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

[rsingler]

If it follows the Shearwater conventions, then the Garmin may be displaying controlling tissue compartment supersaturation. Until we know more, I wouldn't think you'd want any percentage greater than your acceptable GFHi. So for me, I think I'd want it to read xx/.92 < 70, or about 64.
But it's really hard to deal with an undefined datum called "%". Anything more about that field in the manual?

Your one screenshot shows that the % seems to track the edge graphic called "Tissue Load Level". That suggests that it's indeed CTC-SS.

 

[ATJ]

But it's really hard to deal with an undefined datum called "%". Anything more about that field in the manual?

Nah, the manual (as are most Garmin manuals) is lacking detail.
Descent Mk1 - Viewing the Surface Interval Widget

Your one screenshot shows that the % seems to track the edge graphic called "Tissue Load Level". That suggests that it's indeed CTC-SS.

I have reached out to a contact I have within Garmin for a more detailed explanation of what the percentage means.

I need to do a dive where I get close to NDL (or even a bit of deco) and see what the percentage is immediately after surfacing. That would at least show me what percentage is considered "safe".

 

[rsingler]

I need to do a dive where I get close to NDL (or even a bit of deco) and see what the percentage is immediately after surfacing.

One thing you might also watch is what happens to the number in the last 15 feet of your dive, before you surface, assuming you can see that field. If it's the leading compartment supersaturation, it should increase rapidly in the last 15 feet, after your safety stop. If instead, it's a predicted surfacing saturation, it should stay relatively constant on ascent, or even decrease as you offgas during your safety stop, but not increase during final ascent.
If it's neither of those, I dunno...

And if it's a Surface Interval field only, then yeah, you'll have to wait.

 

[ATJ]

One thing you might also watch is what happens to the number in the last 15 feet of your dive, before you surface, assuming you can see that field. If it's the leading compartment supersaturation, it should increase rapidly in the last 15 feet, after your safety stop. If instead, it's a predicted surfacing saturation, it should stay relatively constant on ascent, or even decrease as you offgas during your safety stop, but not increase during final ascent.
If it's neither of those, I dunno...

And if it's a Surface Interval field only, then yeah, you'll have to wait.

The actual percentage is only shown on the Surface Interval widget so only available after finishing the dive.

There is a graphic displayed on the dive screen which may be the same thing. I'll try to remember to watch this.
Descent Mk1 - Dive Data Screens | Garmin Support

 

[ATJ]

The guy from Garmin never got back to me. It is sad that Garmin have so little interest in providing information to their customers.

Today on my dive I got to within 30 minutes of NDL and the graphic showed my tissue loading was >80%. I didn't get a chance to check the Surface Interval widget until 6 minutes after I surface and the N2 was 74%. 14 minutes after the dive ended it was 66%.

There is also a nice tissue graph (coincides with 66% N2):

 

[Geo7]

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.

Hi all, great thread, and also serves as a good reminder of this issue. A chamber operator in Catalina Island I was talking to this weekend mentioned that one of their technicians got a (temporarily) numb arm while accompanying a patient for treatment from San Pedro to Northridge a few years back, so this stuff is definetely real and should not be taken lightly.

With regards to a potential computer: The german company Heinrichs-Weikamp have posted on their forum that the Bühlmann model was created with aerospace applications in mind, and is valid below atmospheric pressure. They say this is clearly stated in Bühlmanns book, but my copy is in a moving box and I have to take their word for it. The old Uwatec Aladins had a wide range for altitude diving, and also used to calculate "no-fly" times, as does the current Heinrichs-Weikamp OSTC software (except OSTC4). According to their post on the OSTC forum, the Bühlmann no-fly times are based on the worst case low pressure of 0.6 bar (much less than in a passenger plane).

Having said that, Shearwater is in touch with the hyperbaric medical community, and if Shearwater is not convinced of the accuracy of no-fly predictions, that does give me pause...

One thing to consider could be breathing nitrox / oxygen on the last stop or on the surface to clear the tissues.

 

[adrian_diving]

Hi all, great thread, and also serves as a good reminder of this issue. A chamber operator in Catalina Island I was talking to this weekend mentioned that one of their technicians got a (temporarily) numb arm while accompanying a patient for treatment from San Pedro to Northridge a few years back, so this stuff is definetely real and should not be taken lightly.

With regards to a potential computer: The german company Heinrichs-Weikamp have posted on their forum that the Bühlmann model was created with aerospace applications in mind, and is valid below atmospheric pressure. They say this is clearly stated in Bühlmanns book, but my copy is in a moving box and I have to take their word for it. The old Uwatec Aladins had a wide range for altitude diving, and also used to calculate "no-fly" times, as does the current Heinrichs-Weikamp OSTC software (except OSTC4). According to their post on the OSTC forum, the Bühlmann no-fly times are based on the worst case low pressure of 0.6 bar (much less than in a passenger plane).

Having said that, Shearwater is in touch with the hyperbaric medical community, and if Shearwater is not convinced of the accuracy of no-fly predictions, that does give me pause...

One thing to consider could be breathing nitrox / oxygen on the last stop or on the surface to clear the tissues.

Great thread. I own a Shearwater Teric, and I can confirm that it seems that Teric is updating tissue loading information in real time after the dive, taking into account the current atmospheric pressure. I live at altitude, and partial pressures will converge to the saturation at that altitude after a while. I live at 3000ft, and I have also driven up to 7000ft, and even 14000ft, while watching the tissues plot and GF99, and it does update in real time.

A dive computer should update tissues in real-time by taking into account altitude in order to correctly compute tissue loading during your surface interval so that you know where the next dive starts. This is important when diving to altitude. Say, you dove at 3000ft, and then during your surface interval you spent it at 0ft (sea level), before going back to 3000ft to dive. If the computer does not take into account that you have more nitrogen in your tissues because your surface interval was at 0ft, the computer will dangerously overestimate how much you can stay under water at 3000ft during your next dive. I am not sure how dive computers normally do this, but Shearwater Teric correctly tracks tissue loading relative to air pressure during the surface interval. Shearwater Teric for instance will give you more time underwater at sea level if you live at 3000ft, because your tissues will start will less Nitrogen saturation because you live at 3000ft.

Given that Shearwater computers track tissue loading correctly relative to altitude, you could use the tissue plot and GF99 to decide if it is safe to drive to altitude. Shearwater however does not endorse this, and they should not. This is because even though the theoretical model does tell you it is safe to drive up, there is no experimental research proving that indeed following this model is going to not result in DCS. What if there are some other factors at play, such as how fast you go to altitude (flying), or how safe it is to drive up after you already have bubbles and pre-DCS from your dive? You may be ok to stay at sea level, but not ok to drive up.

In a typical recreational dive DCS bubbles would form right when you exit the water, not during the dive. The GF99 gradient is typically very low until you exit the water over the last 3ft of water. So if you develop any pre-DCS, it would be at sea level. Staying at sea level during your surface interval would not worsen that situation, and eventually your system will take care of any tiny bubbles. But driving to altitude or flying would make those bubbles bigger, potentially creating a problem. You can already see that even if the theoretical model is the same, driving up to altitude exposes you a different kind of risk than on a typical recreational dive.

That said, technical deco divers do take this kind of risks frequently. They experience high gradient factors not just as they surface, but at depth, as they push the limits so that they decompress as fast as possible at depth under water. Tech divers do push the limits of the decompression algorithms. Recreational divers DO NOT experience those kinds of stresses under water, but only a moderate stress as they surface.

Using your dive computer's decompression algorithm to make decisions about when to drive to altitude is more risky than the risks taken by normal recreational divers, but way less risky than what tech divers do. But driving up to altitude while not following the PADI / DAN recommendation without considering your decompression status is even more risky, probably more risky than tech diving. I would prefer to use a dive computer to check tissue loading before making the decision to drive up soon after a dive, then using nothing at all.

 

[Geo7]

Given that Shearwater computers track tissue loading correctly relative to altitude, you could use the tissue plot and GF99 to decide if it is safe to drive to altitude.

Great idea using the G99.

Shearwater however does not endorse this, and they should not. This is because even though the theoretical model does tell you it is safe to drive up, there is no experimental research proving that indeed following this model is going to not result in DCS.

That's where someone more knowledegable on hyperbaric medicine should chime in than me (even though I own the Bühlmann book...), since I was under the impression that Bühlmann's work specifically covered the topic of low pressure after diving. Heinrichs-Weikamp currently implement it into the OSTC for a reason. Many of us grew up flying after diving when the Uwatec Aladins permitted because they tracked residual nitrogen loading and pressure. You could say your surface stop is just another deco stop before ascending higher. I have trust in the modern OSTC do-not-fly calculations with some safety margin like for everything else.
But perhaps you are right when you extend this model into the mixed gas diving range and the extreme exposures found into today's recreation diving. And pre-DCS microbubbles of course are a complication that likely was not covered in Bühlmann's work. Where are our experts to chime in?

 

[adrian_diving]

Great idea using the G99.



That's where someone more knowledegable on hyperbaric medicine should chime in than me (even though I own the Bühlmann book...), since I was under the impression that Bühlmann's work specifically covered the topic of low pressure after diving. Heinrichs-Weikamp currently implement it into the OSTC for a reason. Many of us grew up flying after diving when the Uwated Aladins permitted because they tracked residual nitrogen loading and pressure. You could say your surface stop is just another deco stop before ascending higher. I have trust in the modern OSTC do-not-fly calculations with some safety margin like for everything else.
But perhaps you are right when you extend this model into the mixed gas diving range and the extreme exposures found into today's recreation diving. And pre-DCS microbubbles of course are a complication that likely was not covered in Bühlmann's work. Where are our experts to chime in?

I am not an expert, but my views are similar to yours. I was wondering why cannot we treat driving up to say 7000 ft as an extended altitude dive in which you start at 7000 ft, drive down to 0 ft, wait there until you saturate at 0 ft, do your dive, surface, wait a bit, and then drive up to 7000 ft. One can modify deco software to calculate this dive profile, and determine how much surface interval to have before driving up to 7000 ft in the last segment (in fact, I DID modify an implementation of Bühlmann and used it to determine how long to wait before driving up). This is similar to what tech divers use to plan their dive. This should be no less safe than what tech divers do, and in fact you can build extra safety by waiting even longer than the model says.

I think it's all good, except that you only have a theoretical model to guide you, and no actual data from many divers doing this safely. When you use a dive computer for recreational diving, you not only rely on the decompression model, but also on a very large number of successful dives that used that algorithm in the same way. Tech divers also use dive profile that are used widely (albeit in a much smaller tech diver community), and are aware when they are trying something that has never been done before. Whereas using this model for driving to altitude -- you are the experiment.

Another option is to use the US Navy Dive Tables. I am absolutely sure they were also derived from a similar decompression algorithm on paper. But then US Navy has some experience using them and testing them, and likely adjusting them if they resulted in DCS. The catch is that US Navy is probably more willing to take risks than recreational divers. US Navy has young fit divers who may be able to withstand more stress.

All in all, I still think that watching your decompression status before driving to altitude (and while driving to altitude) should be safe enough, especially if your dive was not risky. In my case, I am diving Nitrox, and watching SurfGF during my safety stop, and I am spending extra time at 15ft, air permitting, to reduce SurfGF as much as possible. I am usually able to get it down to under 20%, or even under 10%, which is extremely low stress when surfacing. This reduces the risk of forming dangerous bubbles when surfacing, so then I should be able to go by what the decompression model says about driving to altitude.

 

[broncobowsher]

I tried this once a few years back. San Diego back to Arizona can be done 2 different ways. Direct I8 that goes over a 5000' pass. Or drive up to LA and cross over on I10, horrible traffic that adds hours and a lot of miles.

Taking the I8 route I was finding the off-gassing was faster than what altitude would matter. Give it I didn't just out of the water and start driving either. There was the boat ride back, load up the truck, get a bite to eat, now hit the road.