Perdix AI alternatives?

[tbone1004]

@Barmaglot
If you want a nerd now, helmet mount it. There are a couple recent threads where I posted some pictures of how he has it mounted. The issue there is unless you know which CCR you're getting, you won't know if you need/want a Fischer or DiveCAN NERD, so that's still a bit of a tossup.
There are no other computers that will do what you want them to do other than the Teric or Perdix AI.
That said, as mentioned above, they only give 15 second countdowns, but why your instructor mandated having seconds is beyond me, it's all close enough.

 

[Barmaglot]

About the seconds display - the way I was taught, at least on the training dives, is to synchronize decompression stage callouts to the minute; i.e. ascend from 45 to 21, level off, switch from air to EAN50, then signal my buddy/instructor (if we're currently in minute 23, and minute 24 of the dive is coming up, or if the gas switch was done just before minute 24 rolled around) "24, level, 3", meaning that we start decompression schedule at minute 24 and wait here for 3 minutes. Then as minute 27 is coming up, signal ascent to next level, and as we're ascending, we're supposed to pass each 3 meter mark (18, 15, 12) at an even minute mark, at which points I'm supposed to signal to go to next level. We had a 1 minute stop at 9 meters, then 16 minutes at 6 meters, and I was supposed to signal those at even minute marks as well. Without a seconds display, or something else showing me the minute progression, I would have to have my nose buried in the computer to see when the minutes change, which is a big no-no in and of itself. Does this have any relevance on how "real" decompression dives are conducted, or do people just trust their computers to tell them when to ascend while following the rough outline of a plan?

Regarding NERD and CCR - I'm looking at Poseidon Se7en; that would mean that I need a DiveCAN model, right? Wearing a helmet while diving on a reef would look super goofy though...

 

[tbone1004]

@Barmaglot
The Se7en does not use Shearwater DiveCAN, it uses a proprietary canbus system. The Poseidon does not have an analog out cable either, so you will have to have the 3rd cell holder and you will only have access to that single cell.
You only use the helmet when diving OC sidemount, but it's a good place to put a bunch of stuff anyway.

I do not know anyone who still conducts decompression like that. It is simply not necessary. If you are 30 seconds off, the computer will just keep tracking everything in real time and adjust accordingly. Staying at a stop for an extra minute is not going to do anything for you either good or bad. Now, it's important that you are paying attention and are capable of diving with that level of precision, but I do not know anyone who dives like that in the real world.

 

[Barmaglot]

The Se7en does not use Shearwater DiveCAN, it uses a proprietary canbus system. The Poseidon does not have an analog out cable either, so you will have to have the 3rd cell holder and you will only have access to that single cell.

Is that the same attachment point as for Poseidon's own M28 computer, or does that one have deeper integration with the Se7en? If a NERD2 would plug into that third sensor, would it use a DiveCAN or Fischer connector, or would that depend on the specific sensor model? Is NERD2 compatible with Poseison's solid state sensors?

You only use the helmet when diving OC sidemount, but it's a good place to put a bunch of stuff anyway.

Something like this? Light Monkey Diving Helmet | Dive Gear Express®

I do not know anyone who still conducts decompression like that. It is simply not necessary. If you are 30 seconds off, the computer will just keep tracking everything in real time and adjust accordingly. Staying at a stop for an extra minute is not going to do anything for you either good or bad. Now, it's important that you are paying attention and are capable of diving with that level of precision, but I do not know anyone who dives like that in the real world.

So basically, the way a real-world tech dive is run is:

1. Make a plan to account for gas consumption (bailout if diving CCR) and ascent time/turn pressure
2. Put the gases you're bringing (air, nitrox, oxygen, trimix, whatever) into your computer
3. Dive, spend your time at the bottom, ascend, stop when the computer tells you to, switch gases as planned, ascend further when the computer tells you it's okay

Do I have that right? My instructor was really pressing the point that delaying at depth for whatever reason (missing schedule, fumbling gas switches, etc) results in significantly increased decompression time in the shallows.

In general I'm starting to lean towards just biting the bullet and getting a Perdix AI. The NERD2/CCR integration is a lot more complex than I initially thought, and I may need to rethink that plan. Even if I do go that way, it will be useful to have two computers running the same software and deco algorithm.

Another question - Shearwater's manual for AI recommends using a special second transmitter if running two of them in parallel; the one that they conveniently sell in yellow - per their claim, the two transmitters need to have different timing on their signal, or conflicts can arise, producing loss of connectivity for up to 20 minutes. Oceanic/Aeris manuals make no mention of this, and they run up to three transmitters in parallel. Does this actually happen in the real world, or is Shearwater being needlessly paranoid? I already have a grey transmitter with my Aeris T3, and more used ones are available for significantly less than a brand new Shearwater yellow unit.

 

[RyanT]

Do I have that right? My instructor was really pressing the point that delaying at depth for whatever reason (missing schedule, fumbling gas switches, etc) results in significantly increased decompression time in the shallows.

On a deep dive, let's say below 150', delaying you ascent can add a surprising amount of unplanned deco time. Leaving a minute late from, let's say your 50' stop, isn't going to be a big deal, especially if you're already on your 50%, as you should be. As others have already alluded to, definitely plan for precision, but don't fret if your ascent schedule isn't exact compared to your plan.

 

[Barmaglot]

On a deep dive, let's say below 150', delaying you ascent can add a surprising amount of unplanned deco time. Leaving a minute late from, let's say your 50' stop, isn't going to be a big deal, especially if you're already on your 50%, as you should be. As others have already alluded to, definitely plan for precision, but don't fret if your ascent schedule isn't exact compared to your plan.

So an Oceanic OCi (currently available with a transmitter for $490 on ebay) is a perfectly adequate alternative to a Perdix?

 

[tursiops]

So an Oceanic OCi (currently available with a transmitter for $490 on ebay) is a perfectly adequate alternative to a Perdix?

If you are comfortable with a different algorithm....close, but different.

 

[Barmaglot]

If you are comfortable with a different algorithm....close, but different.

As I understand it, my Aeris T3 runs DSAT, while OCi offers a choice between DSAT and "Pelagic Z+ (Buhlmann ZHL-16C data basis)". Perdix runs ZHL-16C too, so it seems like OCi is fairly well matched to run primary with Aeris T3 as backup (both in DSAT) and as backup to a Shearwater Perdix/NERD (both in Buhlmann ZHL-16C). Is this correct, or am I missing something? How great are the differences between Shearwater and Oceanic implementations of ZHL-16C? Oceanic website doesn't mention anything about gradient factors, but if I understand it correctly, they're just extra safety on top of ZHL-16C - a Perdix may tell me to stop and do some deco where the OCi will tell me that it's safe to ascend, but the OCi won't give me a significantly longer decompression schedule than a Perdix, right?

 

[tursiops]

There is no documentation I'm aware of that explains the safety factors used in the PZ+ implementation. There are guesses, and experiments, for NDL dives. See Oceanic DSAT or PZ+ on backup computer for example.
The game gets murkier for deco dives.
DSAT is DSAT so you can certainly match your Aeris and your OCi.
You can't run the OCi with both algorithms simultaneously; you have to choose one for the dive.

 

[Barmaglot]

You can't run the OCi with both algorithms simultaneously; you have to choose one for the dive.

Of course, but I'm not planning on running three computers - my thinking is to run OCi primary/Aeris backup while I'm building up tec experience on open circuit, then get a Shearwater computer for primary and relegate the OCi to backup when I crossover to CCR.