Perdix AI alternatives?

[scubadada]

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

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.

Hi @Barmaglot

I am not a technical diver, however, about 5% of my dives are light deco, <10 min, without gas switch. I have 18 years, 1860 dives experience diving DSAT. For the last 10 years, 1460 dives, I have used an Oceanic VT3, same computer as your Aeris Elite T3. Over the last 4 years, 700 dives, I have used a computer running Buhlmann with GF as a backup, first a Dive Rite Nitek Q, and now a Teric.

I ended up diving both Buhlmann computers at a GF high of 95 to best match DSAT for no stop dives. It should be noted that the match is far from perfect and that there are often differences in first dive and/or in repetitive dives. For no stop, I follow the more conservative. For a variety of reasons I will not list, I ended up diving the Buhlmann computers at a GF low of 75 or 80. I have not had a deco stop below 10 feet. For my diving, the DSAT and Buhlmann short deco times have matched reasonably closely. I clear both and then pad the stop by 3-5 min. The SurfGF function on my Teric has led to some changes in some of my dives.

I have no good idea how DSAT handles deco, but know that the algorithm was not developed for deco diving. If I were to consider doing deeper, longer dives, with more deco and gas switches, I would be very hesitant to use the DSAT deco algorithm. DSAT has served me well within the bounds of my own diving, but I would not be confident outside those limits. When my VT3 finally dies, it will be replaced by another computer running Buhlmann.

If I were in your shoes, rather than buying another computer running DSAT, and then replacing it again later, I would simply make the investment in a computer running Buhlmann now. I would also have to carefully consider if the GFs I'm diving now would be appropriate for "real" deco diving. I have a strong feeling that there would be changes.

Very best and good diving, Craig

 

[tursiops]

Hi @Barmaglot

I am not a technical diver, however, about 5% of my dives are light deco, <10 min, without gas switch. I have 18 years, 1860 dives experience diving DSAT. For the last 10 years, 1460 dives, I have used an Oceanic VT3, same computer as your Aeris Elite T3. Over the last 4 years, 700 dives, I have used a computer running Buhlmann with GF as a backup, first a Dive Rite Nitek Q, and now a Teric.

I ended up diving both Buhlmann computers at a GF high of 95 to best match DSAT for no stop dives. It should be noted that the match is far from perfect and that there are often differences in first dive and/or in repetitive dives. For no stop, I follow the more conservative. For a variety of reasons I will not list, I ended up diving the Buhlmann computers at a GF low of 75 or 80. I have not had a deco stop below 10 feet. For my diving, the DSAT and Buhlmann short deco times have matched reasonably closely. I clear both and then pad the stop by 3-5 min. The SurfGF function on my Teric has led to some changes in some of my dives.

I have no good idea how DSAT handles deco, but know that the algorithm was not developed for deco diving. If I were to consider doing deeper, longer dives, with more deco and gas switches, I would be very hesitant to use the DSAT deco algorithm. DSAT has served me well within the bounds of my own diving, but I would not be confident outside those limits. When my VT3 finally dies, it will be replaced by another computer running Buhlmann.

If I were in your shoes, rather than buying another computer running DSAT, and then replacing it again later, I would simply make the investment in a computer running Buhlmann now. I would also have to carefully consider if the GFs I'm diving now would be appropriate for "real" deco diving. I have a strong feeling that there would be changes.

Very best and good diving, Craig

I have in fact used DSAT for deco diving....back when I took my AN/DP and ER courses, and had an Aeris Savant and an Oceanic Versa Pro. They were horrible. Endless deco time compared to either of my instructor's DiveRite computers. DSAT is not designed for, or tested for, deco diving, so LOTS of conservatism is built in.....

 

[Stoo]

Does the Perdix even have seconds? I have one and I'm pretty sure it doesn't, if not, there's a LOT of Tec divers out there that get by without them. Exactly what do you need to do that's critical to the second?

Exactly. This Instructor seems a little anal... As they always say, decompression is just a theory, so times are simply not that precise.

Having said that, the Perdix (and all SWs except Teric I think) have a countdown "progress bar" that runs under the HH:mm display. Four bars, so 15 seconds each. Close enough for horseshoes, hand grenades and deco stops.

 

[broncobowsher]

OK, here is the plan for you. Get the Shearwater now. Don't care what flavor, just not the NERD (yet).
Dive away with it.
When you get a rebreather and get the NERD, get a rebreather version that can read cells. To pay for it, sell the first Shearwater.
A Shearwater holds its value good enough that this is actually a viable plan.

 

[rjack321]

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...

Not to supplant what you were taught but...

Do the "full" time at your first stop. This could in theory end up being up to 59 extra seconds. whoopdie dooooo
Then use runtime to establish where on the remainder of the stops you should be.

If you focus this much on time and seconds all sorts of other stuff is going to have to be in your periphery which is unhelpful.

 

[dmaziuk]

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?

In theory if these things just broadcast with random delays, the question is how random is their "random". In computers it's well known to be not very random and heavily dependent on the initial seed: a single run will be random enough, but two independent single runs side by side aren't anywhere near that. So if your two signals clash, they'll likely keep clashing all the way.

You'd have to ask whoever knows what's inside these transmitters and how it works... and I expect folks at Shearwater know better then us here.

 

[tursiops]

In theory if these things just broadcast with random delays, the question is how random is their "random". In computers it's well known to be not very random and heavily dependent on the initial seed: a single run will be random enough, but two independent single runs side by side aren't anywhere near that. So if your two signals clash, they'll likely keep clashing all the way.

You'd have to ask whoever knows what's inside these transmitters and how it works... and I expect folks at Shearwater know better then us here.

This was from Shearwater, in a 2017 thread on SB:

We currently only sell one transmitter interval, but there are two others available (both still have FCC ID MH8A):
* Green housing - 4.80 seconds
* Grey/'platinum' housing - 5.00 seconds (standard OEM issue for other brands as well)
* Yellow housing - 5.25 seconds
Note the green housing does not necessarily designate 'O2 safe'. All units are capable of being O2 clean, although we don't currently sell them that way either.

Normally, using transmitters with the same interval does not pose a problem. However, we have on occasion seen (and had reports of) signal interference when using two identical transmitters. Turning one transmitter off for 2 minutes will normally resolve the problem, but of course this is not ideal.

Mitch Burton
Shearwater Research​

So a 5 sec interval is standard. It is not a random interval. If one turned on one grey transmitter and then exactly one minute later turned on a second grey transmitter, unless one of the clocks has a LOT of drift, you won't get any interference. They will never be much closer than 1 second apart. I try and turn mine on 2.5 sec apart.....and don't have any problems.

 

[ATJ]

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.

I have 2 (well 3 including my backup regs) Oceanic branded PPS transmitters (which would be transmitting at the standard 5 second interval).

I frequently dive with two tanks and have used combinations of all three of my transmitters over 100s of dives and have never lost signal (other than the occasional drop out which also occurs with a single tank) with my Perdix AI.

 

[tbone1004]

@Barmaglot the 3rd cell holder can have a fischer cable outlet if you order it that way. The Se7en canbus system will not talk to the DiveCAN system. Technically the systems could, but they aren't and likely won't anytime soon.

yes to helmet

Decompression dives work the way you choose for them to work. The ratios for delayed time at depth is dependent on how deep you are going, but once you hit deco, the difference a minute makes is irrelevant. Run the plan in the software program of your choice and get a general plan so you know what you should be doing, that's how you plan the dive. You should know what that plan is, and ideally have it written in your wetnotes. I choose to ride the computer on the way up though since it knows exactly what I did, not what I think I did. They are reliable enough now that you can largely trust them. Not blindly trust them, but if your tables say 17 minutes, and the computer says 15 or says 18, the difference is irrelevant in the grand scheme of things. Just as you shouldn't blindly trust your computer, you shouldn't blindly trust your instructor. Run what you consider a "normal" dive plan for you, then adjust some things by a minute or two and see what happens. You'll be surprised how small the difference really is by changing the decompression schedule. Delaying your initial ascent though can be quite bad depending on how deep you're doing.
If you believe that is level of timing is important per your instructor, then I would HIGHLY recommend buying anything but a computer running true Buhlmann ZHL-16 with GF. Reason for that is you can't actually plan your dives on anything else because no one knows what their actual algorithms are so you can't predict the dive plan.

As long as you turn the second transmitter on outside of the frequency of transmission, you are OK, but since one transmits every 5 seconds and the other every 7 or whatever it actually is, the risk is less with different colors. I know many that dive dual gray and it's never been an issue.

 

[Barmaglot]

So I guess Perdix AI it is (used, with a grey transmitter), provided I can get to New York next week and not get stuck in the Philippines.