Mass confusion about computers????
- Thread starter Eric Sedletzky
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I personally think that is a legitimate concern, in general. Especially with a Shearwater. They are so reliable that it is (or becomes) very tempting to just rely on what it says and not bother to do any mental gymnastics to sanity check it. After a few years of diving with one, I could easily see getting so "relaxed" that you get really rusty on said mental gymnastics.
However, I think it only takes a small amount of self-discipline to overcome that. And, in your specific case, I suspect that you would find the mental gymnastics so ingrained now that you would always be sanity checking the computer and staying sharp.
Also, if you are really worried about it, you could dive the Shearwater in gauge mode on dives where maximizing BT is not as high a priority, to keep yourself in good practice. Switch it to dive mode for the times where you're with in your buddies and want to make sure you don't become the bottleneck.
If you're going to buy brand new, I would highly recommend getting the AI model, even if you aren't going to buy a transmitter. If you don't want the AI model, I would suggest looking for a used Perdix non-AI that is less than a year old. There are still people out there wanting to upgrade to an AI model, so you can get one for a big discount and still have over a year left on the factory warranty.
For what you can get a used non-AI for, it just doesn't make financial sense (to ME) to buy a brand new non-AI. The resale value on an AI is also going to be a LOT better, compared to buying a new non-AI.
I don't have any "real", specific examples. What I'm talking about is that the algorithm is proprietary, so you don't KNOW what it will do in every scenario. An example of what I'm talking about is what you said: "it quickly adds time to the shallow stop."
With a Shearwater you can predict what it will do. You can even see, during a dive, what it will do if you stay at your current depth for 5 more minutes. You cannot do that with an Oceanic. The unexpected or unwanted result might be that you are looking at your computer and you think that if you stay just 3 more minutes, you will end up with 5 minutes of deco. But then, when you do it, you end up with 20 minutes of deco. That would be unwanted result, at least.
You, Craig, are extremely detail-oriented and have enough experience with yours to know pretty well what it's going to do in pretty much any dive scenario you will be in. But, that is not the situation most people, including Eric, would be in.
For the vast majority of recreational divers, I would recommend an Oceanic with DSAT without hesitation. If I had not started technical diving, I would still be diving my Atom with DSAT 100% of the time (and still do dive it a lot). But, I just feel like, from what Eric has posted and what I know of his experience and diving, I think this is an exceptional case where I would think he would ultimately be more happy with a Perdix than with a DSAT computer.
It's on Amazon right now for about $33.
Amazon.com: Buying Choices: Deco for Divers: A Diver's Guide to Decompression Theory and Physiology
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- Messages
- 20,022
- Reaction score
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- Location
- Philadelphia and Boynton Beach
- # of dives
- 1000 - 2499
Hi Stuart,
Your points are well taken. DSAT was not designed for deco diving. I do not advise others to dive like I do. I would not do longer deco than I currently do, except in an emergency. You cannot do deco planning, either on the computer or with software. For recreational diving, the intended purpose for the algorithm, I cannot think of any significant defect in DSAT.
As you know, I also have reasonably extensive experience diving a Nitek Q, running Buhlmann ZH-L16C with GFs, as a backup to my Oceanic computer. That would be a whole other discussion
Good diving, Craig
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CAPTAIN SINBAD
Contributor
So here is my understanding of this issue. Suunto, Mares and Cressi are RGBM (Reduced Gradient Bubble Model). Each manufacturer has added their own conservatism to RGBM which they do not often disclose so you have no idea what you are dealing with. What exactly makes a dive profile or computer conservative is a topic that has already been debated elsewhere but I am skeptical of any conservatism generated by RGBM.
This RGBM algorithm was developed at a time when scientific community was evenly divided between dissolved gas models (Buhlmann) and bubble mechanics models (VPM.) The dissolved gas community had been grounded in (wrong) assumption that inert gases are stored inside muscles in a dissolved gas state and when a certain pressure difference is reached between tissue pressure and ambient pressure then gas turns to bubbles. This assumption was proven to be wrong when Doppler technology came about but the tables and limits it generated helped reduce incidents of DCS. Bubble mechanics theorists emerged after Doppler technology showed for the first time that bubbles were present in divers who apparently had no symptoms of DCS. Bubble mechanics community argued that since bubbles are present in every dive, ascent rates should focus more on bubble control and limiting bubble growth rather than dodging that fictitious point where dissolved gas supposedly becomes bubble.
For practical purposes this meant that decompression divers were split into “GET OFF THE BOTTOM AND DO A LONG SHALLOW STOP” (Dissolved gas / Buhlmann) and “DO DEEP STOPS AND REDUCE TIME AT THE SHALLOWEST” (Bubble mechanics /VPM) crowd. John Chatterton was in the first group. In the words of John Chatterton, “Regardless, there is one, and only one, single cause of decompression sickness, and that is........ not doing enough decompression!! If you do enough decompression, you do not get bent.” Bubble mechanics community (VPM and RGBM) disagreed and argued that this approach will get you bent. You have to do deep stops all the way up and if you have been doing those deep stops then you do not need to decompress at that last shallow stop for extended period of time.
Notice that making a computer that is “conservative” under one model would be “dangerous” under the other. This is why we have to be skeptical of any computer that markets itself as “conservative.” The world conservative must also be followed by “under which decompression algorithm?”
RGBM was developed as a dual phase algorithm. It was designed to merge these two approaches into a single dive. So if you have a Mares, Suunto or Cressi Leonardo strapped to your wrist, then Dr. Buhlmann and the Dissolved Gas theorists will be monitoring the point at which gas turns into bubbles while Dr. Yount and bubble mechanics community will be monitoring the point at which micro bubbles that already exist in the blood will grow to become dangerous. So when you finish your dive, it is Dr. Yount the founder of VPM that will generate the first stop! This will be generated at a depth that Dr. Buhlmann will regard to be a dangerous depth to do a first stop so Dr. Buhlmann and Dr. Yount will start arguing with each other. Your RGBM computer will overrule Dr. Buhlmann and will let Dr.Yount determine the first stop depth. Once you are cleared by Dr. Yount then you will be given the green light to ascend. When you are at your shallowest, you will meet Dr. Buhlmann who will be throwing a mad fit! He will penalize you for listening to Dr. Yount at that deeper stop and he will give you an elongated stop time at your shallowest. Dr. Yount will find the additional time at shallow depth to be unnecessary and they will start fighting with each other. Your RGBM computer will overrule Dr. Yount at that shallow stop and listen to an extremely pissed off Dr. Buhlmann who will make you do a longer shallow stop. This is dual phase model! Before you exit the water, there is one more person who will be consulted and that is Eric Baker.
Since decompression is not an exact science, Eric Baker developed “Gradient Factors” that would enable users to decide how close they would like to be with the limits set by Buhlmann. The problem is that in order for gradient factors to be effectively applied you need to know divers age, fitness level, cold water stress, dehydration and a whole bunch of things that your computer does not know about you. So different manufacturers will make broad assumptions about their user to “reduce the gradient.”
RGBM is used in these computers because it is great for avoiding a lawsuit. If anyone gets bent using Suunto, Mares, Cressi Leonardo computers then the defense lawyer will have a great opening line. “We incorporated all research into designing this computer!” This is where the death waiver that you sign at the start of your dive will be shown to you and you will be told that the manufacturer listened to every single expert in the history of decompression research and you knew that by taking up diving you were doing something that was inherently risky.
While it is a great tool to avoid lawsuits, RGBM does not offer anything to the user. If the purpose of having a computer over a dive table is to have more bottom time then RGBM loses that advantage quite significantly over repetitive dives. At decompression depths it calls for deep stops and at recreational depths it becomes ridiculously conservative on repetitive dives. You may find that people diving on Buhlmann dive tables may actually have longer dive times on some profiles because the table limits on Buhlmann tables are gradient factors of 100/100 which would be aggressive. Of course no one dives a square profile so your actual gradient upon surfacing from a table dive will be lesser but may not be as less than the manufacturer recommended one on RGBM.
A search online seems to reveal that RGBM is the preferred algo for recreational computers. Suunto, Cressi, Mares, three of the big names run RGBM. People in technical diving community dont seem to like RGBM much. Single Algorithm computers that are marketed towards technical divers (Dive Rite Nitek, Hollis TX) give you Buhlmann with gradient factors.
I believe that we should train divers to start diving on dive tables first instead of computers. The second step in diving education should be a brief introduction of decompression algorithms like Buhlmann, VPM and RGBM etc. This should then be followed by the third step, purchase of the first dive computer because you are not buying a computer but you are buying an algorithm. How can you make that purchase when you have no formal education in what VPM and RGBM etc are? Unfortunately, we are doing this quite the other way around. We are training open water divers on computers from day one! Once you actually train a diver to read and follow the computer then steps one and two are unnecessary and everyone lives happily ever after.
This RGBM algorithm was developed at a time when scientific community was evenly divided between dissolved gas models (Buhlmann) and bubble mechanics models (VPM.) The dissolved gas community had been grounded in (wrong) assumption that inert gases are stored inside muscles in a dissolved gas state and when a certain pressure difference is reached between tissue pressure and ambient pressure then gas turns to bubbles. This assumption was proven to be wrong when Doppler technology came about but the tables and limits it generated helped reduce incidents of DCS. Bubble mechanics theorists emerged after Doppler technology showed for the first time that bubbles were present in divers who apparently had no symptoms of DCS. Bubble mechanics community argued that since bubbles are present in every dive, ascent rates should focus more on bubble control and limiting bubble growth rather than dodging that fictitious point where dissolved gas supposedly becomes bubble.
For practical purposes this meant that decompression divers were split into “GET OFF THE BOTTOM AND DO A LONG SHALLOW STOP” (Dissolved gas / Buhlmann) and “DO DEEP STOPS AND REDUCE TIME AT THE SHALLOWEST” (Bubble mechanics /VPM) crowd. John Chatterton was in the first group. In the words of John Chatterton, “Regardless, there is one, and only one, single cause of decompression sickness, and that is........ not doing enough decompression!! If you do enough decompression, you do not get bent.” Bubble mechanics community (VPM and RGBM) disagreed and argued that this approach will get you bent. You have to do deep stops all the way up and if you have been doing those deep stops then you do not need to decompress at that last shallow stop for extended period of time.
Notice that making a computer that is “conservative” under one model would be “dangerous” under the other. This is why we have to be skeptical of any computer that markets itself as “conservative.” The world conservative must also be followed by “under which decompression algorithm?”
RGBM was developed as a dual phase algorithm. It was designed to merge these two approaches into a single dive. So if you have a Mares, Suunto or Cressi Leonardo strapped to your wrist, then Dr. Buhlmann and the Dissolved Gas theorists will be monitoring the point at which gas turns into bubbles while Dr. Yount and bubble mechanics community will be monitoring the point at which micro bubbles that already exist in the blood will grow to become dangerous. So when you finish your dive, it is Dr. Yount the founder of VPM that will generate the first stop! This will be generated at a depth that Dr. Buhlmann will regard to be a dangerous depth to do a first stop so Dr. Buhlmann and Dr. Yount will start arguing with each other. Your RGBM computer will overrule Dr. Buhlmann and will let Dr.Yount determine the first stop depth. Once you are cleared by Dr. Yount then you will be given the green light to ascend. When you are at your shallowest, you will meet Dr. Buhlmann who will be throwing a mad fit! He will penalize you for listening to Dr. Yount at that deeper stop and he will give you an elongated stop time at your shallowest. Dr. Yount will find the additional time at shallow depth to be unnecessary and they will start fighting with each other. Your RGBM computer will overrule Dr. Yount at that shallow stop and listen to an extremely pissed off Dr. Buhlmann who will make you do a longer shallow stop. This is dual phase model! Before you exit the water, there is one more person who will be consulted and that is Eric Baker.
Since decompression is not an exact science, Eric Baker developed “Gradient Factors” that would enable users to decide how close they would like to be with the limits set by Buhlmann. The problem is that in order for gradient factors to be effectively applied you need to know divers age, fitness level, cold water stress, dehydration and a whole bunch of things that your computer does not know about you. So different manufacturers will make broad assumptions about their user to “reduce the gradient.”
RGBM is used in these computers because it is great for avoiding a lawsuit. If anyone gets bent using Suunto, Mares, Cressi Leonardo computers then the defense lawyer will have a great opening line. “We incorporated all research into designing this computer!” This is where the death waiver that you sign at the start of your dive will be shown to you and you will be told that the manufacturer listened to every single expert in the history of decompression research and you knew that by taking up diving you were doing something that was inherently risky.
While it is a great tool to avoid lawsuits, RGBM does not offer anything to the user. If the purpose of having a computer over a dive table is to have more bottom time then RGBM loses that advantage quite significantly over repetitive dives. At decompression depths it calls for deep stops and at recreational depths it becomes ridiculously conservative on repetitive dives. You may find that people diving on Buhlmann dive tables may actually have longer dive times on some profiles because the table limits on Buhlmann tables are gradient factors of 100/100 which would be aggressive. Of course no one dives a square profile so your actual gradient upon surfacing from a table dive will be lesser but may not be as less than the manufacturer recommended one on RGBM.
A search online seems to reveal that RGBM is the preferred algo for recreational computers. Suunto, Cressi, Mares, three of the big names run RGBM. People in technical diving community dont seem to like RGBM much. Single Algorithm computers that are marketed towards technical divers (Dive Rite Nitek, Hollis TX) give you Buhlmann with gradient factors.
I believe that we should train divers to start diving on dive tables first instead of computers. The second step in diving education should be a brief introduction of decompression algorithms like Buhlmann, VPM and RGBM etc. This should then be followed by the third step, purchase of the first dive computer because you are not buying a computer but you are buying an algorithm. How can you make that purchase when you have no formal education in what VPM and RGBM etc are? Unfortunately, we are doing this quite the other way around. We are training open water divers on computers from day one! Once you actually train a diver to read and follow the computer then steps one and two are unnecessary and everyone lives happily ever after.
Based on the last sentence, why do you feel like we "should" train (new, presumably) divers in the first 2 steps you outlined?
Personally, I think leaving tables and all but the briefest discussion of algorithms out of OW training is perfectly fine. I think people should just be taught some basic concept of how much more there is to learn and then leave it for them to either seek continuing education or do their own research.
To me, if they say "I bought my first computer having no idea that there were different algorithms" that is not good.
If they say "I bought my first computer knowing there are different algorithms but not knowing what the differences are", that is fine. Then it's on them that they didn't take the time to learn more before they spent their money. Basically, all the recreational computers available these days are safe enough that them buying without doing that research does not represent a safety issue.
I had an Oceanic Versa Pro when I started a deco class many years ago. The first real deco dive was an eye-opener; I don't remember the exact numbers, but I spent at least 10 minutes longer at 20 ft and 20 minutes longer at 10 ft than did my instructor, on his DiveRite Nitek. He was, well, a bit distraught. That was the last deco dive that Versa Pro ever saw. The algorithm was pure DSAT, no PZ+ option at that time. I now have an Oceanic OC1 that I wear as a backup for rec dives, keep it on PZ+ to better match my primary SW, and have never put it into deco. I don't know what the PZ+ "better deco algorithm" does with deco. For planned deco, I use two SW (my old Petrel 2 and a Perdix AI). I've passed through multiple DiveRite computers (still have a fine Duo and a Plus, both great computers), some Aeris (same as Oceanic), and probably others.
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Hi @CAPTAIN SINBAD
Good try. There are major differences between recreational (no deco) and technical (deco) diving.
For rec diving, your computer and its algorithm simply tells you how long you can stay underwater and then ascend directly to the surface at an acceptable rate. Some algorithms give more time underwater and some give less. Most would conclude that all recreational computers are generally safe. We add the safety stop to pad that safety.
For tec diving, you can no longer ascend directly to the surface. There are very significant differences in how various computers and algorithms control your ascent back to the surface, mainly stop depth and time. This is frequently a very contentious topic.
Good try. There are major differences between recreational (no deco) and technical (deco) diving.
For rec diving, your computer and its algorithm simply tells you how long you can stay underwater and then ascend directly to the surface at an acceptable rate. Some algorithms give more time underwater and some give less. Most would conclude that all recreational computers are generally safe. We add the safety stop to pad that safety.
For tec diving, you can no longer ascend directly to the surface. There are very significant differences in how various computers and algorithms control your ascent back to the surface, mainly stop depth and time. This is frequently a very contentious topic.
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