depth and time you look at it. As you ascend it will slowly decrease because the pressure in the CTC is off gassing and therefore the numerator in the equation above is decreasing faster than the base (p_mv - sp). SurGF should always be higher than GF99 because the sp is always lower than the ambient pressure during the dive.

The opposite is happening with GF99. It is increasing as you go shallower. Because of the half-time of the CTC p_ctc is not decreasing as fast as the ambient pressure. This suggests that GF99 and SurGF are converging as the diver ascends from depth. In fact, this is exactly what happens because when the diver surfaces, p_amb in the equation for GF99 becomes the sp and both equations must yield the same result.

Another thing I would like to point out is that in the equation for GF99 in order for the numerator to be positive, the p_ctc must be higher than p_amb. This implies that the CTC is off gassing. When the p_ctc is less than ambient pressure, the numerator being negative, the CTC is on gassing.

Since SurGF is always measured against the surface pressure it becomes an indication of how much inert gas the CTC contains. This is why you'll see a value for SurGF while GF99 reads on gas. GF99 is measured against ambient pressure and is indicative of the rate of off gassing. The higher the GF99 the higher amount of gas in the CTC and the higher the rate.

Edit 1: added definition for sp and cleaned up the last sentence.
Edit 2: added last paragraph.

One thing I'm not getting here, what exactly does the M Value pressure represent?

 

[inquis]

One thing I'm not getting here, what exactly does the M Value pressure represent?

A GF of 100% is where Buhlmann didn't need to find more goats.

 

[Hodg]

A GF of 100% is where Buhlmann didn't need to find more goats.

It's the units that were throwing me. I was thinking m values were percents but that's gradient factors.

 

[EFX]

The m_value stands for maximum pressure. It should be understood as the maximum pressure difference between either itself and the surface pressure or between itself and ambient pressure. Since we need to keep the off gassing rate below a safe value it makes more sense to take the pressure difference between itself and ambient pressure. This value is the denominator in the following equation:

GF99 = (p_ctc - p_amb) / (p_mv - p_amb)

where p_ctc is the pressure in the controlling tissue compartment (CTC), p_amb is the ambient pressure of the surrounding water, and p_mv is the m_value. If p_ctc is equal to p_amb in the equation above then GF99 equals 0. If p_ctc is at the m_value then GF99 equals 1. Multiplying these values by 100 gives a percentage of the m_value pressure equal to 0% and 100% respectively.

Gradient factors (GF's) should not be confused with GF99 and SurGF. GF99 and SurGF are calculations done on the amount of inert gas in the CTC. GF's are values you enter for GFLo and GFHi and are percentages applied to 100% of the m_value. Therefore, the GFHi value becomes the m_value for the dive upon surfacing. GFLo is a percentage of the m_value and is used to determine the first stop depth for deco dives.

The actual pressure units used in Erik Baker's fortran code and in my spreadsheet are in feet of sea water (fsw), feet of fresh water (ffw), meters of salt water (msw), or meters of fresh water (mfw). Here is an explanation taken from my spreadsheet:

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).

 

[Hodg]

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).

Thanks, EFX! That last section was really what I was asking about. Appreciate the explanation.