RonR
Contributor
Sorry I was confusing. The gas time remaining calculation always makes the same assumptions in both single gas and multi-gas dives:
- breathing rate consistent with the current dive (this is heavily weighted to recent measurements, so if you suddenly start breathing hard, the gas time remaining will decrease),
- 30 fpm ascent,
- making all stops,
- using only the primary mix to surface.
Anything that changes in any of those parameters will alter the actual gas time remaining. We feel this represents a useful reality check, particularly for newer divers who may tend to underestimate the gas required to slowly ascend from a deeper dive, or for divers who pick up deep or deco stops. Or for a warm water diver diving in our neighborhood, who doesn't recognize how much more air they will need in freezing cold water....
But like any other calculated values in a dive computer, it isn't gospel, it's an estimate based on best data currently available. Your personal safety margin is adjusted by the reserve value you set- this is what you want to surface with. In planned multi-gas diving, though, this number may not be as meaningful- it depends on the scenario. If you enter a planned gas switch, the Cobalt uses that switch to calculate your surfacing schedule, but gas time remaining is based on using the primary mix only.
In no-deco dives with a switch on ascent gas time remaining will be a conservative estimate. However, it still serves as a secondary reality check- if you don't go past zero and something goes south with the planned switch, you should still have enough gas in the primary supply to reach the surface with the planned reserve you have set. You could ignore the alert, but you would be depending on the gas switch happening. It is conceivable that some profiles- for instance if you were on the edge of deco and failed to make a planned switch at depth- could result in your entering deco on ascent- that would change the gas time remaining, though probably not by much. Such stops would likely be short and shallow. That's where entering in an appropriate reserve value for the planned dive is a good idea.
But in a multi-gas deco dive, the surfacing schedule is being calculated based on making the planned gas switch(s)- it may vary quite a lot from the schedule required if only the primary mix is used to surface. If you program in a switch to EAN 50 at 50 feet, that's what the deco calculation is based on. If the diver doesn't make a planned gas switch, the surfacing schedule will immediately recalculate based on the primary mix (or whatever mix is currently in use). A (presumably) longer surfacing schedule will increase the gas requirements.
Because of the increased number of variables in multi-gas decompression diving, once you enter a decompression schedule, the gas time remaining number is probably not very meaningful. Cobalt does always assume for gas time remaining calculations that you will make the entire deco schedule on backgas alone- but if you have a switch entered, it is not using the surfacing schedule the backgas would require.
In order to do what I think you suggest- assume an ascent profile based on the primary mix deco schedule and calculating gas time from that schedule- we would have to keep two deco schedules updating- with and without gas switches. For a variety of reasons this just isn't practical, at least in the current Cobalt. It is a calculation that a decompression diver should be making on their own, as part of their plan.
When we developed the Cobalt initially we did try to provide on screen gas usage estimates for alternate mixes, based on the current breathing rate and the depth. We dropped this for a variety of reasons- mainly that it might encourage divers to believe the Cobalt's estimates rather than doing what they should do, check the SPG. We also calculated gas time remaining to the planned switch depth- we changed this as well to be more conservative and not assume a switch will happen. The simulator on the Cobalt does a good job of predicting gas requirements on multi-gas dives, and we felt it best to leave the gas time remaining calculations simple and consistent. We also acknowledge the limitations of the Cobalt's single sensor by greying out the pressure/ gas time numbers whenever an alternate mix is enabled.
We have considered eliminating the gas time remaining number when gas switching is planned, or greying it out. But for many near or light deco situations it still has value, and we don't know ahead of time what the dive will look like. Divers doing multi-gas deco should be relying on a clear plan for their dive, and this is something the Cobalt's on board planner can help generate. The Cobalt is pretty clearly a recreational, not tech, computer, so I don't know how much use these functions actually see. I'd be curious what divers doing deco on the Cobalt would like to have for multi-gas situations. In many cases these are not questions with a clearly "right" answer, but they are ones every AI computer that allows multiple gasses has to deal with.
Ron
