Hello All, I just picked up a Cobalt and used it for the first time this past weekend along with a Suunto Gekko as a back up, which I have used in the past to back up some other computers. I have to say I really do like this new Cobalt but the difference in legal time was unusally different. I was diving in 75ft using twin 100's with a 33 mix. First dive was about a 55 minutes with 5 minute hang.. The Gekko was held me for 2-3 minutes longer... no big deal there. I did about 1.5 hour SI and went back for another 55-58 minuts. The the Cobalt at this point still gave me 15 minutes legal and the Gekko gave me a 13 minute penalty. That is just too different... Needles to say I stayed to clear the Gekko.
Does anyone have any thoughts on why they are so differnt? Could there be something wrong with the Cobalt? Ohh and I did max out on lobsters.. it was a good dinner
There could be differences in the settings- the Cobalt adjusts the algorithm for your age, conservatism settings, and workload. But to some extent you are seeing the differences between the algorithms- Suunto has the reputation of being quite conservative, particularly when it comes to repetitive or deeper than previous dives, multi-day diving, or repetitive deco dives. There are reduction factors that the algorithm uses to shorten allowable no-deco time or lengthen stops, Suunto uses slightly more conservative factors. The Cobalt is fairly middle-of-the-road as to conservatism. You can use the simulator in the Cobalt to see what it would do for any series of dives.
But the biggest factor in what you are seeing is due to the fact that you are doing a long, shallow (air equivalent) dive that is not much below the depth where you would never get a deco obligation. This question comes up often regarding shallow dives or dives that are approaching a lengthy no-deco limit (at 75' around an hour on a 33% mix, equivalent to a 45 foot or so air dive). It has to do with the binary assignment of deco- no deco times that all dive computers do. If you envision the tissue saturation as a rising line across a graph left to right, on these dives it has a very shallow slope. That slope is defined by the algorithm conservatism. Exactly where it crosses the line into deco can vary a lot in minutes with a very tiny difference in the slope. So you might see a 10-15 minute variation, but the fact is that even if you cross into "deco" on one computer, you are just barely into deco in it, and almost in deco on the other- even if the times displayed seem quite different. It's a problem that comes up because we treat "no-deco" limits as if they were an on-off switch that got pulled. In fact it's a very fuzzy area, particularly on longer, shallow dives. It helps to remember that "all dives are deco dives". I'm betting that if you did dives with a shorter no-deco time- either an air dive, or a deeper dive- you would see the variation between the two computers shrink considerably. The slope of the line defining saturation would be much steeper, even though the two computers might not be the same, in terms of minutes of no-deco time they would seem much more alike. With shallow dives, even though the variation in times that the two computers gave you seems large in terms of minutes, in terms of tissue calculations it is probably very slight.
I'd be happy to look at the profiles if you want a review. You can step through the profile on the Cobalt and see exactly what for no deco time was at any point. But I think that most of what you see is due to the fact that long, shallow, repetitive dives will magnify slight differences between algorithms.
Ron
