Shearwater Perdix AI wish list

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Thanks Bone, I'm going to have to read what you said a couple of times. In some ways, I know like a little kid asking how a turbine engine works on this stuff. I don't understand it but I know I don't want to be passed through it! The engineer knows how it works, but doesn't know quite where to begin explaining. At least on this subject, I'm pretty humble.

If it's been many years since you read the basic padi book on nitrox, it reads like "This stuff is great AND YER GUNNNA DIE!" Now I'm told it's being heavy handed, and this stuff is really more complex than the basic book wants to communicate. I hope that oxygen clock article will clear it up a bit. Still, I hope the above will help you understand the book made me pretty nervous.

But no, I don't find that combative, it looks like it's all good information. I'll be back when I've read it most closely to follow up. In the meanwhile, my slate is a super basic slate that usually stays in my bcd pocket. Main reason I want to see a compass more is a lot of my dives go like this. I follow the more experienced guy, we keep up on each other's air supply, I wonder how we will ever find our way back, and suddenly I find we ARE back.
(I'm planning to take the navigation specialty on my next trip.) I want to see my compass regularly in the hopes I'll start being able to keep track of my position better.
 
@GreyVR I'm a NAUI instructor, so have never actually read PADI's nitrox stuff.
Here's the good thing with nitrox/mod/cns.
At a ppO2 of 1.6 you are technically allowed 45 minutes according to the NOAA table. You should have learned that you really don't want to exceed a ppO2 of 1.4 during the dive portion which gives you 150 minutes. You can't dive to any reasonable depth for 150 minutes on a single tank, and even during multiple dive days, you probably aren't going to get close to that number.

You are only certified to dive up to EAN40, but unless you are a scientific diver, you are almost guaranteed to never dive mixes that rich. The MOD for that is 82/99 FSW for a ppO2 of 1.4/1.6
The most you'll encounter is EAN36 which is 95/114. So long as you aren't planning on going deeper than 100ft, and have a mix that is at or below EAN36, you don't have to worry about MOD or CNS.
EAN32 is arguably the most common nitrox mix which is 111/132. That conveniently coincides with your actual depth limit as a recreational diver.
EAN30 is common off the coast of NC and in some parts of cave country for dives planned to be just under 100ft and the MOD for that is 121/143

The important part here is to have your computer set to warn you at 1.4 or even at 1.3, and then you know you're real bloody close to your MOD but you can see above that there is a pretty wide buffer in depth.

Now on the navigation bit. Unfortunately what is near impossible underwater is to gauge distance. We can relatively accurately gauge distance on land because we are not generally affected by things like the wind, we have traction, and our steps are usually pretty close in size. In the water, kicks can be very different since we no longer have traction, are affected by the current, and the "size" of the kick may not always be the same. Point of that is just because you are gauging the direction, you don't necessarily know how long each of the legs are so getting back may be impossible without a map to gauge where exactly you are. What type of location are you conducting these nav dives?
 
Yes, this is pretty much what one of my instructors was saying at the airport back from the last trip. At the time I wanted more detail, but things were rushed. You guys make me feel better about it then the book did. The idea of hitting oxygen toxicity suddenly and with very little warning is quite frightening.

Now, top help you understand my overall position for wanting more information, information I most likely do not need, it's because at some point when you are learning, you start to see how all these things fit together. You want to see the whole board and how it all moves. As the mind gets more and more oppertunities to contemplate the information, it really starts to understand it.

I'm rather planning (if I can find a deep enough pool) to set my computer to 99%O2 and leave it at the bottom for awhile just to watch it's reactions play out. Meanwhile my actual self will be sitting on the edge of the pool in perfect safety.

Next one will be Bonaire, and it was suggested by one of the other divers as being a good location for nav.
 
Side note, I wish it would run the calculation for PSI SAC to CUFT SAC in the ipad app for me if I plug in cylinder size.
 
@GreyVR if you email @Shearwater and ask nicely, Tyler may be able to send you a video of what the screen does when the CNS clock goes off. Basically it's like any other warning where it flashes red and asks you to confirm. They have done simulation videos for a lot of the features of the computer, so it shouldn't be that difficult for them to show you what it looks like.
I will say that I have seen that warning many many times, and it is truly unavoidable for most of the technical dives that are done and we are still here. The only physiological symptom that I have personally noticed from high ppO2 is when I am on O2 for just a touch longer than I normally like to be. I typically do 15 on, 5 off, but if I have a 20-25min deco, I used to not do an air break. My lungs would start to feel a bit weird and my sinuses would be creating a lot more mucus than normal and felt dry, that's from the vasoconstriction aspect of exposure to high ppO2's. At the time, I was also not doing 10ft stops which I have since incorporated in addition to doing the 15:5 cycle when the ppO2 is 1.6.

another good article on the subject. It is important to keep in mind that the NOAA CNS clock is hotly debated especially as technical dives have put divers at 200-300% or more of their allotted O2, and have had no symptoms to speak of. For recreational diving, it really isn't something you should ever have to worry about.
Alert Diver | Understanding Oxygen Toxicity
Air Breaks… what are they, and do people take them for the wrong reason?

On the CFM conversion, it would be nice, but Shearwater seems to always focus on the actual computers and their desktop software has always been lacking in comparison. The new partnership with Deepblu will hopefully jumpstart progress on that front though. It's always been very much functional, but there's been a lot of features that they could throw in there that would be nice.

You mentioned wanting to see and understand how everything works together. What variables in particular are you wanting clarification on?
 
KK, you are most likely dead on that I am looking to understand more than I need to understand just to do rec diving, but I think you are the type to get it when someone wants to grasp the bigger picture. :wink:

And if I ever run out of stuff to look at at less then 130 feet of depth, I might just have to start technical diving. That will be LONG while though, if it ever happens.

Right now, what I really want is to be able to display all my oxygen data. Which is mostly seems this computer will do, but at the cost of the minicompass. It won't show my mod, but in another thread someone told me that I can use white vinyl electrical tape on the side of my computer, and I'll write down my MOD on that and stick it on the side of the perdix. (I know more advanced guys likely have all that stuff memorized. On my last dive trip I was a little task loaded since it was my first time in current and my first time bringing a camera, so I want a cheat sheet.) Some of this is sort of like the old question 'how long is a piece of string?' and the more experience you get keeping an eye on your gauges, the more you can ballpark the result.

I'm kind of thinking I want to mount a regular compass right next to the perdix, (same wrist so I can get all information simultaneously) but I am fearful that the perdix will throw the compass off. Do you have any information for me on that issue?

You might say it boils down into two things. I want to understand my relationship to oxygen really well and gain experience reading my oxygen meters, and I want to learn to keep much better track of where I am and where I am going at the same time.
 
KK, you are most likely dead on that I am looking to understand more than I need to understand just to do rec diving, but I think you are the type to get it when someone wants to grasp the bigger picture. :wink:

And if I ever run out of stuff to look at at less then 130 feet of depth, I might just have to start technical diving. That will be LONG while though, if it ever happens.

Right now, what I really want is to be able to display all my oxygen data. Which is mostly seems this computer will do, but at the cost of the minicompass. It won't show my mod, but in another thread someone told me that I can use white vinyl electrical tape on the side of my computer, and I'll write down my MOD on that and stick it on the side of the perdix. (I know more advanced guys likely have all that stuff memorized. On my last dive trip I was a little task loaded since it was my first time in current and my first time bringing a camera, so I want a cheat sheet.) Some of this is sort of like the old question 'how long is a piece of string?' and the more experience you get keeping an eye on your gauges, the more you can ballpark the result.

I'm kind of thinking I want to mount a regular compass right next to the perdix, (same wrist so I can get all information simultaneously) but I am fearful that the perdix will throw the compass off. Do you have any information for me on that issue?

You might say it boils down into two things. I want to understand my relationship to oxygen really well and gain experience reading my oxygen meters, and I want to learn to keep much better track of where I am and where I am going at the same time.
I wear the computer on the right and my compass on the left.

Consider a wrist slate as well.
 
@GreyVR why do you think the "oxygen meter" matters in the scope of recreational diving to the point that you want to see that on the main screen vs. flipping to the second or third screen to see it?

If you are using the compass to navigate, you'll want something like a nav slate, but usually the compass goes on the left wrist, computer on the right so they won't interfere
 
That's just a personal preference thing. I want to put everything in one place, which might not be possible.... doesn't stop me wanting to.

As to the rest, I want to keep an eye on it because I'm still learning and the darn padi book put the fear of oxygen into me. Does it really seem weird that I want to keep all my information display in the same place, and that I want to keep track?
 
@GreyVR not weird at all. That said, the key is to find the balance between how much information that you see, and how relevant it is. Ideally you can see and process all information instantly. Unfortunately, we can't, so you have to prioritize what you see, when you see it, and why you see it.

Example. You don't need to see MOD while diving, because you can see the ppO2. The ppO2 changes with depth, and when you hit the ppO2 limit that you set, the computer will flash and that means that you have hit your MOD. We put MOD on stage/deco bottles to make sure we don't breathe out of a bottle that is toxic at depth, but it's not something we consult the computer on.

You can look at the tissue graphs for all 16 compartments, but it's a lot easier if it just gives you the most important compartment which is either represented by NDL, or by your total deco time depending on which way you're going.

The CNS number just gives you a percentage based on your tissue loading. This percentage moves exponentially as a function of ppO2. This number is accessible on the second screen alongside the temp and gf settings. If you deem that more inconvenient and more important than having the compass heading, or some other information, then that is your choose to have that displayed. For most of us, it is not a number that we really worry about outside of dive planning because you can't really do anything about it once you're in the water. Since it is dependent upon ppO2, along with MOD, it is usually something that we plan around, and just let it tick away. Quick example. PADI RDP gives you 20 minutes at 100ft. On EAN36 that is a ppO2 of 1.45. Above what you would normally choose for that dive which would be EAN32. Now, at a ppO2 of 1.45, for a 20 minute dive, the CNS loading is 15%. Since the clock resets every 24 hours, you would have to complete that dive 7 times within a 24 hour period to violate the CNS clock *and this is assuming no surface interval*. You obviously can't do that within NDL's, nor would you ever do that with a deco obligation either, so it is a moot point.
When you choose your nitrox gases for the dive, you are weighing CNS toxicity against decompression obligations. The higher the nitrox mix, the lower the inert gas, the longer the NDL, the faster the CNS clock ticks.
On a high level for dive planning, you have to choose how much you are diving in a 24 hour period, which variables are most restricting, and that will determine your plan.
If you are diving EAN32, which as mentioned above is the most common "standard" nitrox gas, then within the confines of accepted recreational depths, you are allowed to dive to 5ata or 130ish ft, which is also a ppO2 of 1.6 where you are only allowed 45 minutes of total time. The USN tables *which allow the most bottom time for the first dive vs. PADI who has more forgiving repetitive diving* only allow 10 minutes at that depth before decompression so you would be limited by NDL before you are limited by CNS. Unless you can do 4 of those dives within a 24 hour period, you don't have anything to worry about. If you stay to 4ata or 100ft, then your CNS clock allows for 180 minutes since the ppO2 is 1.3, and the USN tables allow for 25 minutes. You would have to complete 7 of those dives within a 24 hour period for you to come close to seeing that CNS box flash 100.

So, what to do. I know personally that the Shearwaters get all sorts of angry at you when you exceed the CNS clocks and that they will flash a warning *though I can't remember where on the screen* that is obnoxious and requires you to confirm the warning. Since you are spooked by your instructor, I suggest you leave CNS on the screen for a "normal" dive day to see that it doesn't really move all that much. Once that fear is alleviated, replace it with the compass, or something a bit more useful to your actual dive. In recreational diving that is probably the clock *so your wife doesn't get mad at you*, and the compass. Once you start doing decompression dives, the other cells become a bit more useful. Particularly the @+5 and GF99 cells
 
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