Difference between MB levels and Gradient Factors

[dmaziuk]

What I am saying is that people are hyping features of a computer, including this feature, that have been added to that computer with no evidence that they do any good and which may, in fact, do harm.

As distinct from, say, displaying SurfGF or G99? Granted, it's hard to imagine how those could possibly cause harm, but then again, I don't think anyone ever called Pyle stops (which is what PDIS effectively is) harmful either. Especially if one doesn't take them.

(Speaking as a not-a-fan of SurfGF)

 

[boulderjohn]

I don't think anyone ever called Pyle stops (which is what PDIS effectively is) harmful either.

I think a lot of people say that Pyle stops done without compensation by extending shallow stops are potenetially harmful.

I am not saying anything one way or the other about deep stops in NDL diving. I am just saying we don't know enough to say. In decompression diving, we have more of a sense that they are not so good.

 

[Wibble]

I don't think I ever said it is a good thing, but I don't think that is the end of the world for most recreational divers either.

Rebreather and technical divers are a small percentage of divers buying computers...I will be the first to say that the G2 is not the computer they want. I don't see anybody (ok, I don't see any sensible person!) trying to convince CCR and technical divers to drop their Shearwaters and replace them with a G2 either. For non-ccr and non-technical divers, proprietary algorithms based on Buhlman or RGBM are not using the "devil's" algorithm like some people make them out to be. For a recreational diver, the G2 is a nice computer and is cheaper than the Shearwater here in the USA. G2 + Transmitter = $1,350. Perdix 3 with Swift Transmitter = $1,475.

The G2 Tek which is running ZHL-16 GF is not a bad technical dive computer either. I use it as a backup to my Shearwater Petrel 3 when on CCR. I am able to keep some of my checklists on the G2 Tek. It tracks the primary controller just about as good as the Perdix 2.

The Shearwater's a no-brainer for a few bucks -- $125 -- more.

Personally I despise the damn fools who decided on brick mode for Suunto. My first dive computer, the Suunto D9tx -- that's a trimix "technical" 'puter says the Suunto blurb. Excited that I get to dive it for the first time on holiday. On arrival I jump into the pool with it in Gauge mode and quickly discover that the damn thing won't work in Dive mode for 48 hours. Also discovered that 36 hours isn't 48 hours, so it was a bloody brick for 4 days

Subsequently discovered what a turd it was for technical diving. Too complex and way too hard to use underwater... and bricks if you don't do the Suunto tax (extra time due to the proprietary algorithm even if it's in the most "aggressive" mode).

Agreed, this G2 computer's a little bit better than Suunto, but it's completely outclassed by the Perdix.

BTW why Swift? Why not save about $125 and buy the Aqualung transmitter which is very compatible with the Perdix (I use one on my rebreather). Oh, then the price is the same!

 

[dmaziuk]

I think a lot of people say that Pyle stops done without compensation by extending shallow stops are potenetially harmful.

I think a significant percentage of them were saying Pyle stops were actually only slowing his ascent, and were too short to incur significant extra loading. (Unlike the bubble-models' "deep stops".) That was supposed to affect mainly fast tissues, as they're the first to get overpressured on ascent, so I find this:

PDIS is different in that it determines the depth where the leading compartments (minus the two fastest compartments) used for the decompression calculation switch from on-gassing to off-gassing.

puzzling. I'm sure there is a point to it that I am not seeing.

 

[Sevenrider860]

The Shearwater's a no-brainer for a few bucks -- $125 -- more.

Personally I despise the damn fools who decided on brick mode for Suunto. My first dive computer, the Suunto D9tx -- that's a trimix "technical" 'puter says the Suunto blurb. Excited that I get to dive it for the first time on holiday. On arrival I jump into the pool with it in Gauge mode and quickly discover that the damn thing won't work in Dive mode for 48 hours. Also discovered that 36 hours isn't 48 hours, so it was a bloody brick for 4 days

Subsequently discovered what a turd it was for technical diving. Too complex and way too hard to use underwater... and bricks if you don't do the Suunto tax (extra time due to the proprietary algorithm even if it's in the most "aggressive" mode).

Agreed, this G2 computer's a little bit better than Suunto, but it's completely outclassed by the Perdix.

BTW why Swift? Why not save about $125 and buy the Aqualung transmitter which is very compatible with the Perdix (I use one on my rebreather). Oh, then the price is the same!

I used the Suunto DX for about a year...MacDive tells me I used it on 215 dives. Never had an issue while wearing a Shearwater at the same time even though they used different algorithms. The commonly stated "Suunto penalty" was never seen. It could very well be that the 9tx used the Technical RGBM algorithm and the DX used the Fused RGBM Algorithm.

For the average diver, I am not sure the $125 difference is worth it. For a technical diver, I agree...it is a no brainer. For the "run of the mill" recreational diver the value is not so clear cut.

 

[Sevenrider860]

I think a lot of people say that Pyle stops done without compensation by extending shallow stops are potenetially harmful.

I am not saying anything one way or the other about deep stops in NDL diving. I am just saying we don't know enough to say. In decompression diving, we have more of a sense that they are not so good.

John,

You are a technical instructor. What do you teach your students about ascent rates? Do you state 30 feet / 10 M per minute 100% of the time?

When I started my technical diving I was taught "no slower than 30 feet/min" until I reached the off-gassing depth and then "no faster than 30 feet/min" after that depth. V-Planner and MultiDeco provide the Decozone start depth where you switch from on-gassing to off-gassing. That would always be a depth I put on my manual schedules and circled. To me this depth is significant information. Basically it was a "don't be screwing around until you are above this depth" value that was always annotated.

I'm curious because I can see where some training may make knowledge of the on-gassing /off-gassing depth of no value where some training may highlight the value of having this knowledge.

 

[Wibble]

I used the Suunto DX for about a year...MacDive tells me I used it on 215 dives. Never had an issue while wearing a Shearwater at the same time even though they used different algorithms. The commonly stated "Suunto penalty" was never seen. It could very well be that the 9tx used the Technical RGBM algorithm and the DX used the Fused RGBM Algorithm.

When doing decompression diving, the Suunto D9tx was the backup computer to the Shearwater Perdix.

Used -- and still use -- 50:80 gradient factors on the Perdix and -2 "attitude" on the D9tx. Regularly found that I had to do a few minutes more with the Suunto still saying that it had not cleared when the Perdix had cleared.

Of course it's down to the different decompression algorithms.

Obviously I'd hang around for the few mins waiting for the Suunto to clear as there's no reason not to. But it still niggled that I had to hang around otherwise the Suunto would brick itself.

 

[Sevenrider860]

The Shearwater's a no-brainer for a few bucks -- $125 -- more.

Personally I despise the damn fools who decided on brick mode for Suunto. My first dive computer, the Suunto D9tx -- that's a trimix "technical" 'puter says the Suunto blurb. Excited that I get to dive it for the first time on holiday. On arrival I jump into the pool with it in Gauge mode and quickly discover that the damn thing won't work in Dive mode for 48 hours. Also discovered that 36 hours isn't 48 hours, so it was a bloody brick for 4 days

Subsequently discovered what a turd it was for technical diving. Too complex and way too hard to use underwater... and bricks if you don't do the Suunto tax (extra time due to the proprietary algorithm even if it's in the most "aggressive" mode).

Agreed, this G2 computer's a little bit better than Suunto, but it's completely outclassed by the Perdix.

BTW why Swift? Why not save about $125 and buy the Aqualung transmitter which is very compatible with the Perdix (I use one on my rebreather). Oh, then the price is the same!

Are we talking the educated diver or the average diver? The average diver is going to purchase what the dealer is selling. The majority of Shearwater dealers in the USA are selling the Shearwater Swift transmitter with the Shearwater computers. It may be different in the UK. I think we already determined that @Wibble would never buy anything but Shearwater...but @Wibble is also not the average diver that is shopping for a recreational dive computer are you? So relax, you settled on Shearwater and nobody that I know would argue with your decision.

 

[Sevenrider860]

Here is a far-flung analogy to explain my last point.

Years ago, I was working in a high school that, like most high schools, was perplexed by the percentage of students habitually failing classes. One teacher was listening to students talking about their part time jobs, and he decided that the students were working way too many hours, and he theorized that we could help our failure problem significantly by finding a way to limit the number of hours students worked during the school year. Everyone was enthused with the idea. I was tasked with dong the preliminary research of surveying student work schedules and correlating them with grades. To everyone's shock, the results were the precise opposite of what was expected--in general, the more hours a student worked, the higher the grade point average. The students who were habitually failing were pretty much not working outside of school.

When Wienke and Yount promoted their algorithms (RGBM and VPM) years ago, they pushed the theories hard, and almost everyone bought into it. I did. They had no research supporting those theories, but everyone was sure that was just a matter of time. It wasn't. No research has supported their approaches on decompression dives, and they are now no longer in favor.

No one doubts that there are more factors involved in decompression that are included in current algorithms. It's just that no one really knows enough about how they are involved to make any definitive statements. As such, I am leery of any computer algorithm that includes factors that adjust the approaches that have been studied for more than a century without some indication that those adjustments actually help.

I think this is where Scubapro drops the ball on their "Human Factor Diving" approach. I don't know what specific studies DSAT performed, but the PADI RDP does state that in cold water or strenuous conditions, plan your dive assuming a depth 10 feet deeper than actual and I have not seen a community uproar over that General Rule...although the current number of divers using the RDP is probably a pretty small group. Open Water students are taught "If you get cold or exercise a lot during a dive, you may end up with more dissolved nitrogen than calculated by your dive computer or tables. This could increase your decompression sickness risk. Stay well within the no stop limits, being even more conservative than normal by adding an extra safety margin." So I don't think the concept or being more conservative based on being cold or strenuous activity is voodoo science nor does it seem to turn a century of decompression studies upside down.

The issue for me is that there is zero information provided by Scubapro on what values result in dive profile changes. What skin temperature is the point at which the profile is changed? What pulse rate or respiration rate triggers the change? And most importantly...how does the dive profile get changed? I have never seen those details documented. So the changes the computer will make to your dive profile remain a complete mystery until they occur. This is why the "Workload" feature remains off on my G2. Workload is off by default on the G2 and to get the full "integration" you will need the $139 Scubapro Heart Rate Belt, so the vast majority of divers using the G2 are not being affected by this feature anyway.

However, for the recreational diver that has no dive plan other than watching what their computer tells them and is fully equipped for the G2 Workload options, the only thing that is going to happen is that if the cold or pulse/respiration workload "triggers" are met, the computer will provide NDL settings that are shorter than if the workload setting were not enabled. What benefit does that provide? Who knows how much, if any benefit is provided in each instance, but we know that there is precedence for a shorter NDL because you are cold and had a strenuous activity during the dive. If Scubapro would state when skin temperature is "X" or respiration/pulse rate is "Y", the NDL calculations will be adjusted as if the diver was 10 feet deeper than the actual depth...that would be better than the way it is now.

 

[boulderjohn]

When I started my technical diving I was taught "no slower than 30 feet/min" until I reached the off-gassing depth and then "no faster than 30 feet/min" after that depth.

What agency teaches this?

As for what I taught, I taught my students to research all materials to the greatest degree possible, after which we would discuss what they had found and then arrive at a conclusion. I don't recall anyone finding anything at all suggesting that we do anything other than 30 FPM.