Feedback on recent two-tank and dive limits

[tursiops]

in the overwhelming majority of cases rec divers will run out of NDL well before running out of gas

Only true for deeper dives, typically greater than 50-60 ft or so.

 

[David Carron]

Only true for deeper dives, typically greater than 50 ft or so.

You are absolutely correct, doubly so if you take into account a depth-appropriate gas mix. However, at all depths and for all gas consumptions an 'average rec diver' is being led around by a DM, questioned regularly on their remaining gas and is in any case limited by the programmed maximum dive time. The last thing they care about is if they're going to run out of gas or NDL.

Yet again, don't get me wrong: I really don't have much of an opinion on what's actually useful vs what we've been led to believe is useful, I'm just trying to summarise what I believe to be @old frogman's relatively minimalist and pragmatic position.

 

[AsherN]

This is really very interesting to me and, again, don't get me wrong: I'm just trying to summarise my understanding of @old frogman's position.

Realistically, the TTS for a "rec" diver is between one and three (all cases round to two, realistically) minutes of ascent plus the safety stop duration: you might just as well display only the safety stop duration instead of displaying both. I've already covered GTR (which I assume is gas time remaining) as in the overwhelming majority of cases rec divers will run out of NDL well before running out of gas, and in any case "surface when you hit reserve" is at least a good a rule as "try to get down to the last few bars in the tank". Water temp, well, it doesn't do any harm except perhaps begin unnecessary, but it also doesn't do any good: it's not like you can do anything about it or - unless you know better than your computer - somehow integrate it into your own parallel deco calculations.

I'm sure that peregrines do a splendid job of optimising power consumption but the datasheets and experience should be telling you those bright displays result in a machine for which battery life becomes a thing which must be taken into account as opposed to a typical black-on-grey display for which it simply isn't. A killer thing? probably not. Just a thing which you're either willing to live with given the (marketed or real) advantages or you're not.

As I said, one column, the one that has temp, is fun to have. As far as TTS, Yes it varies, yes it is between 3 and some other small number. GTR matters as well, because you are wrong. Rec divers will run out of gas before they run out of NDL. I've never come close to "running out of NDL". When NDL is low at a given depth, you ascend a bit. Gas is the limiting factor.

The advantage of a large colour lit display over a monochrome backlit LCD is not a marketing gimmick. The backlit is even worse in low light conditions when you need to press a button to light up the display as opposed to just glancing at it.

 

[AsherN]

You are absolutely correct, doubly so if you take into account a depth-appropriate gas mix. However, at all depths and for all gas consumptions an 'average rec diver' is being led around by a DM, questioned regularly on their remaining gas and is in any case limited by the programmed maximum dive time. The last thing they care about is if they're going to run out of gas or NDL.

I've been on plenty of group dives planned for 50-60 minutes and have some newer divers run out of gas after 30-40 minutes. Gas is a factor more than NDL.

 

[grantmac]

The "simple black on grey" display is all but useless the moment you run into low visibility. Which where I dive happens most of the year, especially at 6m and above.
Then let's look at night diving with a display which requires backlight to be activated.

I learned to dive in an extremely conservative (military) environment using bottom timers and they now use the Perdix because it's simply better in every way.

 

[old frogman]

The "simple black on grey" display is all but useless the moment you run into low visibility. Which where I dive happens most of the year, especially at 6m and above.
Then let's look at night diving with a display which requires backlight to be activated.

I learned to dive in an extremely conservative (military) environment using bottom timers and they now use the Perdix because it's simply better in every way.

In low vis diving and night dives I use a head mounted small light so I can read my dive computer and all other associated dive instruments, for example, contents gauge, compass, wet notes etc.

I believe that Shearwater make dive computers with the Thalman algorithm for the US Navy and DCIEM algorithm for the Canadian Navy. The Belgian Navy adopted the Perdix with Buhlmann GF however they found the 30/70 suboptimal for dives up to 60metres (200ft) and recommended symmetrical GF, for example 90/90 and to decrease symmetrically as required for added safety.

Reference - Paper "Selecting optimal air diving gradient factors for Belgian military divers: more conservative settings are not necessarily safer." VIPER Research Unit, Royal Military Academy, Brussels, Belgium. Centre for Hyperbaric Oxygen Therapy, Military Hospital, Brussels, Belgium.

Keep it simple and do not put all your eggs in one basket.

 

[David Carron]

As I said, one column, the one that has temp, is fun to have. As far as TTS, Yes it varies, yes it is between 3 and some other small number. GTR matters as well, because you are wrong. Rec divers will run out of gas before they run out of NDL. I've never come close to "running out of NDL". When NDL is low at a given depth, you ascend a bit. Gas is the limiting factor.

The advantage of a large colour lit display over a monochrome backlit LCD is not a marketing gimmick. The backlit is even worse in low light conditions when you need to press a button to light up the display as opposed to just glancing at it.

I'd be curious to see a picture of the simplest possible "rec mode" configuration of the perdix display.

Large, permanently illuminated displays certainly do have advantages but they come at the cost of transforming "don't even really think about the battery" to "battery is a concern". The computers that I've worked on all have a screen brightness setting of 'basically off except when you press a button'

 

[Belzelbub]

I'd be curious to see a picture of the simplest possible "rec mode" configuration of the perdix display.

Large, permanently illuminated displays certainly do have advantages but they come at the cost of transforming "don't even really think about the battery" to "battery is a concern". The computers that I've worked on all have a screen brightness setting of 'basically off except when you press a button'

It's essentially the same as the Peregrine display. Only difference is really the name of the DC on the side. Rec Mode Perdix display shows the exact same info with the center and right positions of the bottom row being user adjustable.

As the current Perdix is air integrated, assuming the diver is using a transmitter, then having the pressure displayed in one of those spots is probably a good idea. I use the center spot for pressure.

For simplicity, there is a mini-compass for basic direction that could be put in the other slot, or you could leave it blank.

 

[tursiops]

For simplicity, there is a mini-compass for basic direction that could be put in the other slot, or you could leave it blank.

Nothing beats the floating North-arrow compass on the Teric. That is terrific.