Definition of Gas Time in the Cobalt

[Hatul]

Can I ask, what is the definition of "Gas Time" in the Atomic Cobalt?

 

[Sevenrider860]

It is the time remaining (minutes) calculated from tank pressure and breathing rate. This is the time you can remain at the current depth, and still allow for a safe ascent to the surface, making all stops, and arriving with your preset safety reserve.

 

[Hatul]

It is the time remaining (minutes) calculated from tank pressure and breathing rate. This is the time you can remain at the current depth, and still allow for a safe ascent to the surface, making all stops, and arriving with your preset safety reserve.

That is the better implementation of "air time" and it was not clear from browsing through the online manual. Where did you get that information?

 

[Scared Silly]

Where did you get that information?

That is what is commonly used with most if not all "air" integrated computers.

 

[Sevenrider860]

That is the better implementation of "air time" and it was not clear from browsing through the online manual. Where did you get that information?

Page 33 of the Cobalt manual.

 

[RonR]

It is the time remaining (minutes) calculated from tank pressure and breathing rate. This is the time you can remain at the current depth, and still allow for a safe ascent to the surface, making all stops, and arriving with your preset safety reserve.

This is exactly right. The Cobalt assumes you will make all Deep, Safety, and Deco stops called for in the current schedule, and ascend at 30 ft per minute. It knows your breathing rate for the current dive, and will use that rate to calculate the gas required to reach the surface with whatever safety reserve value you have defined in the Dive Settings screen (only values between 300 and 999 psi are allowed). This isn't a trivial calculation, and it is constantly updating- so going into deco, or if you pick up a deep stop, you may see the gas time remaining change abruptly. The Cobalt alerts by flashing the Gas Time number- and beeping- as it approaches zero.

There are also two alerts for gas pressure, these can be set to go off at essentially any value. These alerts will flash the Gas Pressure number (with optional sound). So a diver wanting to follow a rule of thirds, for instance, would be able to set alerts for the appropriate pressures. But for Gas Time Remaining, we feel we need to take the entire ascent into account, and particularly the breathing rate on the actual dive. In deeper or deco dives, this can serve as a secondary reality check- though divers should always be making their own estimations of gas requirements as well.

One other note concerns dives with planned gas switching. If you have planned a switch to a secondary mix during ascent, that obviously changes the "real" gas time remaining. However, we err on the side of conservatism here, and always base gas time remaining on surfacing with only the primary mix (the mix on the sensor/ hose), not just reaching the planned switch depth. We don't know if a planned switch will actually happen, so we assume it won't for the purposes of this calculation.

In the case of multi-gas decompression diving, the gas time remaining number is not going to be too informative. The decompression schedule will be based on a planned switch to a deco mix at the specified depth. If that planned switch not confirmed, the Cobalt will immediately recalculate a decompression schedule based on the gas actually being used. This will obviously change the gas time remaining calculations, as the surfacing schedule will change. The built in simulator (under planning) in the Cobalt will make gas requirements predictions based on either your historical breathing rates or any other rate you want to use. For multi-gas diving it would be better to base planning off these values.

Hope this clarifies the calculation.

Ron

 

[Hatul]

Thanks for all the replies and Ron's thoughtful reply as always. I did miss the statement on page 33 of the manual. I really like AI since getting the Galileo Sol and can see the safety value of this. There are in fact two implementations of the air time number. Suunto and Liquivision do not take into account gas requirements in the ascent, while Uwatec, Oceanic and and the Cobalt do. The latter type is much superior in helping the diver avoid OOA situations, at least in the open-water setting.

 

[Sevenrider860]

I was surprised to find that the Lynx has such a poor implementation of airtime...it has nothing to do with predictive ascent calculations, it is all about setting 700 PSI as zero. The Cobalt has a much better implementation. From the Lynx manual...

"The Air Time reading will reach zero once your tank pressure hits 700 PSI/48 BAR. This normally sufficient to ascend from a no decompression dive recreational diving depth while respecting proper ascent rates and a 3 minute Safety Stop at 6 ft/1.5m, but it will likely not be sufficient gas to complete a decompression schedule."

 

[Liquivision]

I was surprised to find that the Lynx has such a poor implementation of airtime...it has nothing to do with predictive ascent calculations, it is all about setting 700 PSI as zero. The Cobalt has a much better implementation. From the Lynx manual...

"The Air Time reading will reach zero once your tank pressure hits 700 PSI/48 BAR. This normally sufficient to ascend from a no decompression dive recreational diving depth while respecting proper ascent rates and a 3 minute Safety Stop at 6 ft/1.5m, but it will likely not be sufficient gas to complete a decompression schedule."

Our implementation is the similar to the Suunto implementation except the Suunto places zero air time at 500 PSI whereas we use 700 PSI. Our belief is that there are too many factors that prevent the computer from estimating the gas needed for the ascent. In fact, in our beta software we did use a full ascent gas calculation, but it was exceedingly complicated and very difficult to verify that it would work in all possible scenarios. So we opted for a simpler approach which we feel is more reliable.


Eric Fattah
Liquivision Products

 

[Dr. Lecter]

Awesome, we all love data; please provide some examples of scenarios in which a full ascent gas calculation was found unreliable. The ones that come to mind involve failures or faulty assumptions (consumption rate is higher than programmed in; pressures are lower to start with than programmed in; gas is lost due to a failure), which are potential problems inherent in any gas time remaining estimate. Which is not to say for a recreational-only computer like the Lynx the simpler approach isn't better.

Ron, I'm having trouble reconciling these two statements:

"If you have planned a switch to a secondary mix during ascent, that obviously changes the "real" gas time remaining. However, we err on the side of conservatism here, andalways base gas time remaining on surfacing with only the primary mix (the mix on the sensor/ hose), not just reaching the planned switch depth. We don't know if a planned switch will actually happen, so we assume it won't for the purposes of this calculation."

vs.

"In the case of multi-gas decompression diving, the gas time remaining number is not going to be too informative. The decompression schedule will be based on a planned switch to a deco mix at the specified depth. If that planned switch not confirmed, the Cobalt will immediately recalculate a decompression schedule based on the gas actually being used. This will obviously change the gas time remaining calculations, as the surfacing schedule will change."

Are you saying that the gas time remaining calculation assumes a single gas dive for NDL dives, but once you go into deco the gas time remaining calculation stops being conservative and starts assuming a deco schedule being completed involving multiple mixes? Since the Cobalt doesn't know anything about gas time for mixes other than the primary mix (I guess it could estimate from programmed pressures/volumes and historical RVM values), how does that work?

I would have thought it just assumed you'd have to complete your deco schedule on backgas alone and displayed gas time remaining accordingly. Which may or may not make it less than useful for multigas deco, depending on whether the diver is planning for a worst-case scenario (incur no more deco obligation than you have backgas to complete assuming the same model/conservatism and the longer stop times that would be indicated) or not.