Ambitious newb: Researching PDCs and relevant background info

[Peter Guy]

Stuartv -- I'm one of those people who have an AI wrist computer and really like it. BUT, when (not if) it dies, it won't be replaced with one. It is a "nice to have" feature but not one I'd spend very much time (or money) on.

The important things, in order (IMHO -- oh, and must be wrist mount with bungee):

a. READABILITY in light AND dark water;

b. Displays the information I want in a format that works for me (information I want -- depth, time, NDL (well actually, not really!), avg depth)

c. Is Nitrox compatible

d. Is downloadable

e. Has gauge mode

Anything else is merely gravy

 

[Steve_C]

Buy the computer. Read the manual including the deco part carefully. Make sure your computer does not lock you out during a dive.

From my own experience: I had an aggressive dive guide on the Spiegle Grove early on in my diving shortly after I bought a new computer. I broke off from the group on the second deep dive and headed to the up line but ran out of NDL before I hit the line. I was not going to do a free ascent in strong current from 100 ft. Fortunately I had carefully read the manual and understood what it was telling me about extra stops and ceilings. Spent an extra 15 minutes playing flag and got back safely on board with a cleared computer.

When I travel I always use my own computer ( and mask and usually my own regs). I understand what all the data means and I understand quickly without having to think about it.

{I will add that breaking off earlier was not an option since we were going through a swim through.)

 

[stuartv]

Okay, now I have a couple of new questions.

1) I've seen some computers that have a bungee mount option (including the Hollis). I have inferred that the bungee mount is used so that when your wetsuit compresses at depth you don't have to tighten up a wrist strap to keep it from flopping around. And, I guess more importantly, you don't have to loosen it on ascent to prevent having a tourniquet on your wrist.

But, I have also been reading about wetsuit options. Including the entire Wetwear website. And there, it talks about Rubatex neoprene which left me thinking that Rubatex doesn't compress significantly. So, if you're using that top-shelf neoprene and only diving to Rec depths, does it really matter if you have a bungee mount?

2) I'm still trying to sort the input on wireless AI.

I've read a number of posts here on SB that boil down to "you don't need and shouldn't use anything but a depth gauge, a timer, a dive plan that was thoroughly nailed down topside, a slate to write it down on, and an SPG that you normally never even look at."

And then, obviously, there's a whole bunch of posts recommending PDCs - even from the serious Tech divers.

These attitudes seem contradictory to me. I assume that means I just don't understand the subject well enough yet. It SEEMS like, once you make the decision to totally rely on a PDC for your dive plan - which seems to be what all Rec (or at least all new Rec) divers do, why would you balk at AI? It seems like your depth gauge is more important than your SPG, and you're relying on your PDC for that. I expect someone will say "yeah, but I still have a backup on my console if the PDC croaks." To which I respond "okay, but what good does that do you? Did you do a dive plan? Do you know how long you can stay down? Do you have a backup timer, too?" These things make me think that, for the vast majority of Rec divers, having a brass'n'glass depth gauge is a complete waste. If their PDC croaks, it's time to go, period. And if it doesn't croak, why have the mechanical depth gauge.

And if you're going to rely solely on a PDC for Depth and dive plan, why even hesitate to also rely on it as a SPG?

I would totally expect that when wireless AI was new there were problems with reliability. But, all the reviews I've been reading seem to indicate that current tech is solid. Worst case, you have to hold the PDC up in viewing position and wait 3 or 4 seconds. No worries about flat out not being able to get an accurate reading. At least, no more so than worries about the PDC itself malfunctioning. And, if you change the battery in the transmitter whenever you change the battery in the PDC, you don't have to worry about that, either.

And for the Tech guys who want redundancy, and yet use a Petrel or other PDC, why not use a PDC with AI as a complete primary set of instruments, and then just have one mechanical console with a complete set of redundant gauges?

Anyway, I'm not sure if I'll shell out for it. $250 for a transmitter is still a lot, compared to a $65 single gauge console. But, I do have to admit I'm a gadget nut, and I really like streamlining and the KISS principle. So having a 1st stage with only 2 hoses coming off it (a primary and a hybrid secondary) has a lot of appeal to my inner OCD.

If any of you can enlighten me on the seeming contradiction I described, that would be cool!

 

[Dhboner]

Tech divers who use computers have also cut tables using deco planning software and carry the tables on the dive. If the computer fails they have the tables as a back-up. An SPG is a robust piece of kit that rarely (never in my experience) fails on a dive...an AI computer on the other hand can experience signal loss and battery issues so almost no diver undertaking a technical dive would ever use one...to say nothing about all the transmitters you would need for your doubles and stages As an aside my technical instructor (Steve Lewis) trains us to have a pretty decent idea of our remaining gas at any time based on our rate of breathing, time and depth...it's something I practice and it is surprisingly accurate!

 

[lv2dive]

I dive a Petrel... Honestly it's some of the best $ I've spent on kit. And of the dozens of people I know who have one, everyone would buy them again.... That is saying something.

I don't think most people would have a problem relying on a *hosed* AI computer for pressure readings but no way would I trust a wireless AI DC. Although... I guess I take it back... If the battery goes out on any kind of AI DC with a digital readout, dive is over. Otoh, If you have an analog spg, a watch, and you know your depth (and you aren't relying on the computer but are running your own dive) you are good to go.

---------- Post added September 5th, 2014 at 09:38 PM ----------

PS- I don't make a decision to totally rely on DC for anything. It is a tool on recreational dives that I use to validate my predive plan

 

[TSandM]

You like detail and analysis. Think about it this way: Your depth can change in seconds. Having your depth on your wrist where you can monitor it constantly is a very useful thing, particularly on ascents, when things are very dynamic.

On the other hand, unless you are one of the brand new divers with an insane SAC rate (the ones where you can almost see the sides of the tank collapse with each breath) your pressure changes over minutes. How onerous is it to pull up a gauge and check your pressure every five minutes? Especially since, as you gain experience, the SPG becomes a double-check, because you have a very good gut sense of what the gauge should show before you ever touch it.

Then ask yourself, if you want to spend several hundred dollars on dive gear, is it better spent on a bright computer display (or even on a good dive light, or an SMB and spool) rather than a transmitter that lets you be lazy?

 

[stuartv]

I am guilty of distracting the conversation. For the moment, forget Tech diving and forget wireless AI. If your argument against using any instrument on a dive is based on the possibility of a battery issue, then how do you justify diving using a PDC?

You KNOW that virtually every new diver you recommend a PDC to is going to use it in lieu of preparing a dive plan topside. I imagine most won't do any more pre-dive planning than to find out the depth at the dive site. And they'll use their PDC as their depth gauge and timer and most likely won't even have a backup timer.

As I understand it, a depth gauge and a timer are the two most important instruments a diver takes with him/her. So, how can you shun a less important instrument on the basis of "battery issues" and then recommend combining the two most important instruments (plus the dive plan itself) into 1 battery powered device and then dive without a backup for one, two, or all three of those?

And again, this not about Tech diving or what you experienced divers do. It's about what you recommend to new divers.

---------- Post added September 6th, 2014 at 01:41 AM ----------

I don't think most people would have a problem relying on a *hosed* AI computer for pressure readings but no way would I trust a wireless AI DC. Although... I guess I take it back... If the battery goes out on any kind of AI DC with a digital readout, dive is over. Otoh, If you have an analog spg, a watch, and you know your depth (and you aren't relying on the computer but are running your own dive) you are good to go.

So, do you dive with your Petrel and a second equally capable DC? If you do, then obviously if your primary PDC (the Petrel) conks out, you can fallback to the secondary and abort the dive safely.

But, if you are using a Petrel and your fallback is your analog gauges and your dive plan, then that means you cannot vary from the dive plan at all. Because as soon as you decide on a variance that the Petrel could handle just fine you've then put yourself in the position where if the Petrel conks out, your pre-calculated plan is out the window, and you have to just figure it out on the fly and hope you get it right, right? So, if you use any PDC and your fallback is analog gauges, then why bother with the PDC at all? You're gonna spend $850 just to use the Petrel in Gauge mode?

And it sounds like Petrel owners tend to think there is no other computer that is equally capable, for the same money or less (unless it's an older Shearwater, like a Predator), so are you all diving with 2 Petrels or a Petrel and a Predator? If not, how is the Petrel worth the money to you over a second set of analog gauges?

Are you taking advantage of the Petrel's ability to recalculate deco on the fly and just risking it that it won't conk out and leave you having to recalculate your own dive plan in the middle of a dive?

---------- Post added September 6th, 2014 at 01:50 AM ----------

to say nothing about all the transmitters you would need for your doubles and stages

That certainly could add a lot of expense. But, it could also enhance safety, right? From reading owner's manuals on some of these PDCs, it looks like they will alert you if you switch the PDC to a new gas but it doesn't detect any air flow from the bottle you told it you switched to. I.e. It will tell you if you mistakenly switched to a different bottle than you selected on the DC. A non-AI DC couldn't do that.

I'm not saying the extra safety is worth all that extra money. The monetary value of increased safety is something every individual has to decide for themselves.

 

[Dr. Lecter]

And it sounds like Petrel owners tend to think there is no other computer that is equally capable, for the same money or less (unless it's an older Shearwater, like a Predator), so are you all diving with 2 Petrels or a Petrel and a Predator?

A Petrel and a Predator controller, yep. And an analog depth gauge, an analog dive watch, and pre-cut tables. Multiple backups are good when you're several hours away from being able to surface.

If not, how is the Petrel worth the money to you over a second set of analog gauges? Are you taking advantage of the Petrel's ability to recalculate deco on the fly and just risking it that it won't conk out and leave you having to recalculate your own dive plan in the middle of a dive?

This isn't as complicated as you're making it out to be: you plan and cut tables for the deepest, longest dive you might do and then you dive within the plan. If all goes well, the Petrel has you out of the water before the plan would because nobody's doing a perfectly square profile; if all goes to you don't have to recalculate anything, you just dive no deeper or longer than originally planned and wind up doing a little extra deco.

 

[Dhboner]

While I think your enthusiasm is admirable at this point, I have to say come back to this conversation when you have a few hundred dives under your belt ;-)

 

[lv2dive]

I am guilty of distracting the conversation. For the moment, forget Tech diving and forget wireless AI. If your argument against using any instrument on a dive is based on the possibility of a battery issue, then how do you justify diving using a PDC?

You KNOW that virtually every new diver you recommend a PDC to is going to use it in lieu of preparing a dive plan topside. I imagine most won't do any more pre-dive planning than to find out the depth at the dive site. And they'll use their PDC as their depth gauge and timer and most likely won't even have a backup timer.

As I understand it, a depth gauge and a timer are the two most important instruments a diver takes with him/her. So, how can you shun a less important instrument on the basis of "battery issues" and then recommend combining the two most important instruments (plus the dive plan itself) into 1 battery powered device and then dive without a backup for one, two, or all three of those?

And again, this not about Tech diving or what you experienced divers do. It's about what you recommend to new divers.

As a rec diver, I justify diving a DC as a backup to what I planned for the dive. For example, by my rough calculations* for a dive to 70 feet on Nitrox 32 (EAN32) I have about a 60 minute limit before I hit M(inimum) Deco Limits or MDL. And I can tell you those limits at any depth without blinking that because I dove (dived? LOL) for a really long time without a computer, with a bottom timer/depth gauge and SPG. The computer backs up my brain.

*and let's agree this thread isn't the place for how to calculate MDL/NDL - we can do that in a different thread if you like!!

So, do you dive with your Petrel and a second equally capable DC? If you do, then obviously if your primary PDC (the Petrel) conks out, you can fallback to the secondary and abort the dive safely.

But, if you are using a Petrel and your fallback is your analog gauges and your dive plan, then that means you cannot vary from the dive plan at all. Because as soon as you decide on a variance that the Petrel could handle just fine you've then put yourself in the position where if the Petrel conks out, your pre-calculated plan is out the window, and you have to just figure it out on the fly and hope you get it right, right? So, if you use any PDC and your fallback is analog gauges, then why bother with the PDC at all? You're gonna spend $850 just to use the Petrel in Gauge mode?

Someone else did a pretty good job of answering this, but most of your questions are answered with no.
On rec dives, I dive with my brain and Petrel as a backup and to record the dive -it has a pretty nice software and the interface to my computer is bluetooth which is OH SO EASY.
On most tech/cave dives, I dive with brain (of course) and also tables, and the Petrel, and my buddy, and, in rare cases, a borrowed additional Petrel depending on the dive. If the Petrel fails, I am reviewing my schedule and most likely falling back to my buddies deco schedule because almost always his is longer than mine anyway . YOur statement about not deviating from the dive plan was answered by another poster and the fact of the matter is, we just can't explain everything about contingency planning for tech diving in an short thread about computers. But suffice it to say, I haven't really heard about people getting bent or dead in tech diving from computer failures.

And it sounds like Petrel owners tend to think there is no other computer that is equally capable, for the same money or less (unless it's an older Shearwater, like a Predator), so are you all diving with 2 Petrels or a Petrel and a Predator? If not, how is the Petrel worth the money to you over a second set of analog gauges?

Are you taking advantage of the Petrel's ability to recalculate deco on the fly and just risking it that it won't conk out and leave you having to recalculate your own dive plan in the middle of a dive?

I do not think there is another DC out there that even comes close, especially when you take into account Shearwater's customer service. I have steered several friends towards Petrels as well and they all have been very pleased. Part of having a DC is using the ability of the computer to calculate deco as you go. I start to think about having a second computer for dives over four hours. There is no need for a second set of analog gauges.

That certainly could add a lot of expense. But, it could also enhance safety, right? From reading owner's manuals on some of these PDCs, it looks like they will alert you if you switch the PDC to a new gas but it doesn't detect any air flow from the bottle you told it you switched to. I.e. It will tell you if you mistakenly switched to a different bottle than you selected on the DC. A non-AI DC couldn't do that.

I'm not saying the extra safety is worth all that extra money. The monetary value of increased safety is something every individual has to decide for themselves.

Where is AJ when you need him with his one-word quips? Bollucks! Is the best thing I can come up with! Most simply put - the last thing you want to do, is to be relying on a computer to tell you anything like what you just wrote above. By the time you get to the point in your diving where this would be an "option" - and it would not surprise me if you get there sooner than later - you will look back on this and smile! I promise!