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

[rsingler]

My question re. using a dive computer as suggested would be if the tissues are constantly being updated when not diving. I suspect many do not, but just recalculate them at the beginning of the next dive or when queried about something like no-stop times. So if this works in real time as a meter is dependent on the computer.

My assumption was that this tissue bar graph, which you can bring up after a dive, recomputes in real time.
What I am hoping @Shearwater will tell us is whether the recalculations occur compared against newly measured barometric pressure (as when one drives up the mountain), or are calculated against the barometric pressure registered at the beginning of the dive.

 

[dmaziuk]

View attachment 563681
My assumption was that this tissue bar graph, which you can bring up after a dive, recomputes in real time.
What I am hoping @Shearwater will tell us is whether the recalculations occur compared against newly measured barometric pressure (as when one drives up the mountain), or are calculated against the barometric pressure registered at the beginning of the dive.

But the second part of it: tissue loading, also needs to be recomputed in real time as opposed to on-demand e.g. when one turns on the dive planner or hits the water on the next dive.

 

[YZed426]

I am not aware of any dive computers that will do what you want. Most dive computers only have a 24 hour count down timer that tells you when it is safe to fly after your last dive.

There is a way to manually calculate the time required to wait before ascending to altitude using the NOAA or US Navy Dive Tables. The bad news is that you also have to figure your last repetitive dive group by using the NOAA or US Navy tables for all of your dives for that day. There are no dive computers that will tell you what your repetitive dive group is at the end of a dive.

Attached is a copy of the US Navy Air Dive Tables (you need a different table if you are using Nitrox) and the US Navy Ascent to Altitude Table.

CAUTION -Do not use any other tables (NAUI, PADI, SSI) and the Ascent to Altitude table. The repetitive dive group letters from other agencies may be different than the US Navy Dive Tables.

Would it be feasible to use your computers "plan" option to cross reference your pressure group? (i.e. "plan" a dive to 60', it says you're good for 29min, therefore you're PG "E" on the Navy table, reference that to the altitude table and you can fly your private plane home as long as you stay under 6000') I know there is an issue with different computers using different algorithms, DSAT vs Pelagic etc, but would it be close enough? Maybe throw in a one or 2 PG buffer?

 

[RonR]

I expect you can't turn "dive mode" on with a button on a teric/perdix? Or do they even have "modes" as in "dive mode", "SI mode", etc.

And yes, I guess you're right: if you split you ascent into progressively thinner slices, a-la integral, you can get to the point where the change within each slice is linear for all practical purposes, regardless of the medium. On any modern microprocessor the sampling/recalculation rate should make the slices plenty thin enough. Though I'm not sure that would scale sensibly to tissue half times: what's the meaningful, in terms of on/off-gassing, time slice for 5-minute TC, 2 minutes?

I don't have any insight into how SW is set up, but for Cobalt on the surface we recalculate tissues whenever we need to provide the diver with information, say when looking at no-stop times, or when entering the dive screen. Originally we recalculated and stored tissues on the surface every 10 minutes, but that ended up not really being worth the overhead.
During dives we check pressure and recalculate every 2 seconds, so even "fast" tissues are pretty slow in relation to the sample rate. I'm sure other computers are doing something similar.
-Ron

 

[KenGordon]

It is easy to calculate a negative ceiling in Buhlmann. This can be converted from say -1m of water to however many 1000s of feet of altitude.

However...

I don’t believe this model was not developed for altitude. He did other work about that. So to extrapolate the limits like this may be a mistake.

I think there is a lot of thinking about how to manipulate the model here, when really it is the physical person that matters.

Going back to the OP. Depending on the weather the BSAC88 tables allow a return to 600m after an hour or, in the case of a depression, 90 minutes assuming the last dive was a deco dive.

Having said that, BSAC88 tables are quite aggressive.

 

[dmaziuk]

Originally we recalculated and stored tissues on the surface every 10 minutes, but that ended up not really being worth the overhead.

The good news is if you're not underwater, you can the code on your smart-laplet-phoatch, all you need to do is get the logs from the DC and recompute tissue loadings (downloading them direct from DC would be even better but I doubt any of them exports those).

 

[ATJ]

The Garmin Descent Mk1 may be the computer you're looking (although information is difficult to get).

The Surface Interval screen on the Mk1 displays N2/He %:

This is updated in real time such that 7 hours later it shows:

Now, I wondered if atmospheric pressure made a difference, so I stuck my Mk1 in an old camera housing and sucked out around -30kPa. The N2 went to 46%. This shows it will update if you were returning to altitude after a dive.

What does the percent mean? The manual barely says anything other than "tissue load". Perhaps 100% means saturated.

An interesting thing happened when I let air back into the housing. My Mk1 went into dive mode and had me on a dive at 2.5m. It was a bit of a challenge to get it out of dive mode. Basically, I had to suck the air out again so that it ended the dive and then very slowly, over 30 minutes or so, let the air back in.

I don't know how we can get an explanation from Garmin on the true meaning of the N2% but I think it might be useful.

 

[dmaziuk]

An interesting thing happened when I let air back into the housing. My Mk1 went into dive mode and had me on a dive at 2.5m. It was a bit of a challenge to get it out of dive mode. Basically, I had to suck the air out again so that it ended the dive and then very slowly, over 30 minutes or so, let the air back in.

I don't know how we can get an explanation from Garmin on the true meaning of the N2% but I think it might be useful.

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.

 

[rsingler]

View attachment 563681
My assumption was that this tissue bar graph, which you can bring up after a dive, recomputes in real time.
What I am hoping @Shearwater will tell us is whether the recalculations occur compared against newly measured barometric pressure (as when one drives up the mountain), or are calculated against the barometric pressure registered at the beginning of the dive.

But the second part of it: tissue loading, also needs to be recomputed in real time as opposed to on-demand e.g. when one turns on the dive planner or hits the water on the next dive.

See this interesting reply from Shearwater. It looks like, if we simply turn the computer off when driving up the mountain, and turn it back on when we want to check, it samples a new reference altitude and compares that with your tissue load, which has been offgassing since you hit the surface. The only error will be the difference in offgassing rate between sea level and say, 1000 feet, which will be in your favor.
Driving home to altitude

 

[rongoodman]

My assumption was that this tissue bar graph, which you can bring up after a dive, recomputes in real time.
What I am hoping @Shearwater will tell us is whether the recalculations occur compared against newly measured barometric pressure (as when one drives up the mountain, or are calculated against the barometric pressure registered at the beginning of the dive.

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