Dive computers... SO many choices!

[LandonL]

@stuartv Ah yeah, no worries. I wasn't taking it directly. lol

Should have been more specific with the sentence about "high values like 95"... i mean both low and high GF values that are numerically high.

Im wondering if the DSAT profiles versus a ZHL-16C GF High of 95 are just comparing square profiles, or multilevel dives? Im thinking about drift between the algorithms when the active tissue compartments start to shift, I don't know much about how DSAT is structured and calculated.

 

[KenGordon]

Well... technically, the GF High determines when you begin your ascent (if you are observing NDLs). GF Lo just determines where your first stop is once you've gone into deco and you decide when to start your ascent.

Don't believe me? Fire up the dive planner on your Shearwater and set your GF to 99/99 then plan a dive to 130' for 1 minute. It will tell you what your NDL is. Then change the GF to 30/99. You will see that you still have the same NDL - because the GF Hi determines when you have to begin your ascent.

If you go into deco, then you decide when to begin your ascent, not the computer, and the GF Lo just determines where the first stop is.

Subsurface dive plan
based on Bühlmann ZHL-16C with GFlow = 75 and GFhigh = 75
Runtime: 63min

depth duration runtime gas
➘ 15m 2min 2min air
➙ 15m 59min 61min
➚ 0m 3min 64min

CNS: 8%
OTU: 5

Gas consumption:
3093ℓ/279bar of air (79ℓ/7bar in planned ascent) — Warning: this is more gas than available in the specified cylinder!

Subsurface dive plan
based on Bühlmann ZHL-16C with GFlow = 25 and GFhigh = 75
Runtime: 64min

depth duration runtime gas
➘ 15m 2min 2min air
➙ 15m 59min 61min
➚ 3m 2min 63min
➚ 0m 1min 64min

CNS: 8%
OTU: 5

Gas consumption:
3094ℓ/279bar of air (80ℓ/7bar in planned ascent) — Warning: this is more gas than available in the specified cylinder!

 

[KenGordon]

@stuartv Ah yeah, no worries. I wasn't taking it directly.

Should have been more specific with the sentence about "high values like 95"... i mean both low and high GF values that are numerically high.

Im wondering if the DSAT profiles versus a ZHL-16C GF High of 95 is just comparing square profiles, or over multilevel dives? Im thinking about drift between the algorithms when the active tissue compartments start to shift, I don't know much about how DSAT is structured and calculated.

DSAT only has m0 (surface m value) values. No slope. It is pretty hard to compare how it would behave vs a ZHL model which depends on depth. I can only guess how it deals with high tissue loads at depth. Maybe it just assumes a 10m/m ascent and if you get to the surface and m0 is not exceeded you get to surface, otherwise you need to wait until you could ascend. I am only guessing...

See table 1 http://www.ddplan.com/reference/mvalues.pdf

The 95% value being pushed by the DSAT fans is really just a guestimate based on NDL times I think.

 

[KenGordon]

I never said the low GF has any effect on the NDL, but yes 45/99 or 99/99 are exactly the same UNTIL you pass the NDL, so if someone does that accidentally I wouldn't suggest decompressing on a 99/99, and I wouldn't really want to be a manufacturer producing a recreational computer that allowed such. You do have to consider your insurance for producing such a product that would do this. If someone were diving that profile they are effectively diving a straight Buhlmann M-line, and have eliminated the safety margin.

How are you calculating the first ceiling? Is it at the limit of GF lo? The limit of GF hi?

What is a no stop dive? One where the tissue saturation is ok to reach the surface directly? ie the ceiling is at or above the surface. Or is one where if you ascent at 10m/m you will not reach the ceiling? (ie you race the ceiling to the surface)

Is the diver hed at a stop until they no longer exceed the GF for that depth or for the next stop up - so for a 10/90 dive is with the first stop at 6m can they leave 6 at 50 or only at 10? assuming another stop at 3)

 

[LandonL]

How are you calculating the first ceiling? Is it at the limit of GF lo? The limit of GF hi?

The first ceiling is the first stop, so it is a function of GF Low.

What is a no stop dive? One where the tissue saturation is ok to reach the surface directly? ie the ceiling is at or above the surface. Or is one where if you ascent at 10m/m you will not reach the ceiling? (ie you race the ceiling to the surface)

Well mathematically, you do not enter decompression until the ceiling drops below the surface. That being said you shouldn't outpace the safe ascent rate.

Is the diver held at a stop until they no longer exceed the GF for that depth or for the next stop up - so for a 10/90 dive is with the first stop at 6m can they leave 6 at 50 or only at 10?

So, the diver spends time in the stop until partial pressure drops in the tissue compartments enough for enabling ascent to the next stop, which has a bit higher GF than the stop before it. You are sliding down the GF line from the GF low, to the GF high. So the GF gradually increases.

 

[KenGordon]

The first ceiling is the first stop, so it is a function of GF Low.



Well mathematically, you do not enter decompression until the ceiling drops below the surface. That being said you shouldn't outpace the safe ascent rate.



So, the diver spends time in the stop until partial pressure drops in the tissue compartments enough for enabling ascent to the next stop, which has a bit higher GF than the stop before it. You are sliding down the GF line from the GF low, to the GF high. So the GF gradually increases.

So is a no stop dive any dive where the ceiling stays above the surface? So if the GF low brings the first stop below the surface then is it still a no stop dive?

 

[stuartv]

Subsurface dive plan

Do you dive Subsurface? Or a dive computer?

I quoted the Engineering Manager from Shearwater in the post I linked you to. He explicitly states that it's only GF Hi that determines when you go into deco.

 

[LandonL]

So is a no stop dive any dive where the ceiling stays above the surface?

Yes, a no stop dive, by definition, is a dive that does not require decompression stops. This means that the ceiling is above the surface.

Again though, there is a difference between theory, and reality. Even if it is a no stop dive, you should still use the safe ascent rate. Consider the safe ascent rate a virtual ceiling that is a constant distance in front of you on your way to the surface.

So if the GF low brings the first stop below the surface then is it still a no stop dive?

No. Once you have required decompression stops, it is no longer a no stop dive, you have passed the NDL (No Decompression Limit).

The NDL is determined by your GF High value, and your first stop is defined by the GF Low.

See the article here: Gradient Factors | Dive Rite
It has a really good explanation with some graphs that should help.

 

[stuartv]

Im wondering if the DSAT profiles versus a ZHL-16C GF High of 95 are just comparing square profiles, or multilevel dives? Im thinking about drift between the algorithms when the active tissue compartments start to shift, I don't know much about how DSAT is structured and calculated.

DSAT only has m0 (surface m value) values. No slope. It is pretty hard to compare how it would behave vs a ZHL model which depends on depth.

@scubadada has been diving DSAT for a long time and more recently diving with a Nitek Q added to his kit so that he can see how DSAT and Buhlmann w/GF compare to each other in the real world. When you do that, it's actually not so hard to compare them.

He can elaborate if he cares to, but from what he has told me, GF Hi of 90 - 95 gives ABOUT the same NDLs as DSAT, depending on the dive details.

Regardless, the way some computers work, including Shearwater, they only have "m0 (surface m value)" also - until you exceed the NDL. So, it should not actually be that hard to compare DSAT to a Shearwater.

 

[stuartv]

The NDL is determined by your GF High value, and your first stop is defined by the GF Low.

This is the part KenGordon will not read or believe.