Off-gassing time countdown. Huh?

[comodino]

To whoever may concern and/or wants to contribute.
This is not a complain against a specific product/brand, rather an attempt to clarify some of what may are my knowledge misconceptions.
I just cannot get my head over the following. Maybe because I am missing pieces of information.


I am planning to buy a SW Petrel not having owned/used one before, and I'd like it to function as my do-it-all rec, tec and teaching tool.
So in a recent email exchange with a superbly kind Lynn from Shearwater Researches I was asking some questions regarding the device's features.
And one little detail struck me: there is no time-to-fly countdown (nor a desaturation time expectancy for what it matters). Maybe it's a common thing among tech computers, I don't know.
I asked why, considering that the computer actually shows a live tissue loading graph during surface intervals as well as when diving.
Since the algorithm is tracking the level of off gassing and showing the results via a graphic bar, I assume it knows exactly when the time to show a normal surface-saturated tissue will come. Right?
I know some cpus/algorithm interpretations distinguish between desaturation time and no fly time (Oceanic's Pelagic), others don't (Suunto's RGBM). So let's just say they are the same thing, and whichever is the longest time shown is the correct one.


Well, this is Shearwater's position, and I quote:
"We do not show a time to fly setting. The reason is that there is no scientific data on this particular subject. We recommend that divers follow the time to fly recommendations from DAN and that is what we do ourselves."
"Until there is some scientific evidence regarding time to fly, it is unlikely that we will offer a countdown timer for this."


So why is there a graph at all?
If I wanted to be polemic for the sake of it I could ask: "if there is no scientific data, where do DAN's recommendations come from, and why should I trust them?". But I won't, if anything because I do trust DAN's data collection.
I imagine this data collection process happens also in post-dive surveys.
We all know a dive is not safely completed until we can state with reasonable certainty that there is no sign of DCI in the diver. And that confirmation happens way after the diver comes out of the water.


To come to my point:
I know how much time I should and will wait after a dive before jumping on a plane or going for a freediving excursion, but I'd like to compare the results to my data cruncher to see what the algorithm calculates. Same as I would do during the dive regarding NDL or deco obligations.
Plus.
I find the "there is no scientific data on off gassing time requirements" statement rather baffling, considering that the whole decompression theory is based on mathematical models and empirical data collection. Before, during and after the dive.
So an algorithm can approximate with decent accuracy what happens to the bubbles in my body when I am subjected to alien pressures and placed in a different environment, but it cannot attempt a prediction on off-gassing on land?
I suppose it can. Is it based on pure conjectures with no scientific data whatsoever backing up these findings?
If the answer is yes, then I don't understand why we should rely on the algorithm's anyway.
Does the basis for VPM,RGBM,Buhlmann & co. suddenly become unproven gibberish when the diver is not submerged anymore?
Shearwater Petrel's manual says "No technology will keep you alive. Knowledge, skill, and practiced procedures are your best defense". I agree.
I can't imagine anybody owning a Petrel (or any other computer) staring at the display's countdown like new year's eve in Times Square and immediately get a hot bath massage in a non pressurised airplane cabin when the numbers zero out. Although I've seen people doing popcorn ascents all the way up to the surface as soon as the safety stop countdown was over.
Is the aforementioned company's statement the consequence of potential liability risks similar to the microwave company being sued by the guy who blew up the cat in it?


I'll appreciate any reply. Please understand that I am not trying to create a flame nor criticising a product I don't own yet, but simply asking for an explanation from people with better knowledge and experience.
Thanks.

 

[Herb-alaska]

To whoever may concern and/or wants to contribute.
This is not a complain against a specific product/brand, rather an attempt to clarify some of what may are my knowledge misconceptions.
I just cannot get my head over the following. Maybe because I am missing pieces of information.


I am planning to buy a SW Petrel not having owned/used one before, and I'd like it to function as my do-it-all rec, tec and teaching tool.
So in a recent email exchange with a superbly kind Lynn from Shearwater Researches I was asking some questions regarding the device's features.
And one little detail struck me: there is no time-to-fly countdown (nor a desaturation time expectancy for what it matters). Maybe it's a common thing among tech computers, I don't know.
I asked why, considering that the computer actually shows a live tissue loading graph during surface intervals as well as when diving.
Since the algorithm is tracking the level of off gassing and showing the results via a graphic bar, I assume it knows exactly when the time to show a normal surface-saturated tissue will come. Right?
I know some cpus/algorithm interpretations distinguish between desaturation time and no fly time (Oceanic's Pelagic), others don't (Suunto's RGBM). So let's just say they are the same thing, and whichever is the longest time shown is the correct one.


Well, this is Shearwater's position, and I quote:
"We do not show a time to fly setting. The reason is that there is no scientific data on this particular subject. We recommend that divers follow the time to fly recommendations from DAN and that is what we do ourselves."
"Until there is some scientific evidence regarding time to fly, it is unlikely that we will offer a countdown timer for this."


So why is there a graph at all?
If I wanted to be polemic for the sake of it I could ask: "if there is no scientific data, where do DAN's recommendations come from, and why should I trust them?". But I won't, if anything because I do trust DAN's data collection.
I imagine this data collection process happens also in post-dive surveys.
We all know a dive is not safely completed until we can state with reasonable certainty that there is no sign of DCI in the diver. And that confirmation happens way after the diver comes out of the water.


To come to my point:
I know how much time I should and will wait after a dive before jumping on a plane or going for a freediving excursion, but I'd like to compare the results to my data cruncher to see what the algorithm calculates. Same as I would do during the dive regarding NDL or deco obligations.
Plus.
I find the "there is no scientific data on off gassing time requirements" statement rather baffling, considering that the whole decompression theory is based on mathematical models and empirical data collection. Before, during and after the dive.
So an algorithm can approximate with decent accuracy what happens to the bubbles in my body when I am subjected to alien pressures and placed in a different environment, but it cannot attempt a prediction on off-gassing on land?
I suppose it can. Is it based on pure conjectures with no scientific data whatsoever backing up these findings?
If the answer is yes, then I don't understand why we should rely on the algorithm's anyway.
Does the basis for VPM,RGBM,Buhlmann & co. suddenly become unproven gibberish when the diver is not submerged anymore?
Shearwater Petrel's manual says "No technology will keep you alive. Knowledge, skill, and practiced procedures are your best defense". I agree.
I can't imagine anybody owning a Petrel (or any other computer) staring at the display's countdown like new year's eve in Times Square and immediately get a hot bath massage in a non pressurised airplane cabin when the numbers zero out. Although I've seen people doing popcorn ascents all the way up to the surface as soon as the safety stop countdown was over.
Is the aforementioned company's statement the consequence of potential liability risks similar to the microwave company being sued by the guy who blew up the cat in it?


I'll appreciate any reply. Please understand that I am not trying to create a flame nor criticising a product I don't own yet, but simply asking for an explanation from people with better knowledge and experience.
Thanks.

I believe what they mean is that the time to fly is somewhat a flexibility by person. Is it 12 18 or 24 hours. Everyone makes their own decisions although you have strong recommendations out there. If they put a 24 hour count down some would be upset about the delay from 18 hours if they go shorter then they could incur a legal liability if you took a hit after your computer cleared you to fly. I think they are more comfortable giving you the data and allowing you to choose when it is same to fly.

 

[rongoodman]

I don't understand your complaint--you have this very detailed tissue saturation graph if you want the information, so what additional information is a count-down display going to provide?

 

[redacted]

The 24 hour time to fly provided by my older Oceanic computers is useless (and harmless). My newer Oceanics provide a forecasted desat time which I initially thought might be useful. It really is not unless if you just need the reassurance. My flight time was scheduled well before I make decisions on my last dive so even the desat time is of little use. With my older Oceanics, I usually note the percentage of residual loading based on remaining ticks as I get ready to board. That is marginally useful for my next trip if I want to adjust the time and profile for my last dive.

 

[cbrich]

Shearwater does show you surface interval since your last dive. It's up to you on what that surface interval should be before flying.

 

[Gill]

Shearwater does show you surface interval since your last dive. It's up to you on what that surface interval should be before flying.

I second this.

 

[comodino]

Thank you all for your replies.
It seems to me that the focus of your answers is on the time-to-fly function usage or lack of thereof, while my questions are more on the scientific side of the thing.

The 24 hour time to fly [...] It really is not unless if you just need the reassurance.

That's exactly my point. It'd be good as a reassurance information.

I believe what they mean is that the time to fly is somewhat a flexibility by person. Is it 12 18 or 24 hours. Everyone makes their own decisions although you have strong recommendations out there. If they put a 24 hour count down some would be upset about the delay from 18 hours if they go shorter then they could incur a legal liability if you took a hit after your computer cleared you to fly. I think they are more comfortable giving you the data and allowing you to choose when it is same to fly.

That is the only conclusion I can come to, but I could argue that it's the same for any other kind of deco indication given by the unit.
So why change the computer's behaviour between decompression at depth and decompression on the surface?
And, although technical diving involves many more risks, I'd suspect the chance to get a bent user because of his idiocy are much higher in the recreational world (I've seen a few), therefore all the companies producing rec computers should adopt the same philosophy and steer clear from a no-fly clock.
Is Shearwater position a bold statement on other companies' cowdung or the consequence of an overly worried lawyer's suggestion?

I don't understand your complaint--you have this very detailed tissue saturation graph if you want the information, so what additional information is a count-down display going to provide?

I am not complaining, I thought that was clear.
The "very detailed tissue saturation graph" is excellent while diving, and it can prove a valuable piece of information for dive planning once on the surface.
The additional information provided by a count down timer would be to actually know, according to the same algorithm that shows me the nice graph, when my tissues will have off-gassed completely so that I can double check my own conclusions without turning the device on to analyse the graph every now and then.
Same function of the TAT or a deco stop countdown.
Hey, the computer doesn't have the feature, that's fine with me. I can live with it. What struck me was the motivation behind the choice of not including the gimmick.
I am interested in deco theory and I read quite a bit on it, although I am not a mathematician nor a physicist.
And this is the first time I hear about a lack of scientific data so tantalising to force a trusted brand to entirely avoid a feature.
Which for what I know might be missing from every single tech computer and I've never known that because I've never owned one.

Shearwater does show you surface interval since your last dive. It's up to you on what that surface interval should be before flying.

Following this reasoning then all I needed would be a Seiko Divers analog watch and a depth gauge for my diving. GUE guys would be happy.
Once again, I am not criticising Shearwater for not putting a countdown. I am trying to understand the "absence of scientific data" issue.


To conclude, I am a big boy with reasonable diving experience to make my own calls, and I am not worried that I won't be able to understand when to take a plane.
I wanted to use the Petrel as a teaching tool during Open Water lessons, since the manuals include questions on "no fly mode" or "desaturation time".
I guess I'll rely on the shop's Gekkos for that.
Still, my questions on the scientific aspect of it are open to anybody with the knowledge to answer constructively.

 

[BabyDuck]

i bet a part of it is a lack of data on the plane you plan to take. by the time you've input the type of plane and planned altitude of the flight and the planned cabin pressurization, just wait the freakin' 24 hours. because then you show up at the airport, and they've changed the type of plane! and now your information is invalid!

just like the table vs computer argument. the computer is more precise, the table treats worst case (deepest for longest). the 24 hours until flight is a table.

 

[cbrich]

Following this reasoning then all I needed would be a Seiko Divers analog watch and a depth gauge for my diving. GUE guys would be happy.
Once again, I am not criticising Shearwater for not putting a countdown. I am trying to understand the "absence of scientific data" issue.

Then why did you buy a Shearwater????? Sounds like you are better off with a Scubapro, Suunto, or Oceanic, because these give you the "no fly time."

Shearwater makes these computers for people with a certain mind set within scuba diving; these computers are not for everyone. Could they add a 24 hour timer on the Predator and Petrel, you bet they could; however the folks these computers are intended for, did not see a need for this function.

IMO a "no fly time" is a useless piece of information. If I can not determine 24 hours from my last dive, then I should not be diving. When I go on vacation, I know when my flight leaves, thus I know when 24 hours is before my flight.

 

[comodino]

Then why did you buy a Shearwater????? Sounds like you are better off with a Scubapro, Suunto, or Oceanic, because these give you the "no fly time."

I speak Aramaic or you don't read what I write.
Either way, thanks for trying.
I hope this thread can be useful for other potential customers, and don't want to ruin it opening a personal argument with you sir.
Have a good day.


BabyDuck, you went quite close to an email I received today from Shearwater:


"There isn't much scientific data to back up a no-fly calculation. It is relatively simple to extend the decompression models to calculate a no-fly time. However, this is stretching the models to places they were neither designed nor validated to go.[…]
While we could implement a simple timer based on the DAN recommendations, we don't feel it offers much value since it is so easy to schedule manually. […]
There is however one aspect of a no-fly time display that I do think provides value, and that is that it educates divers that a no-fly buffer after diving is necessary. However, we feel that the education within the dive community on this issue is sufficient that this reason also does not warrant inclusion of the feature. […]"


At this point I don't see any other reason to continue the discussion here. I probably should've posted a question on no-fly calculation implications directly in the Deco Theory section of the forum.
Thank you everyone for your time and thanks Shearwater Researches for the readiness, availability, and kindness of their customer service. It is a relief to know that some companies still care about customers.