Shearwater Perdix AI

[Kevrumbo]

. . .SAC is the rate of change of the tank pressure over time, normalized to 1 ATA. Let me try to put that in simple enough terms for you. SAC is the number of PSI that your tank drops in 1 minute, converted to what it would be if you were at the surface. If you're at 33' (which is 2ATA) and your tank drops 50 psi in 1 minute, then that's a SAC of 25 psi/min.

So, yes, as a matter of fact, SAC DOES change, for a given diver, if they change to a larger or smaller tank.

And where did I EVER say that a given diver's RMV changes based on tank size? I said that a SAC of 25 means a lower RMV if you're using an AL80 vs an HP120. I didn't say a given person's RMV was going to change if they get a bigger tank. I said IF their SAC is 25 with an AL80, then their RMV is lower than IF their SAC is 25 with an HP120.

And my original post that you have been jumping all over - but never quite hitting - was in response to another poster who said something about needing to enter tank sizes into the Perdix AI in order for it to tell you your SAC. I was explaining why you didn't need to do that and why you would need to, if you wanted the Perdix to tell you RMV.

The Perdix AI manual (p.16 Sec 7.1) clearly states that RMV is independent of tank size, and pressure SAC rate is not transferable to different tank sizes. Therefore @stuartv you cannot argue nor does it make any sense when you post:

I said IF their SAC is 25 with an AL80, then their RMV is lower than IF their SAC is 25 with an HP120.

This is the correct way to understand, interpret and comprehend it all:

A beginner OW student with a correctly normalized max RMV of 1 cubic feet per minute is determining which two tanks he's interested in buying:

An AL80 actually has 77.4 cubic feet and would last at the surface 77.4 minutes;
An HP120 has 120 cubic feet and would last at the surface 120 minutes.

Pressure SAC for an AL80 is simply 1cf/min divided-by 77.4cf/3000psi equals 38.7psi/min.
Pressure SAC for an HP120 is simply 1cf/min divided-by 120cf/3442psi equals 28.7psi/min.

Qualitatively, it makes better sense that the HP120 lasts longer and has a pressure that moves at a slower rate per minute because it has a greater volume capacity than an AL80, for a given RMV of 1cf/min.

28.7 psi/min multiplied-by 120 minutes equals 3444 psi -HP120 working pressure;
38.7 psi/min multiplied-by 77.4 minutes equals 2995 psi -AL80 working pressure.
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Another easy intuitive example with same RMV of 1cf/min:

Single AL80 which lasts on the surface for 77.4 minutes;
Double AL80's which last on the surface for 154 minutes.

Pressure SAC for an AL80 is simply 1cf/min divided-by 77.4cf/3000psi equals 38.7psi/min.
Pressure SAC for Double 80's is simply 1cf/min divided-by 154cf/3000psi equals 19.4psi/min

Clearly, it makes more sense that Double AL80's last longer and have a pressure that moves at a slower rate per minute that's half of a single AL80 pressure rate per minute, because the Doubles have twice the volume capacity for a given RMV of 1cf/min.

38.7 psi/min multiplied-by 77.4 min equals 2995 psi -AL80 working pressure;
19.4 psi/min multiplied-by 154 min equals 2987 psi -Double AL80's working pressure.
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And lastly the great car analogy again (from Bob @NWGratefulDiver):

A good analogy is the gas gauge in your car. If you take two cars with exactly the same miles per gallon (your RMV aka volume SAC rate), and put different size tanks in them, the gas gauge will show different values as you drive. If both cars get 25 miles per gallon and you drive 100 miles, both cars will use 4 gallons of gas. But if one car has a 20-gallon gas tank and the other has a 10-gallon gas tank, the former will show that it still has 3/4ths of a tank of gas, while the latter has only a half-tank. They didn't change their gas consumption rate at all ... what changed was the relative measure of how much gas they actually consumed, because the size of their tanks is different.

It works pretty much the same way for calculating gas consumption when you're diving. You might find some of the information in this article useful in helping you understand how it works ... NWGratefulDiver.com

 

[Diving Dubai]

The two of you are from different planets, I'm from the same planet Stuart is from, I understand everything he says . I doubt I'm the only one

Only because I got a bable fish...

 

[stuartv]

Only because I got a bable fish...

And a towel? ;-)

 

[WarrenZ]

Wrong thread take the discussion of bable fish and towels to the HH'er thread in the pub. This thread is about people attacking each others beliefs in weather or not AI is a useful tool for a diver.

I still vote that Im keeping my B&G SPGs over a $350 transmitter because I am cheap. however in the future if I upgrade my computer I would get one with the AI capability.

 

[Lorenzoid]

I think you have misunderstood my point. For the likes of you, and your wife. You understand Rock bottom and your SAC etc. But for the majority of divers (including a fair few on SB) these concepts are alien. They have have no exposure to them, and even if they were included in training, they were forgotten (because of the information overload)

Personally I think its a useful step for people like these to have this information available to help them. It's no different really from the NDL. . . .

Maybe my point of view on the subject comes down to me (and my wife) having drunk the proverbial Kool-Aid about a system of diving recreationally that borrows from the tech diving world. Four years or so ago I asked the Scubanati here a simple question that I see lots of divers asking here from time to time: "What can I do to become a better, safer diver?" The chorus replied: "Take GUE Fundamentals." I guess I have it in my head that every diver on SB would like to go down this same sort of path, if not with GUE then with another agency or just by adopting the principles. Of course, as you point out, that's not true.

At the time, my wife and I dived jacket-style BCs, and she even had an AI computer (console type, on a hose). She liked her AI computer and was hesitant to switch to the so-called brass-and-glass SPG that most GUE divers use. We were hesitant about a lot of things that we had to change in our gear and, of course, our procedures. After all, GUE has been described as a complete system of gear plus procedures. To make a long story short, in hindsight we believe it was the best decision we ever made, and we can't believe we ever dived any other way. Like evangelists, we probably have a hard time seeing other points of view. With our GUE blinders on, we can't see why anyone would pay good money just to have their tank pressure displayed on their computer. If anyone wants to accuse me of having drunk the proverbial Kool-Aid, then okay, I am willing to chalk up all my critical posts here to the Kool-Aid effect. For those who have gone down the path that my wife and I have, AI is but an interesting and perhaps as yet unproven gadget. For others, it's something more. I don't mean to sound sanctimonious or something--I only mean that this way of diving, where concepts like SAC, Rock Bottom, etc., play a real role--may not be for everyone, and may not even be for the "for the majority of divers (including a fair few on SB)," as you said.

 

[Joneill]

Wrong thread take the discussion of bable fish and towels to the HH'er thread in the pub. This thread is about people attacking each others beliefs in weather or not AI is a useful tool for a diver.

Actually, the thread started out as people civilly discussing what they personally perceive as the the benefits of AI and how Shearwater implemented it. Unfortunately, a certain individual then took us off the rails by telling us how silly and unnecessary AI is... and now we've gone further of the rails because that individual is so bent on being right/having the last word that he has not really read what he is responding to (if he had, he'd see that he is saying the same thing that Stuart already has...).

For me, it's March before I get to dive again and use the Perdix AI, so I am interested to getting back to reading experiences from people who have hands-on experience using it. I will also be interested to see if Shearwater makes any updates to the firmware as a result - things like GTR (e.g., factoring in Safety/Deco Stops) or other enhancements. GTR not a big deal to me - I got the Perdix AI for wrist display of pressure and data logging. Hopefully, we can get back to a useful discussion.

 

[Basking Ridge Diver]

For me, it's March before I get to dive again

Are you diving wet? There is still good diving in NJ from now till March... Jan 3rd 2016 it was in the 60's air temp wise...

 

[Joneill]

Are you diving wet? There is still good diving in NJ from now till March... Jan 3rd 2016 it was in the 60's air temp wise...

Yes - I dive wet. I'm really a tropical, warm water diver - I've done several training dives over at Dutch Springs, but not real fan of cold water/low vis diving. I might try some NJ next summer - we'll see? I also have a tough work/travel schedule between now and March so not much opportunity even if I wanted to - but hoping to get 4 Caribbean trips in from March on.

 

[dmaziuk]

Actually, the thread started out as people civilly discussing what they personally perceive as the the benefits of AI and how Shearwater implemented it. Unfortunately, a certain individual then took us off the rails by telling us how silly and unnecessary AI is...

How else do you keep it going for 500+ posts?

Some people like WAI. Shearwater decided to not include safety stop in their Guesstimated Time Remaining display. Shearwater decided to limit their firmware to only 2 tanks. End of discussion.

 

[Texas Torpedo]

Lorenzoid,

Yep, the kool-aid gotcha! That is the price you pay for being 'enlightened' via fundies, etc..

But, I must ask... If money isn't a concern... Why not?

I won't be ditching my analog SPG's, but how can AI hurt, with the exception of people using it outside of their level of training, which they could do with other computers as well? If it came down to being able to monitor tank pressure, I've been in tight-squeeze, no-mount, zero-viz situations where I couldn't have reached / read my SPG's.. However, I could have read the pressure on the computer, if I put it right up on my mask, IF I needed to... (No, I wouldn't have needed to.. Just being hypothetical)

I'm a bit of a gadget geek, so factor that in... But what I am actually most interested in is having my tank pressure recorded in my logs for any type of diving. That is something that I've missed since ditching non-tech computers. Will it make me a better diver? A very very slight 'maybe' by having the ability to review my air consumption during stressful situations, how the cave water flow affected it, etc. Also, I just like historical data.. And historically, I've sucked at writing down details like start / end pressure in my logs.

Bottom line for me.. I was interested in moving from Liquivision to Shearwater before I took a break from diving.. I'm just getting back into it now as the AI version of the Perdix has been released. For me it was a no-brainer to get the AI version.

Maybe my point of view on the subject comes down to me (and my wife) having drunk the proverbial Kool-Aid about a system of diving recreationally that borrows from the tech diving world. Four years or so ago I asked the Scubanati here a simple question that I see lots of divers asking here from time to time: "What can I do to become a better, safer diver?" The chorus replied: "Take GUE Fundamentals." I guess I have it in my head that every diver on SB would like to go down this same sort of path, if not with GUE then with another agency or just by adopting the principles. Of course, as you point out, that's not true.

With our GUE blinders on, we can't see why anyone would pay good money just to have their tank pressure displayed on their computer. If anyone wants to accuse me of having drunk the proverbial Kool-Aid, then okay, I am willing to chalk up all my critical posts here to the Kool-Aid effect. For those who have gone down the path that my wife and I have, AI is but an interesting and perhaps as yet unproven gadget. For others, it's something more. I don't mean to sound sanctimonious or something--I only mean that this way of diving, where concepts like SAC, Rock Bottom, etc., play a real role--may not be for everyone, and may not even be for the "for the majority of divers (including a fair few on SB)," as you said.