Can anyone suggest a computer for returning to altitude after a dive

[rsingler]

These were taken before takeoff and at cruising altitude after leaving Palau the year before last. The slower compartments still show a little overpressure.

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Awesome post! This is exactly what we needed to see.
So if the scale is linear on the tissue graph, and 0% is the green/yellow interface, and 100% is the yellow/red interface, we see a surge to ~20% GF99 at altitude, after a 14-15h surface interval.
Did you turn the computer off between those two pics? (In other words, did the computer use your gradient compared with the surface [with power left on], or did it compute the gradient compared with cabin altitude [if powered up in flight]?)

Also, I'd forgotten that Shearwater tucks a little vertical mini tissue graph next to the NDL number, showing peak tissue GF even when the tissue bar graph isn't displayed.

Thank you, @rongoodman !

 

[rongoodman]

I don't remember if I turned it off or not.

 

[rsingler]

I don't remember if I turned it off or not.

Just looked at the manual: it automatically turns off after 15 minutes at the surface. So it must have resampled atmospheric pressure on the plane when you checked it again. This is very cool!

Now the question is, "What GF should be considered safe when driving to altitude after a suitable interval after surfacing?" Like all GF's, that will likely be a personal decision, but I would think that not exceeding your GFHi would be a start. But considering that we all bubble, and studies have shown a surge in bubbling around 45 min after diving, perhaps a significantly lower acceptable GF surge would be advisable, not to mention waiting at least that long before starting the drive.

Note also that I am definitely NOT recommending this as a way to shorten the interval before flight! You won't know your GF surge until you power up the device at altitude. It is NOT predictive. And by then, it's too late if you've surged into dangerous territory. Much different than driving up 500', powering the toy on and seeing where things have headed. In the car, you can always stop and wait.

 

[ATJ]

Nice. (Armed with 20/20 hindsight) I'd've coupled it with the wet sensor, so pressure sensor alone does not trigger the dive mode. I wonder if others do that.

There are so many ways Garmin could improve dive mode detection. At the moment it can't even tell the difference between freediving and proper diving - it just reverts to the last used mode. I have suggested some rather easy changes but I doubt they will implement.

The increasing pressure thing starting a dive does appear to be dependent on the rate that pressure increases. Some users have no problems at all with flying with the Mk1 while others have had it go into dive mode when the cabin repressurises. I know I was able to stop it going into dive mode by repressurising slowly.

If you can pressurize it to a depth, keep it there until NDL counts to 0, then take it back to surface pressure (safe ascent rate to be on the safe side) and see what the N2% shows, you should/may be able to make a fairly good guess as to what N2% means.

Unfortunately, the only way I can do this is diving. The Mk1 does display a graphic during the dive:

I have definitely got it into the yellow during a dive and I suspect into the red as well (when I've gone into deco but I was paying attention to other graphics plus the Mk1 is not my primary computer).

Looking at the data from the Mk1 from deco dives, and comparing it to the data from my Perdix for the same dive, I suspect it is showing SurfGF, although for some reason, the Mk1 is showing a higher SurfGF than the Perdix at the same point in the dive.

If it is SurfGF, then it should be the same as GF99 when on the surface and so could be used to assess tissue load before heading up the mountain or while heading up the mountain.

 

[NCadiver]

Only way I can see it being done without a special computer being made for you would be turn the computer on at home, dive it once on the surface give it sometime and slowly drive home and have some O2 on hand just in case. Or you could breathe O2 on the way back to add more safety to an unknown variable.

 

[RonR]

Nice. (Armed with 20/20 hindsight) I'd've coupled it with the wet sensor, so pressure sensor alone does not trigger the dive mode. I wonder if others do that.

If you can pressurize it to a depth, keep it there until NDL counts to 0, then take it back to surface pressure (safe ascent rate to be on the safe side) and see what the N2% shows, you should/may be able to make a fairly good guess as to what N2% means.

We use wet contact sensors on the Cobalt, and others do as well. From the description it sounds as if Garmin is just looking at rate of pressure change to decide if it is water or air.

Even with wet contacts preventing dives starting in air it can be complex to determine when exactly a dive begins. You can just use an arbitrary depth- but is the diver just adjusting a strap, or are they really descending? If the former, and they hang out on the surface for a while- as in a long surface swim- then you could have issues with the decompression algorithm seeing the actual dive as a repetitive dive. We use an "start of dive" algorithm that looks at the rate of pressure change and the depth reached to try to get this right and not trigger dive time prematurely.

-Ron

 

[Esprise Me]

What exactly does turning off the Teric mean in this context? Is it enough to exit dive mode and switch to watch mode, or do you have to make the screen go dark? (And if so, do you have to keep turning off the screen and turning it back on to get fresh readings as you ascend?) Because of course even then, it's not really all the way off, as Shearwater themselves confirmed.

 

[rsingler]

What exactly does turning off the Teric mean in this context? Is it enough to exit dive mode and switch to watch mode, or do you have to make the screen go dark? (And if so, do you have to keep turning off the screen and turning it back on to get fresh readings as you ascend?) Because of course even then, it's not really all the way off, as Shearwater themselves confirmed.

That's a good question, @Esprise Me . I'm a Perdix AI guy, so for me it's "Turn Off", which resets the sampling of starting barometric pressure. But as you pointed out, the toy continues to check while it's off.
I'll see if the Teric manual gives us a hint, between watch mode and dark.

 

[rsingler]

From the manual,
"The surface pressure is set based on the lowest pressure the Teric sees in the 10 minutes prior to the start of a dive."

I would interpret that to mean that if you switched to watch mode, it will resample barometric pressure when you next switch back to dive mode as you drive up the mountain. But that's just a guess.

On the other hand, if that second dive never starts...

Conversely, the Perdix manual states,
"The surface pressure is set when the Perdix is turned on."
I don't know if this is only a wording difference between the manuals, or whether the two set barometric pressure differently.

 

[ATJ]

I have confirmation from Garmin that the N2/He % on the Surface Interval screen is updated in real time (probably every minute) based on the actual air pressure. This will change as one drives up the mountain.

I'm still waiting to find out exactly what the percentage represents but I'm guessing you wouldn't want it going over 100%.

The pressure at 700m is around 92% if that at sea level so I would say if the N2/He% is less than 90%, you are good to go, but it is very easy to monitor as you go - and no need to turn anything off.