Dive computers... SO many choices!

[KenGordon]

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.

I try to dive a plan. The computer is secondary really. Otherwise I do not know if I will have enough gas. That is one of the main pints of planning. If you plan and then get unexpected stops you may be in trouble.

I think explicitly is a bit strong. You would need to ask the question like 'is GF lo used AT ALL when calculating NDL?' And you did not. His was a bit of an arm waving excercise about the stops having cleared when you got there.

However I can see that if 30m 45/85 gives a stop after 10 minutes (36 seconds at 6 and 2 minutes at 3 by multi deco) when the competition is still without stops after 18 minutes and your target market is not trusted to do those stops then you might decide to first check the surfacing tissue limit and skip calculating stops if it was ok. However that is not what Erik Baker described.

Do you accept that the GF low prescribes the first stop? I think that was the original point you disputed and you went down the 'it cannot be because my shearwater ignores it for NDL' route to argue that.

You say you make a living from writing software, try writing an implementation. It is an education.

 

[tbone1004]

@Rred you can't/really don't want the computers factoring in ambient temperature because they don't know your core temp. It has no idea if you're in a drysuit in 73f and sweating your balls off, or diving in a bathing suit and shivering the whole time. There has also been no formal studies on thermal stress and how different decompression algorithms should be adjusted, and no correlation between specific ambient temperatures and how that impacts DCS.

i.e. ambient temp doesn't matter, your skin and core temperature does, and the computers don't have a way to measure that, so they shouldn't adjust for it. If one did, it would be a computer that I would never dive and I doubt any of my tech diving buddies would either

remember also that the USN tables are completely different than anything that is really done in the recreational world because they were intended for one long dive per 24 hour period, not repetitive diving, they account for a very high chance of DCS because their S&P's require chamber access to "bend 'em and mend 'em" is OK, and a myriad of other variables that don't really apply to recreational diving. one stop up from the USN tables is the NAUI tables, so do you go another stop up? or just don't worry about it because diving truly square profiles is exceedingly difficult and rare, so you have extra conservatism padded into the dive profile if you're diving tables, and if you're diving a computer, odds are the algorithm is nowhere near as liberal as the dive tables are to begin with so what does it matter?

 

[KenGordon]

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.



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.

If GF low is used to calculate the first stop and the presence or absence of stops determines whether a dive is a deco dive or not then surely the GF low is involved in determining the No deco limit?

Do you determine the NDL by solving for the limiting tissue tension or by testing a bunch of times until there are stops?

(By the way, when I say 'you' I mean you, writing the code for your device, not 'one' or people in general)

 

[stuartv]

I'm going to give you one more go and then I have to get some real work done.

However that is not what Erik Baker described.

Do you accept that the GF low prescribes the first stop? I think that was the original point you disputed and you went down the 'it cannot be because my shearwater ignores it for NDL' route to argue that.

Baker's Fortran code implementation of GF does indeed use GF Lo to determine the depth of the first stop, and that is set regardless of what the GF Hi is. It is quite simplistic, actually, in that the first stop is actually the ceiling. The calculation does not factor in ascent time. So, it could easily give you a list of stops to make that you would never actually need to make because you will off-gas enough during the ascent that your leading compartment tension would be less than the target for that depth before you ever get there.

I have never disputed that GF Lo prescribes the first stop. What I have disputed is whether it is used in the case that a direct ascent would not exceed GF Hi at the surface. Baker's code does use it. Other implementations of his concept do not.

If GF low is used to calculate the first stop and the presence or absence of stops determines whether a dive is a deco dive or not then surely the GF low is involved in determining the No deco limit?

So, one more time, some implementations (e.g. Shearwater's) use the GF Hi to determine at what time you would no longer be able to do a direct ascent without exceeding GF Hi at the surface. That is your NDL time. Once you exceed it, then you will have to make 1 or more deco stops and GF Lo is used to calculate where that first stop is. So, no, GF Lo is NOT NECESSARILY involved in determining the No Deco Limit. It depends on how the implementer chose to implement the algorithm.

Another choice for the implementer is whether to take the ceiling, based on GF Lo, as the first stop, or whether to calculate the time during the ascent where GF Lo would be exceeded and use that as the first stop. Baker's actual code does it the first way.

My opinion is just my opinion and is completely meaningless until such time as I choose to implement a deco algorithm into some code myself. That said, my opinion is that:

- if I can do a direct ascent to the surface without exceeding GF Hi at the surface, then that's what I want my computer to tell me, as far as NDLs go. If I'm not going to exceed GF Hi at the surface (after a 30ft/min ascent), then I really don't care what GF Lo is. Virtually every dive more than 5 or 10 minutes to deeper than 40 or 50 feet is likely to result in surfacing with tissue tensions higher than most GF Lo values (that people actually use) would allow. So what? If I'm diving GF30/70 and a direct ascent will result in having 40% of the M value in my leading compartment at the surface, should I have to do a deco stop at 10'? That seems ridiculous. And it's even more ridiculous if I'm at 60' depth and the 30% ceiling is at 10', so I'm prescribed a 10' stop, even though my leading compartment is only at 25% by the time my ascent gets me to that depth (a la Baker's Fortran implementation).

- Baker's implementation calculates the first stop as the current ceiling, using GF Lo, at the end of the last bottom time dive segment. Again, let's say you're diving GF30/70. You're at 150' and his implementation says that your first stop is at 70'. It has determined that by, essentially, saying that if you magically transported from 150' to 70' instantly, you would need to hold at 70' for some amount of time in order to avoid having your leading compartment exceed 30% of the M value. In reality, by the time you actually ascend that far, you may very well off-gas enough that you could continue to ascend to 60' or higher without having your leading compartment exceed 30% of the M value. So, how much sense does it really make to stop at 70' if you could proceed non-stop to 60' without exceeding 30% of the M-value in your leading compartment?

Now, all that I have said is based on my reading of Baker's code. I have not compiled it and run it to prove that it works the way my reading of it indicates. I could be wrong. If I am, I hope someone will explain to me why.

One thing I am not wrong about is that the Shearwater Dive Planner on my Perdix AI does NOT use the GF Lo value in determining an NDL. A dive to 150' on GF90/90 has exactly the same NDL as a dive to 150' on GF10/90. Of course, using the Planner is not the same as actually doing the dive. And, as SW does not allow changing GF Lo during a dive, I guess there's no way I can prove that it works the same way during a dive as what the Planner says, short of diving 2 Shearwaters together, with different GF settings. In the meantime, I have faith in Shearwater, that the computer will perform the same during a dive as what the Planner predicts. And, I am completely comfortable diving a GF implementation that does not use GF Lo when calculating NDL. YMMV

Lastly, this is a lot of debate about how to do something that Baker's algorithm wasn't really even intended for. I could be wrong, but I believe his algorithm is/was specifically intended to calculate ascents for decompression dives. It was not designed or intended to be used to calculate NDLs. So, how his implementation works when you try to use it for NDL dives may not really be the "best" way to use his algorithm or the best way to calculate NDLs. In other words, even though his implementation always calculates the first stop using GF Lo, that doesn't mean that that is the best way to calculate an NDL. I personally think the way Shearwater and others are doing it (using only GF Hi) is a better way of calculating an NDL.

 

[LandonL]

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.

If GF low is used to calculate the first stop and the presence or absence of stops determines whether a dive is a deco dive or not then surely the GF low is involved in determining the No deco limit?

I do agree with trying to keep things simple for the sake of being practical, he wasn't directly explicit so maybe it was confusing, maybe this is what Ken is focusing on? Am I correct Ken?

So I guess if you really wanted to have an academic argument that is somewhat pointless, the GF Low does have some effect on the whether the planned dive is going into deco or not, if using planning software, because of ascent rate assumptions used by the program, but practically these stops are so short and cleared by arrival, you can ignore them (which is what some of the computer planning models do - why stop for 10 seconds 30 feet above you when your tissue will be clear by the time you get there). Given this fact, is the stop really relevant if you have cleared it before arrival? Which is exactly what is being said in the expanded explanation from Shearwater.

The software makes assumptions about ascent rates that affect stops, so some get glossed over, but they may reappear if they expand enough to fall outside of the ascent rates. It may give the appearance of GF Low affecting the NDL, when in fact it does not.

If we really really wanted to argue semantics, technically every dive is a decompression dive, but we don't run around arguing this point with everyone, and correcting new divers, because while mathematically you are off-gassing after a dive, practically you do not have a ceiling obligation, so the argument is.... academic.

It's kind of like someone saying "I'm standing still" and someone corrects them with "Actually, you are wrong, because you are standing on earth, and its rotating at 1000 mph" and the next guy says "you are both wrong, because even though you are standing still, and the earth is rotating at 1000mph, you are traveling around the sun at 67,000 mph"... and so on, and so on.

Do you determine the NDL by solving for the limiting tissue tension or by testing a bunch of times until there are stops?

(By the way, when I say 'you' I mean you, writing the code for your device, not 'one' or people in general)

The NDL is determined by the intercept of the GF line with the pressure at the surface (1 ATA if assuming sea level). Again, this is determined by the GF High. In planning software, GF low can appear to affect the NDL, because it is possible for you to ascend enough to hit the ceiling during ascent, so it can add a minute or so to total runtime, because it is shifting the saturation downwards, but not really affecting the intercept, and there are some assumptions being made about ascent rates. It is when you hit the GF line, based on those assumptions by the planning software, the intercept changes, because it is being recalculated based on the new dive dynamics.

An example in Subsurface..... plan a dive to 30m, on EAN30, with a 3 minute descent to depth, and a 30 minute time at depth.... do this for 30/85, then change it to 20/85.. you will end up with a brief stop, which changes the run time from 43 to 44 minutes.

In other words, your ascent rates cause you to hit the GF-line because the slope of the GF line changed, not so much the intercept, but when you hit the line, your dive changes slightly, so it may shift the intercept a very very small bit because tissue saturation will be slightly affected due to the change in the dive (your overall ascent rate with minor stop).

 

[LandonL]

Speaking of varying algorithms and times...I see most dive computers can display the ambient temperature, but I haven't seen them mention if they are accounting for that in their calculations.

The "standard" USN dive tables that many of us started with, were officially "warm water tables" and the USN quietly but clearly said "If you need to wear a wet suit or you feel cold, you should use the cold waters tables instead." Which was basically one time/depth group "up", what would be called one additional personal safety/comfort factor now.

So I'm wondering, do any of the computers adjust their tables, based on ambient temperature? In theory, that has to be accounted for in order to calculate safe times. Perhaps that's quietly a part of why some computers are 'too conservative" compared to others?

There isn't any point in calculating for ambient temperature directly. This whole concept is related to physiology. The idea behind it is that if you get cold, your body is going to try and conserve heat by shifting blood flood towards internal organs and less so towards the extremities. Decreases in blood flow to the arms, legs, surface tissues, etc mean that gas exchange is slowed down because blood flow is no longer optimal so off-gassing can be inhibited.

Since everyone is different, you can't directly calculate the changes, because they aren't universally quantifiable. Body fat percentage, age, smoking status, etc will all have some effect as well, but it varies person to person.

I mean, I guess if someone wants to volunteer to have a temperature probe stuck up their exit orifice....

 

[ls1dreams]

Now to your question - technically the 'safest' GF setting would be lowest numbers that you are willing to dive with, as this would keep your tissue saturation minimal throughout the dive and at the surface, although using incredibly low values for both would create some ridiculous dive profiles. Picking GF values is more about picking what you are comfortable diving with. It's all about your comfort level in relation to the M-line.

As a point of clarification, isn't this untrue for the lo gradient? The deep stop studies showed higher levels of DCS when people were trying to do something closer to a reduced bubble model, which is what the lo gradient does with smaller numbers.

"Safe" for the lo gradient is a bit contradictory I believe:

- A low number (0-30) approximates RGBM or deep stops, which is known to not work and increase DCS
- A very high number (90+) risks going past supersaturation limits / M-values though, which is obviously very dangerous if you blow your ceiling at any given time.

For the hi gradient, obviously the lower the number the better - it just impacts time/ascent rate.

So, what makes sense for low conservatism? That's why I thought something like (40-60) / (lowest possible) would be best. I'm not sure what the advantage is of setting all of your low values in the 20-40 range, except for keeping people very far away from the M-values.

 

[stuartv]

The NDL is determined by the intercept of the GF line with the pressure at the surface (1 ATA if assuming sea level).

I agree that this is how I would want it to be. And I note that the intersection of the GF line with the pressure at the surface is always and exactly the GF Hi value. That is by definition. So, if NDL is actually calculated that way, GF Lo would have NO effect on the NDL.

GF Lo does not affect the intersection of the GF line with the pressure surface in any other way than by determining the slope of the line at that intersection.

Example for you and @KenGordon:

You do a dive on air to 60' for 45 minutes, with GF30/90. At the end of that 45 minutes, your leading compartment pressure gradient would be greater than 30% if you were magically, instantly at the surface. But, if you ascend at 30 ft/min, by the time you get to the surface, your leading compartment has never actually exceeded 30% of the M-value. That should not be a deco dive.

But, by Baker's code, and what KenGordon keeps saying, this dive would be a deco dive. Because the calculated ceiling is based on GF Lo, and after 45 minutes it is something like 1'. And once you have a 1' foot ceiling, that means a mandatory stop at 10'. I think that is, well, definitely not how I would want my computer to work. For reference, NOAA Air tables give a 60 minute NDL at 60'. If my GF Hi is 90, I definitely would not want to be told I'm in deco after 45 minutes.

 

[stuartv]

I mean, I guess if someone wants to volunteer to have a temperature probe stuck up their exit orifice....

As I noted earlier, some ScubaPro computers come with a heart rate monitor that is also a skin temperature sensor, both of which pieces of data are somehow incorporated in their algorithm. Since there are plenty of devices out these days that can measure a person's temperature without being inserted anywhere, I would guess that the SP skin temp sensor at least has a chance of providing reliable body temp info. I don't know. I don't really know anything about body temp sensing tech, so like I said, it's just a guess.

 

[LandonL]

Example for you and @KenGordon:

Uh? Why do I need an example? What I have said is correct. I think maybe I am not getting my point across. Would be more than happy to explain it verbally though... lol