analog vs digital gauge (SPG vs Air Integrated Dive Computer)

[tbone1004]

that's the problem. You don't know. Lots of companies use "customer algorithms" where they pad them with a whole bunch of crap. You'll lose time for short surface intervals, fast ascents, sawtooth dives, etc etc and you can't predict it.

So with the Petrel you get to choose the gradient factors that determine how NDL's and decompression stops are calculated. For recreational divers the important one is GF-High where that number correlates to the theoretical tissue loading when you surface. This algorithm is public and is available in many different decompression planning programs so you can accurately plan dives. The Mares uses a proprietary algorithm and luckily has a dive planner out so you can actually plan your dives, but you have no control over the conservatism of the computer. If you want to dive more conservatively, you have to cut dives short, and even then you don't know. If you set GF high to 85, you are theoretically coming out of the water with your tissues 85% loaded. If that isn't conservative enough, you can drop it to 75, or 70, or whatever, or if you want to be more aggressive you can increase it to 90/95/99 whatever. *PADI tables roughly correlate to 45/95*. You can figure out what gradient factor somewhat correlates to the Mares algorithm, but it's trial and error until you figure out what the approximate GF is. This is true across all of the recreational dive computers btw.

Oceanic Dual Algorithm | Simply Scuba Blog
You can read about the oceanic algorithms here and see where they differ from each other.
https://www.shearwater.com/news/flexible-control-of-decompression-stress/
Little bit about Gradient Factors there


Jim is talking about using VPM with conservatism settings. I don't really like VPM because of the deep stops, but to each his own. I use Buhlmann with Gradient Factors to control mine. With the Buhlmann side you have almost unlimited ability to set the GF's since they are in +_1 increments on both sides. VPM is just one of the five settings. +4 is about where I dive most of the time, but I have bad knees so I'll deal with a little extra deco. In recreational mode you can choose low, 45/95 *similar to PADI tables*, 40/85 which is similar to what ratio deco comes out with, and high of 35/75 which is comparable to what a lot of tech divers use for OC diving. VPM +2 correlates to something like 20/90. Too aggressive for me. This is all way above where you are now, but there are lots of papers written about this if you want to dig into them

 

[snsb]

Thank you so much for the insight!



tbone1004

Very good info regarding the brand and model selection. Thanks for the update on company ownership as that will probably affect the distributor and availability here. I will definitely have to research more on this but H3 looks very promising


halocline

That makes sense. I guess its only a matter of preference between dive com or console now. Thank you


DA Aquamaster

Wow, Thanks for the info on battery and the mechanic of SPG.

Good to know what actually caused the failure and how to prevent them.


CuzzA

Sorry for the late reply, I hope you are getting better by now!

Thanks for the detail info, that selection process makes a lot of sense. Mares Icon looks good. But the deal that you got is a lot better than what is currently available. Thanks for the summary and I will definitely look into these models.


dmaziuk

Thank you, that explained a lot. Just too bad for the way it is *-*


Hatul

From reading this forum and your comment, few things that most people agree on seems to be that AI is useful but expensive and wireless sometimes is less reliable. I guess I will go with this option. Thanks


tcphil

That is exactly why I am interested in AI dive console at first. And I will probably get one, and when I am ready for the next level that required redundancy, I guess dive come with wireless or SPG will be a good additional unit. Thank you for sharing

 

[tbone1004]

[video]https://youtu.be/gs-4dta_w-Y[/video]

This is how a pressure gauge works btw. This is the Termo SPG's that almost everyone is using, but they are all basically the same.

 

[scubadada]

So where does the RGBM's least conservative algorithm compare to one of the 5 Petrel settings?

The Petrel rec mode has 3 set levels of conservatism, 45/95 low, 40/85 medium, and 35/75 high. 45/95 is quite liberal. The tec mode has unlimited settings as you choose both GF lo and GF hi.

Shearwater uses the Buhlmann ZHL-16C decompression algorithm, a well characterized algorithm available in the public domain. Mares uses a proprietary RGBM algorithm for which much less is known. The RGBM algorithms are known for punishing some dive practices in ways that are not well described. Comparing computers for multiple dives essentially requires diving them both. The attached report from Scuba Labs (Scuba Diving Magazine) shows results from 4 simulated dives in the Catalina hyperbaric unit and demonstrates the variability of decompression algorithms. Your computer is shown, the Petrel is not http://ads.bonniercorp.com/scuba/PDF/ScubaLab-Computer-Test-September-2014-data.pdf

 

[dmaziuk]

dmaziuk

Thank you, that explained a lot. Just too bad for the way it is *-*

EM waves don't work underwater, esp. under salt water. So they typically use accoustics: ultrasound for communication so you have to design the speaker and microphone in from the start. Every vendor has their own "sekret" transmitters, so you have to design that. And the communication protocol. It's not like you can take bluetooth that's already there and just pair with an off-the-shelf transmitter.

---------- Post added December 8th, 2015 at 11:44 AM ----------

Can you clarify this? If I brought both the Icon and Petrel on the same dive what would be the difference in the NDL clock.

You'd have to dive wearing both and see. RGBM is rocket science that about one person on the planet understands, the model's complex and I'm not sure how much oomph's needed to run it near real time -- certainly at lower end they run some sort of approximation that gives close enough results over some range of depths. Buhlmann is an open and well-understood algorithm and gradient factors are reasonably clear. So when you're programming those in you feel like you know what you're doing.

I'm sure you can set GFs to closely match Icon RGBM on a given dive. Or to be more conservative, or less. Part of the issue is you can tweak them any which way -- as opposed to having "Safety Factor" button going from 0 to 3.

 

[CuzzA]

The Petrel rec mode has 3 set levels of conservatism, 45/95 low, 40/85 medium, and 35/75 high. 45/95 is quite liberal. The tec mode has unlimited settings as you choose both GF lo and GF hi.

Shearwater uses the Buhlmann ZHL-16C decompression algorithm, a well characterized algorithm available in the public domain. Mares uses a proprietary RGBM algorithm for which much less is known. The RGBM algorithms are known for punishing some dive practices in ways that are not well described. Comparing computers for multiple dives essentially requires diving them both. The attached report from Scuba Labs (Scuba Diving Magazine) shows results from 4 simulated dives in the Catalina hyperbaric unit and demonstrates the variability of decompression algorithms. Your computer is shown, the Petrel is not http://ads.bonniercorp.com/scuba/PDF/ScubaLab-Computer-Test-September-2014-data.pdf

So, no surprise the Icon got 5's across the board on their Ergo test, but I'm still trying to wrap my head around some of the data presented on the simulated dives. Specifically the dives comparing the Tusa using Buhlmann ZHL-16C and the Icon using the RGBM. For the most part when comparing the two "fixed" algorithms they are nearly identical +/- a couple minutes. But at 60' and 40' on all of the dives there's a huge difference. What's going on there?

To the best of my memory, when I did my AOW deep dive (100'), two rebreather (Hollis Explorers/PPO2 set to 1.2) divers were wearing Petrels and I (Nitrox 30%, PPO2 1.2 @100') wore the Icon and another diver (Nitrox same mix) wore a DG02. Predive we discussed doing the narcosis drill first and then ascending to 40' at 10 minutes until deco. We all checked our NDL at the same time and every one of our computers were right at 10 minutes. I don't know what to make of that, if anything. Unfortunately, I have no idea what the NDL's were at 40' for everyone and I assume for the rebreathers they were able to increase their PPO2 and blow off nitrogen faster and the Petrel would calculate this. Again, I don't know if there's much to gain from that, but that's the only comparison I can personally add to the conversation.

 

[scubadada]

I currently dive an Oceanic VT3 primary computer and a Geo2 backup. The VT3 runs only DSAT, the Geo2 is dual algorithm, I run DSAT on both. I have been successfully using that decompression algorithm since 2002, 1028 dives. The DSAT decompression algorithm is the basis for the PADI RDP. DSAT is quite liberal on no stop dives, it is quite conservative for deco dives. The only algorithm that I know is more liberal than DSAT, is the Cochran algorithm, derived from the Navy WAL-18 algorithm. I am very familiar with my computers and am aware of the strengths and weaknesses. I detest how Oceanic computers track O2 exposure vs. most other computers.

When you are aware of what you are doing, you can make rational, intelligent choices in your equipment. My next computer will be a Petrel or a H3 or some computer that allows me complete flexibility in adjusting my decompression algorithm and tracks O2 in a reasonable manner.

Personally, I have some difficulty in interpreting the testing table I made available, it was mainly to illustrate the variability in decompression algorithms. I have no idea how different manufacturers have decided to implement the Buhlmann algorithm. I have queried Oceanic regarding the GFs they have used for PZ+, but have never gotten a response. Who knows what Tusa does? Unfortunately, there's a lot of secrecy and lack of transparency in scuba. There's a lot to be said for using a well documented algorithm with flexibility for adjustment depending on one's personal risk tolerance.

 

[tbone1004]

So the explorers don't actually get to "set" their PPO2, it is fixed by the gas mix coming into them. Their PPO2's change with depth, just like yours does on OC. It's a little different due to metabolic oxygen consumption etc etc, but it isn't like CCR where they dive with a fixed PPO2 which will create longer NDL's. The Petrel has 2 modes if it isn't monitoring an O2 sensor. One is fixed FO2 which is CC and SCR diving where you tell it what gas mix you're breathing and it does its thing based on the changing PO2, and in CCR mode you put in your PO2 and it calculates based off of your PO2 *you input your dil mix so it can adjust for helium etc etc.*. On the EXT models it actually measures the PO2 in the loop and here it has a dedicated SCR mode where it will constantly adjust the PO2 based on the readings. Again, very much farther ahead than where you are, but the explorer works basically the same as open circuit, they can't change anything, but the Petrel will do whatever you tell it to do and adjust accordingly.

That said, you aren't really going to be narc'd at 100ft on 30%, your EAD is around 85' and you'll feel something, but certainly not enough to really do narc drills and have a measurable change. Anyway, the differences are explained somewhat in the gradient factor link I posted from Shearwaters site. With you guys diving different algorithms, it is obviously more complex, but basically the computers were calculating your NDL's based off of a theoretical tissue loading limit and if that limit is determined to be different by the different computers, it will show accordingly. This is why it is difficult to dive with guys using different computers with different algorithms that you can't control. You may have plenty of time left on yours, or you might think that "oh my buddy is always 5 minutes ahead of me", but if you're at a different depth and that changes, the computers may get really annoyed and start penalizing you. The Petrel never penalizes you or locks you out, it allows you to do what you want to do and it calculates accordingly, on the other hand many recreational computers can get really grumpy if you violate something and actually lock you out for 24 hours which is idiotic.

 

[dmaziuk]

...many recreational computers can get really grumpy if you violate something and actually lock you out for 24 hours which is idiotic.

especially if you can remove the battery and reset the fscker to clean slate...

 

[CuzzA]

Thanks for the explanation. Yes, I've got a lot of learning to do still so maybe it's a good thing I can't make my computer more liberal, at least until I have a better understanding of what's happening to my body. But the lock out is another good point.

Yes, after 3 dives to 100 ft. in my very short diving career with mixes being between 23 and 30 I still have yet to recognize myself feeling narc'd. Who says partying hard in college won't pay off later on in life. lol