Hard for you to say, because you don't know what you're talking about.
- You have an inconsistent mathematical construct in terms of units: you cannot assign a unit of Volume to a value of Pressure. "25 psi/min" is not a Respiratory Minute Volume (RMV) term.
- Also a given RMV or volume SAC rate is consistent across all tank sizes so your argument, "25 psi/min from an AL80 is a lower RMV than 25 psi/min from an HP120 is exactly and perfectly correct", is not only exactly wrong & incorrect, but makes absolutely no sense.
I had let this go, but since I've let you tempt me into another useless response, let me explain this to you in more simple terms.
I said "25 psi/min from an AL80 is a lower RMV than 25 psi/min from an HP120." You said that is wrong & incorrect [both!].
If your SAC is 25 psi/min and you are using an AL80, then you are consuming 0.64 cu-ft of air per minute (if you are on the surface).
If your SAC is 25 psi/min and you are using an HP120, then you are consuming 0.86 cu-ft of air per minute.
Thus, your RMV is higher with an HP120. The same comparison is true no matter what depth you're talking about. It also doesn't matter whether you assume 25 psi/min is corrected for the surface or is the actual rate at depth. I just chose depth of 0 for convenience of the calculations. At any depth and any consumption rate, if the rate of change of tank pressure over time is the same, then using an HP120 means your RMV is higher than if you're using an AL80. 25 psi corresponds to a specific volume of gas from an AL80 and it will always be less than the volume of gas that is correspondent with 25 psi in an HP120. Period.
What I said before is perfectly accurate and correct. Apparently, you are the only one that couldn't understand it, so again I encourage you to seek help with either reading comprehension or basic gas planning.
Petrel 2 dive computer ... and a Uwatec 330m Bottom Timer or Liquivision Xeo in gauge mode
Let me make sure I understand the points you've been making. I'm going to kind of summarize, to please set me straight if I get this wrong.
A Petrel 2 sells for $850. A Xeo runs $620. A 330m Depth Gauge is $260. So, you have/use roughly $1700 worth of electronics for your diving. Possibly as low as around $1100 worth on any given single dive.
The issues you have with the Perdix AI are practicality, economy, and reliability.
You have mentioned AI dropouts, meaning temporary loss of signal, numerous times. You have also referred to the AI functionality as being "electronic", implying that it has a battery and an O-ring for the battery compartment, extrapolating to the conclusion that it is less reliable than a simply, physical SPG.
You have also said that you don't really need an SPG. An SPG is backup, to confirm your primary SPG in your head.
You have also said (I think), that you calculate your ascent plan in your head and use the Petrel 2 as backup to confirm the plan you come up with.
Now, if you used 2 mechanical depth gauges and 2 wind-up mechanical timers (along with your physical SPG), I could give you kudos for at least being consistent. If you even used one mechanical and one electronic, I could still find consistency. But, I cannot see any consistency in your actual statements.
Maybe this is my inexperience showing, but I feel like the single most important piece of data a diver needs is their depth. Next most important is a timer. But, I can count off a close approximation of seconds in my head. So, if I know my depth and roughly how long I've been down, I can work out some kind of ascent schedule in my head and then follow it somewhat closely.
So, if depth is the single MOST important piece of data, and an SPG reading is not really that important, where is the consistency in saying that you need a physical SPG for the reliability (but you don't need any redundancy), but you are fine with having 2 depth gauges that are both electronic? Both are subject to batteries dying. Both are subject to flooding. Tech divers plan for two failures, right? What if your 2 failures are your 2 computers? You then rely on your buddy? Okay, that is the accepted answer. But, in that case, why is it a problem to have an electronic SPG? Having an SPG is not nearly as important as having a depth gauge and timer, right? If it totally dies, it's no big deal, right? So, who cares if it has a 1 or 2 second dropout every now and then? If an SPG is SO important you need the ultra-reliability of being physical, then why don't you have 2? If it's not THAT important, then what's wrong with having one with occasional brief dropouts? I don't see consistency here.
The single most important piece of data you can have is depth. But why? Because you need to know your depth to determine and control your ascent. If you trust an electronic gizmo (for your primary and backup, no less) to tell you your current depth, max depth, and average depth, and your time, then why would you say it's not okay to trust it as your primary and backup source for your ascent plan. Calculating an ascent plan in your head is subject to mistakes. I assert that it's a LOT more likely to result in a mistake than having a Petrel or Perdix calculate an ascent. But, if you think that a Petrel COULD make a mistake on calculating an ascent (and yet you trust it to tell you your average depth), then why not use a second computer to calculate an ascent and compare the two? If one of them has a problem where it's giving a significantly different ascent than the other, and you can't decide which one to follow, THEN you could use your mental gymnastics to calculate an ascent on your own, to help you decide which computer to follow.
How do you do your pre-dive planning? Do you run any kind of desktop computer software to tell you what your ascent should be? Does it tell you what your gas requirements are, too? Or do you do all of that just using pencil and paper? If you do any of that on a computer, how do you know that it's any more accurate than what a Petrel tells you in the water?
Like I said, maybe it's my inexperience showing. But, I just can't find any consistency in your statements. Using $1100 - 1800 worth of electronics to tell you your most important pieces of data then railing against a single, much cheaper (in comparison) electronic device to tell you a much less important piece of data just doesn't seem to fit together in my brain.
You keep mentioning "economy" and this all starts to seem like a bunch of FUD to rationalize a position that really boils down to "I think AI is too expensive for what you get (and that makes me grumpy)!"
You have a $600+ computer that you use as a backup, in gauge mode, fer cryin' out loud. How practical or economical is that? Most people use computers to maximize their bottom time. The alternative for them is using tables where any kind of multi-level dives would be cut way shorter if you had to end your dive based on tables. But, you actually figure your ascents in your head, not based on tables. So, why have a computer at all? If Ratio Deco or whatever it is you do is All That, where is the economy or practicality in having anything more than 2 of the 330m Bottom Timers? And if electronics is not reliable enough for an SPG, why not have at least 1 mechanical bottom timer, instead of 2 electronic ones?
Nope. No consistency at all. But, given you can't even understand that using 25 psi in a minute out of an AL80 is a higher volumetric consumption than using 25 psi in a minute out of an HP120, I guess I shouldn't expect any more.
