Shearwater Perdix AI

[Lorenzoid]

it's the same thing I come back to every time somebody tells a new diver they should stick to a computer with a more conservative algorithm because it's "safer". Really? Are there any stats to show that computers running DSAT are getting people bent more than computers running Weinke RBGM? If not, then how can you say that the difference is meaningful to the diver's safety? The longer bottom time is clearly meaningful to many divers' enjoyment!

Yes, that's the sort of thinking I was referring to. I haven't seen any stats that show, for example, PADI divers are getting injured more than NAUI divers, GUE divers, or whatever. For a number of reasons, it's difficult if not impossible to compare them. So while I chose certain training for myself, I am not about to criticize any agency's training as being unsafe.

 

[stuartv]

Maybe, if you get the computer, you can use the dive planner to compare the Perdix and AUP algorithms for the same dive.

I have been diving my Atom with DSAT and a SeaBear H3 with Buhlmann+Gradient Factors for a while. And a Petrel 2 before the H3.

My experience has been that DSAT and Buhlmann/GF do not produce identical results. But, the closest you can get to DSAT is using GF of 90/90 or 95/95. With those GF settings, a Petrel or Perdix or H3 will match a DSAT computer pretty closely, most of the time, for NDLs. Once you go into deco, all bets are off (and you shouldn't be using a recreational computer).

Oh, and if you use GF100/100 then, by definition, you should get exactly the same results as straight Buhlmann with no Gradient Factors - whether you're using a dive computer or a desktop computer to determine the tissue loading and ascent plan. Gradient Factors ARE a percentage of straight Buhlmann. GF Hi of 70, for example, means you should arrive at the surface with your leading compartment tissue tension at 70% of the what the M-value says is allowed for the compartment before you start getting bubbles. The M-value being the number that tells you what tissue tension (i.e. dissolved gas partial pressure, essentially) you can have, as compared to ambient pressure, before bubbles start to form. Each Buhlmann "compartment" has a different M-value.

 

[dmaziuk]

Aeris Manual pg 25. "ATR is the time you can remain at the present Depth and still safely surface with the Tank Pressure reserve that you selected during setup (End Pressure Alarm Setting)."

Hollis TX1 Manual pg 39. "GTR is the time your current selected gas will last at the present Depth with the Tank Pressure reserve that you selected during setup (End Pressure Alarm Setting)."

Perdix AI manual Pg 10. "Gas Time Remaining (GTR) is the time in minutes that can be spent at the current depth and SAC rate until a direct ascent to the surface at a rate of 33 feet/min (10 m/min) would result in surfacing with the reserve pressure."

Hollis wins: neither "safely surface" handwaving, nor the need to constantly recalculate gas requirements for direct ascent at 33 fpm. Melikes: simple, stupid, everything you need and nothing you don't.

 

[stuartv]

Hollis wins: neither "safely surface" handwaving, nor the need to constantly recalculate gas requirements for direct ascent at 33 fpm. Melikes: simple, stupid, everything you need and nothing you don't.

I'll take any of them, as long as I know what the number means when I'm looking at it.

[walking away, head down in shame that I didn't know how my TX-1 works until savsuds posted]

 

[Kevrumbo]

Sorry - but you you continually mis-characterize what was said.

He was saying that someone measuring an SAC of 25 (psi/min) while diving an AL 80 would have a lower RMV (cu.ft./min) than someone reporting an SAC of 25 while diving a HP 120. Given that a full AL 80 has 0.026 cu.ft./psi (77.4 cu.ft. @ 3,000 psi) and a HP 120 has 0.035 cu ft/psi (120 cu. f.t @ 3442 psi), then a person with an SAC of 25 on an AL 80 has an RMV of 0.65 while a person with an SAC of 25 on a HP 120 has an RMV of 0.875. That is exactly what Stuart said - why don't you get that?

I think most of us understood that, if the same person dived an AL 80 and an HP 120 under the same conditions, their measured SAC would be lower on the HP 120 than on the Al 80 (but their RMV would be that same) - but that was not the example Stuart used.

@Joneill, @stuartv was referring to one diver in this quote:

. . .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. . .

The Perdix AI obviously would not show this for the same diver given the same breathing rate per minute.

If you claim it's relative to two different divers, then the statement is plausible --but it wasn't posted that way (read the quote again).

 

[Joneill]

@Joneill, @stuartv was referring to one diver in this quote:
The Perdix AI obviously would not show this for the same diver given the same breathing rate per minute.

If you claim it's relative to two different divers, then the statement is plausible --but it wasn't posted that way (read the quote again).

Sorry, I didn't and still don't read it that way. I read it as just stating that the same SAC on 2 different tank sizes results in a higher RMV on the larger tank - which is true.

As the same diver would very likely have a different SAC on different tank sizes (unless conditions/exertion was different) and Stuart seems to understand the concepts, I logically concluded that he was talking about different divers or just speaking in absolutes about SAC versus RMV with different tanks sizes.

 

[Joneill]

I have or have owned 2 AI dive computers in addition to my Perdix AI (have not dove with it yet) the Aeris A300CS OLED and the Hollis TX1 AI dive computers. Aeris calls it Air Time Remaining (ATR) and Hollis calls it Gas Time Remaining (GTR).

Aeris Manual pg 25. "ATR is the time you can remain at the present Depth and still safely surface with the Tank Pressure reserve that you selected during setup (End Pressure Alarm Setting)."

Hollis TX1 Manual pg 39. "GTR is the time your current selected gas will last at the present Depth with the Tank Pressure reserve that you selected during setup (End Pressure Alarm Setting)."

Perdix AI manual Pg 10. "Gas Time Remaining (GTR) is the time in minutes that can be spent at the current depth and SAC rate until a direct ascent to the surface at a rate of 33 feet/min (10 m/min) would result in surfacing with the reserve pressure."

I guess dive computer companies choose to define it however they seem fit.

I am accustomed to the Hollis variant of GTR, which could explain my confusion about the GTR differences.

I would be perfectly happy with GTR defined how Hollis does it.

EDIT: GTR how Aeris and Shearwater defines it seems to leave the calculations more subject to variables than the Hollis variant.

So does the Hollis do anything to account for the ascent + stops or does it leave that up to you to factor into your reserve? It sounds like it just tells you when you reach your preset reserve pressure only?

 

[scubadada]

I've been diving Oceanic AI since 2002. ATR includes safety stop or deco obligation. Why wouldn't any ATR/GTR calculation work this way? Shearwater has a single tank designated for GTR, no reason this could not be worked out. Obviously, of particular utility for single tank, rec divers.

 

[Hatul]

Because not everyone dives the same way you do? Not everyone dives off boats? Not all boats limit your dive time?

If I used an AL80 on the dives I normally do, I would often have to ascend because I was getting low on gas before i got low on NDL time.



Most AI computers use ATR/GTR to tell you when to start your ascent, so that you arrive at the surface with your desired reserve left. The Perdix AI works almost that way - it just doesn't account for gas you'll use during a safety stop. So, with the PAI, if you do a safety stop, you'll hit the surface with a little less gas than you set for your reserve. That is assuming, of course, that you ascend at exactly 30 ft/min.

So, GTR is not a countdown to getting out of the water. It's a countdown to starting your ascent. Possibly more accurate than your estimate of when you should start to ascend in order to have 500 (or 800 or whatever it is you want) psi left when you get to the surface.

The situation with GTR in the Perdix is worse than that, for it does not include mandatory stops.While the amount of gas used in the safety stop is small and can be easily accounted by incresing the reserve pressure, that used in the mandatory stops is not.

 

[Kevrumbo]

Sorry, I didn't and still don't read it that way. I read it as just stating that the same SAC on 2 different tank sizes results in a higher RMV on the larger tank - which is true.

As the same diver would very likely have a different SAC on different tank sizes (unless conditions/exertion was different) and Stuart seems to understand the concepts, I logically concluded that he was talking about different divers or just speaking in absolutes about SAC versus RMV.

@Joneill,

RMV by definition is a diver's personal breathing rate of volume per minute at one atmosphere absolute (1 ATA at the surface): It does not change with different tank sizes and is independent of cylinder size. The only way it varies is with hard physical exertion resulting in an increased breathing rate, or an easy relaxed activity state that results in a lesser breathing rate. Also pressure SAC rates are not transferable to different tank sizes. (See p.16 of the Shearwater Perdix AI user's manual).

By convention you first determine and then pick one RMV value -either relaxed activity or hard exertion- and evaluate it over different tank sizes to give you a different pressure SAC rate for each particular cylinder -->NOT pick one SAC rate evaluated over different cylinders to give different RMV's because this contradicts the definitions above which state that RMV does not change over tank sizes, and SAC rates are not transferable to other tank sizes.

Therefore, it is incorrect to say:

. . .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. . .

This is like claiming:

• 25 psi/min from an AL80 is a lower RMV than 25psi/min in Double AL80's. If your SAC is 25 psi/min and you're using an AL80, then you are consuming a RMV 0.64 cuft per minute; so if you use 25psi/min in Double AL80's you are consuming a RMV 1.28 cuft per minute?

No! This would imply your breathing rate doubles when you combine a single cylinder with a particular volume into double manifolded tanks. This is obviously not true by definition and doesn't make any sense.

Properly applied per definition and convention:

For an exertion RMV of 1cf/min per ATA:

Pressure SAC rate for an AL80 is 1cf/min divided-by 80cf/3000psi equals 37.5psi/min.
Pressure SAC rate for Double 80's is 1cf/min divided-by 160cf/3000psi equals 18.7psi/min.

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