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

[Michael Jastrzebski]

then would watching leading compartment overpressure (GF99) be a good way to assess your safety?

I suspect SW's answer would be firm: NO. Therefore nothing of the sort was implemented. This drive to the mountains is practically an identical problem to flying-after-diving and here is a very informative discussion on the subject on SW's website: Flying After Diving - Shearwater Research
Basically SW is trying to justify in front of some users why there is no fly-after-dive info available on their computers. Even though GF99 is never mentioned in the discussion one can get sense what their thinking is. In other words, after reading this I am beginning to think the GF99 even if updated would be a bit useless (or meaningless) to avoid symptoms of DCS. SW points out that DAN's recommendations to fly-after-dive can't be derived from theoretical tissue load. And since these recommendations can't be derived from any deco algorithms and aren't calculations based - they decided to leave it alone. Opinions may vary but I think it is sensible, if you are a deco 'purist' - this is what you would do.

My friend had some Suunto brand new DC on my trip last week to Truk, his DC was showing him 39-hrs wait before flying. He ignored it and stuck with 24 hrs. I am glad SW is staying away from this mess.

 

[Blackcrusader]

I live 600-700m above sea level and I am trying to research possible dive computers that would let me know when it's safe to return home after a dive.

The variety of computers available is simply mind-boggling, so if anyone has any suggestions, I'd appreciate it.

I'm a newly qualified PADI OWD, but I live in the Canary Islands with ample opportunities to dive, so I would think that I'll be going at least once a month and would like to invest in some kit that would reflect this frequency of diving, regardless of price. I'd rather save up and buy something I'll use for years than have to spend more replacing stuff later.

Many thanks!

Kirsty

I live up near 2400m above sea level. I have a Shearwater Perdix. Never an issue with the computer. Not cheap but if you are planning to do regular diving and want a once of purchase this would do. If you prefer a watch type computer the Shearwater Teric.

However as I dive far away it's always around 36 hours before I get home from my dive sites overseas.

 

[rsingler]

This drive to the mountains is practically an identical problem to flying-after-diving

Respectfully, sir, I would strongly disagree.
It is one thing to take (as the DAN article points out) already bubbling divers and in a single square-wave phenomenon, subject them to a significantly greater offgassing gradient (7000 foot cabin altitude), not to mention bubble-expanding ambient pressure.
Instead, what driving to altitude after diving is closer to, IMO, is ascending from a safety stop. What we see on our dive computer is a bloom in GF 99 as the offgassing gradient increases. Similarly, taking 30 minutes to ascend 1000-2000 feet applies the same gradual increase in decompression stress that ascent from depth does. Therefore, to me it isn't illogical to see if you can quantify that stress, much as we do with gradient factors, and decide what is safe.
Flying after diving, on the other hand, seems more akin to saying, "My dive is a no decompression dive, so I can ascend straight to the surface at 60 FPM with no safety stop. After all, it's a no-stop dive."

That's what we used to do, isn't it? And now we pause. And now we slow our ascents. And now we quantify offgassing stress with things like SurGF. I don't think it's unreasonable to try to look at a gradual ascent to altitude in the same way. No one's going to wait 12h like they would for no-fly time.
Indeed, we've had suggestions in this thread to use tables to figure out an appropriate interval to lower your letter group so that you can ascend to altitude. Isn't that just like diving with the old U.S. Navy tables, before computers came along? I think I'd rather try and look at a real-time solution, where we can expect the same benefits we discovered when we applied computers to table diving.
We just need the right number. And then (just as with early recreational and technical diving), divers will experiment on themselves and we will learn more as a group.

 

[Michael Jastrzebski]

Well, let me disagree with your disagreeing...
Flying is actually a far worse case and in this case we should go after the worst case scenario...
I am a private pilot, after some diving I could jump right into a Cirrus SR22 that I fly regularly and be at 17,000 ft altitude in about 30 mins (far worse scenario than any commercial flying).
If your GF99 is useless in such case ... it is going to be even more useless in a gentle drive to a mountainous village.

 

[dmaziuk]

But tell me your thoughts on this: IF the computer actually recalculated the predicted tissue overpressure in real time based upon measured barometric pressure as you ascended, then would watching leading compartment overpressure (GF99) be a good way to assess your safety?

As you know by now, I am very skeptical about GF99, GFSurf, etc., in general. Yesnomaybe: it should not be any "less safe" than watching it on the way up from safety stop, but I am not convinced that lowering it makes you any "more safe". As long as you don't overshoot 100%, obviously.

 

[RonR]

Maybe I'm missing something here. Like the point of watching GF99 as you drive up the mountain.

1. Surfacing M-value at sea level is calculated with the assumption you're getting (in and) out of the water at sea level. Surfacing M-value at 600 m is calculated based on the assumption you're getting out of the water in a mountain lake in Switzerland at 600 m altitude. M-value line is a straight line because the pressure changes linearly.

In the air M-value line is no longer straight, but your dive computer has no idea. If you're getting out of the water at sea level and driving up to 600 m, the surfacing M-value at 600 m calculated by the computer is wrong for you.

2. Similarly, computer calculates off-gassing between sea level and 600 m up based on the assumption that ambient pressure drop is linear. If you're driving up in air, it isn't, but the computer does not know this. Your computed gas loading is going to be off.

3. GF99, as I understand it, is the ratio of calculated gas loading, as per #2, to the surfacing M-value as per #1.

To me this sounds like two wrongs. They may largely cancel each other out and make an almost-right, but personally I'd take Scubapro's little yellow mountains over GF99 any day.

It may depend on the computer's firmware and how information is displayed as to whether there is any practical information available- can you see tissue saturation updated when you are on the surface? I’d want to know if tissues are continually being updated in relation to actual ambient pressure, or are they just recomputed at the start of a dive? Can the computer directly sense if it is in water or in air?

But essentially you are always looking at tissue loading vs. ambient pressure. Ambient pressure on the surface will change with altitude, and yes, it is not linear. But linearity or non-linearity of ambient pressure change in a medium is irrelevant. That’s because it is being directly measured. The computer has no idea how much your altitude changed, it only knows pressure. We could use the same depth sensor in the computer to make a reasonably accurate altimeter by putting in the right equation and providing a display. But that would have no impact on tissue loading vs. ambient pressure or decompression calculations, because those are purely dependent on saturation and actual ambient pressure (and the gas being breathed, which on the surface we always assume is air). There is always a “ceiling” for any tissue saturation, in air or in water. In GF-terms you could presumably set that to be whatever safety margin you wanted. I’m not sure GF99 would be the ideal.

As a thought experiment, imagine that you increase your speed driving up the mountain such that the rate of pressure change was in fact linear. It wouldn’t matter. The only thing that matters is the tissue saturation vs. ambient pressure.

This brings up an interesting question, which is why the thread caught my attention. When we originally showed the Cobalt electronics/ firmware design to Atomic many years ago, we included a no-fly time calculator. That displayed the time to when the tissue saturation would be “safe” at a 7,000 ft. cabin pressure as the no-fly time. That was nixed by Atomic, I’m sure for liability reasons, as the times displayed were nearly always far less than the DAN 12/24 hour recommendations.

We have always thought about putting back in this calculation as optional information to provide a “ceiling” altitude in air. We have received the request from both private and commercial pilots, who are well aware that the simple countdown clock does not tell them anything useful. We kind of have this information now in the no-stop time calculator, in that if you change altitude the no-stop times change, and at some point if you went high enough they would go to zero. But that’s not predictive, so it doesn’t help those wanting to plan flying or driving back to their mountaintop lair. I wonder what people think about this as a feature?

-Ron

 

[dmaziuk]

But tell me your thoughts on this: IF the computer actually recalculated the predicted tissue overpressure in real time based upon measured barometric pressure as you ascended, then would watching leading compartment overpressure (GF99) be a good way to assess your safety?

On second thought, h*ll no! Watch the flipping road!! Don't GF99 and drive!!!

 

[dmaziuk]

As a thought experiment, imagine that you increase your speed driving up the mountain such that the rate of pressure change was in fact linear. It wouldn’t matter. The only thing that matters is the tissue saturation vs. ambient pressure.

I dunno, that's why I started with "off the top of my head" originally. In Schreiner's equation the rate of change R is delta-pressure divided by time. The assumption is that the pressure changes at a constant rate during that time interval. I can't quite wrap my brain around how it would change things if it didn't, so I err on the side of "garbage in - garbage out": If one of your underlying assumptions is garbage, I would not trust the output.

 

[RonR]

No, that's right. The assumption that pressure changes at a constant rate is an assumption to make things workable, but it's not ever perfectly accurate. After all, the delta-P moving up or down just a foot in water is equivalent to something like changing 1000' in air near sea level. You might ascend in water at quite variable rates, despite the medium itself being linear. The shorter the time intervals between samples and recalculations, the more accurately the assumption will map reality. To a computer, ascending in air at a constant rate would just look like changing pressure / ascending at a very slightly decreasing rate.
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.
But it is perfectly possible to predictively calculate a time interval after which ascending to an altitude would be within desired saturation to ambient pressure values.
-Ron

 

[dmaziuk]

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.

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?