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

[boulderjohn]

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.

If you overrun your NDL and have a deco requirement, you should ascend at the rate your computer calls for and do any required stops below 6m/20 feet for the time the computer tells you. It won't necessarily hurt you to extend a deeper stop, but it may end up requiring a longer shallower stop. When you are above 6m/20 feet, prolonging stops beyond what the computer says won't hurt and will be beneficial.. That's when you can be watching the SurfGF to look for number you like.

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.

Here Lowwall identifies a key difference between a dive within NDLs and a dive with required decompression. As he writes here, once you begin an ascent within NDLs, unless you do some significant stops along the way, it really doesn't matter what you do on your ascent. At some point it is recommended that you do a safety stop, but that's it. When you violate NDLs, how you ascend matters, and any delays along the way add to the total requirements for decompression.

 

[dmaziuk]

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.

Did you really do 45 minutes at "yo-yo within a few metres" of 23 msw on dive 2? What size tank were you using, or are you a petite female with gills?

What kind of computer did they give you?

 

[Joemdown]

Did you really do 45 minutes at "yo-yo within a few metres" of 23 msw on dive 2? What size tank were you using, or are you a petite female with gills?

What kind of computer did they give you?

That’s what the comp said depth was and the time in and out matched.

Worth noting the dive was pretty much just sit still and watch the bull sharks, no swimming or anything of effort just a following them around a little

AL80

I don’t really sip much air, used 190bar =2,755 PSI according to lo

No idea, aqualung something

 

[dmaziuk]

That’s what the comp said depth was and the time in and out matched.

AL80

I don’t really sip much air, used 190bar =2,755 PSI according to log

No idea, aqualung something

Huh. Off the top of my head you maybe could run out of NDL on the 2nd dive, but I don't think you should have, not with 1 hr SIs between the dives.

I'm not familiar with AL's "PZ+" algorithm so I can't comment on that. RGBM stretches the "repetitive dives" window to 48 hours so a Suunto or Cressi could have easily come with the gas loading from the previous renter, that would explain it, but IDK if an AL would've kept that overnight... it could've been set to extra-high conservatism I suppose.

 

[Joemdown]

Huh. Off the top of my head you maybe could run out of NDL on the 2nd dive, but I don't think you should have, not with 1 hr SIs between the dives.

I'm not familiar with AL's "PZ+" algorithm so I can't comment on that. RGBM stretches the "repetitive dives" window to 48 hours so a Suunto or Cressi could have easily come with the gas loading from the previous renter, that would explain it, but IDK if an AL would've kept that overnight... it could've been set to extra-high conservatism I suppose.

Fair enough. I didn’t feel bad or even tired. So no symptoms.

Was just wondering if I got lucky or if the dives truly were within limits.

Worth noting someone else had a cressi and it also went into gauge. But based on him asking the instructor after what that meant, I’m sure he didn’t know how to use his computer properly either.

Anyway, thanks for the responses (everyone) really helpful in understanding some of what the computers are saying/calculating.

 

[lowwall]

Minor correction... when SurfGF hits GFHigh.

I assumed that SurfGF and GF99 were based on the percentage of the GFhigh setting. Do they actually always show a percentage of the full M-value? In other words, if you are diving with a GFhi of 70, does SurfGF display 70% or 100% when NDL hits 0?

It seems to me that it would be confusing to have them as a percentage of the unadjusted M value, especially if you find the computer telling you to do a deco stop while your SurfGF is well below 100. OTOH, I could see the advantage of knowing your unadjusted SurfGF if something happened on the dive that made you want to consider breaking the NDL. Also having a consistent number is more useful if you are trying to narrow in on a range that works best for you.

[Goes and finds a Teric manual]

Aha, "100% means the leading tissue super-saturation is equal to the original M-Value limit in the Bühlmann ZHL-16C model."

To deal with the potential confusion, "If the current GF is greater than GF High, SurfGF will be displayed in Yellow. If the current gradient factor is greater than 100%, SurfGF will be displayed in Red."

As an aside, every time I look into how Shearwater computers function, I come away impressed. Someday I'm going to have to bite the bullet and get one. Maybe when my daughter takes her OW course because we'll be one computer short

 

[dmaziuk]

...
Aha, "100% means the leading tissue super-saturation is equal to the original M-Value limit in the Bühlmann ZHL-16C model."

Well, when you think about it, if SurfGF was anchored to GF High, then by definition 100% SurfGF = 0 NDL. So it would be just another name for NDL and not very useful as a distinct separate metric.

 

[lowwall]

Well, when you think about it, if SurfGF was anchored to GF High, then by definition 100% SurfGF = 0 NDL. So it would be just another name for NDL and not very useful as a distinct separate metric.

Not useful on the bottom, but still useful once you head up.

Anyway, that's not what they do, so it's useful wherever you are. Unless you run your GFhi at 100

 

[StevePix]

Here’s how I apply them in my diving:

• When I get to my safety stop, I want to see GF99 be in the 20-40 range. If it’s only 2%, a safety stop at that depth is doing almost nothing, so I’ll ascend a few feet and watch the GF99 increase. Example: I recently did some deep wreck dives in a current. While hanging on the crowded rope at 17 feet, my GF99 was in the single digits. As soon as space on the rope was available I ascended to about 12 feet, and GF99 increased to the 20’s. That’s a much more efficient, yet still safe, level of off-gassing.
• I use the default GFhi of 85 to determine my NDL, but I’d prefer to surface with my SurfGF no higher then 70 (even better if I can get it no higher than 50 when I surface). So, assuming gas and other conditions are favorable, I will extend my safety stop to continue reducing SurfGF to my preferred range. Recent example: at the end of a Belize blue hole dive, my SurfGF was nearly 80 when I got to the safety stop. I had plenty of gas, so I did an 8 minute safety stop and reduced my SurfGF close to 50, increasing my conservatism margin.

 

[Divin'Papaw]

Here’s how I apply them in my diving:

• When I get to my safety stop, I want to see GF99 be in the 20-40 range. If it’s only 2%, a safety stop at that depth is doing almost nothing, so I’ll ascend a few feet and watch the GF99 increase. Example: I recently did some deep wreck dives in a current. While hanging on the crowded rope at 17 feet, my GF99 was in the single digits. As soon as space on the rope was available I ascended to about 12 feet, and GF99 increased to the 20’s. That’s a much more efficient, yet still safe, level of off-gassing.
• I use the default GFhi of 85 to determine my NDL, but I’d prefer to surface with my SurfGF no higher then 70 (even better if I can get it no higher than 50 when I surface). So, assuming gas and other conditions are favorable, I will extend my safety stop to continue reducing SurfGF to my preferred range. Recent example: at the end of a Belize blue hole dive, my SurfGF was nearly 80 when I got to the safety stop. I had plenty of gas, so I did an 8 minute safety stop and reduced my SurfGF close to 50, increasing my conservatism margin.

I use GF99 and SurfGF in a very similar way. On the Shearwater Perdix & Peregrine, you can add them to a mini-gauge to display on the main screen at all times. Along with available gas (GTR & remaining PSI) I pay attention to NDL for bottom time but then focus almost exclusively on SurfGF and GF99 during my ascent.