Atomic Cobalt computer and DCS

[BugHunterNY]

Thanks for the replies. I just posted this to see if anyone has had any experiences with DCS specifically using the Cobalt. I had it set to the liberal setting with moderate exertion level and my age entered correctly. I completely understand the contributing factors to DCS. I'm going to say the factor in this case was over exertion. Bad current that day, and yes, I guess I'll have to accept the fact that I'm more than twice as old as I was when I got certified! I do like the computer a lot and have since set everything on it as conservative as it will go.

So all your gear isn't for sale?


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[kensuf]

I guess I'll have to accept the fact that I'm more than twice as old as I was when I got certified! I do like the computer a lot and have since set everything on it as conservative as it will go.

If it makes you feel any better, I'm 3 years shy of being four times as old as I was when I got certified..

 

[Lorenzoid]

Still not going to share the dive profile?

 

[whysocold]

I also dive the cobalt and I also suffered DCS recently. I do find that it is more liberal than other computers *cough*Zoop*cough* even though I have it set for the most conservative settings.


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[tazgator]

Thank you for sharing this...

 

[RonR]

Thanks for the replies. I just posted this to see if anyone has had any experiences with DCS specifically using the Cobalt. I had it set to the liberal setting with moderate exertion level and my age entered correctly. I completely understand the contributing factors to DCS. I'm going to say the factor in this case was over exertion. Bad current that day, and yes, I guess I'll have to accept the fact that I'm more than twice as old as I was when I got certified! I do like the computer a lot and have since set everything on it as conservative as it will go.

This is the kind of posting we dread, and I hope you are recovering well.

As designers of the Cobalt (but not the algorithm, though we did implement it and so understand it well), one of the hardest things we have to deal with is the knowledge that some divers will follow the computer and still get DCS. The fact is that many things with very large impacts on DCS risk- hydration, exertion level, fitness, and hidden physiological factors- are things that a computer can’t measure. And decompression is an imperfect field- in Erik Baker’s expression, all algorithms are attempts to draw a bright, clear line through a fuzzy gray area. I’ve personally been diving the exact profile right alongside someone who got hit when I did not. Computers are a guide- usually very good guides- but they just run mathematical models, they can’t know what is actually happening in your body.

To address some of the questions, RGBM is a framework, one that seeks to minimize bubble formation, but the relative conservatism is determined in each manufacturer's implementation by adjusting settings in the algorithm. Prior to releasing the Cobalt a great deal of experimentation and simulation was done to adjust the Atomic algorithm to be about in the middle of the pack for conservatism- as compared against major recreational dive computers and considering a wide variety of dive profiles and multi day, multi- dive scenarios. As has been noted here, generally the Suunto computers are at the conservative end of the spectrum. Others are more liberal than the Cobalt. Over a number of years most dive computer implementations have been getting more and more similar in results, even if they use different models to get there. But what constitutes conservative or liberal is hard to say precisely or for all situations. Is it no-stop times? In repetitive diving? Is it time to surface in deco diving? To what depth and with what mix? There are a lot of variables.

Conservatism differences tend to seem more extreme in shallower, longer, recreational dives. This is because we treat deco/ no deco as a binary function when in reality is is a gradually increasing slope, very gradually increasing at shallow depths. So on shallower dives even a slight divergence in the algorithm conservatism can translate into many minutes difference in no-deco time. The same computers might show much less difference- in minutes of no-deco time- at deeper depths. That probably accounts for some of the subjective differences in experience of how conservative a particular computer is. A six or seven minute difference in NDL’s on a shallow dive may indicate a relatively trivial difference in the algorithms. A two minute divergence at greater depth might indicate a significant difference. At what depth and with what prior dive history are key factors.

You could compare this to hikers climbing a very gradually increasing slope, and deciding at what % grade to turn back. Say one person decides to turn back when they encounter a 30% slope and another at 32%. If they both head straight uphill, they will turn around at almost the same time. But if they traverse the slope at a very shallow angle, the 30% hiker may turn around a long time before the 32% hiker. His turnaround point is no more conservative than it was going straight uphill, his risk no less, but the difference in time between the two is greater because of the angle at which they approached the hill. In the same way, shallow dives tend to magnify- in terms of time- differences in algorithm conservatism.

Speaking as someone involved in computer design and dive equipment design for more years than I like to think about, computers- all of them- should be used as one source of information, not as oracles. I’m gratified that the posts here seem to reflect that attitude. Taking into account all the risk factors the computer doesn’t measure, and adding those in with the information it provides, will often result in deciding to follow a more conservative profile than the computer gives.

Ron

 

[whysocold]

Ron, after my DCS hit, I noticed that the Colbalt was reporting my air pressure differently than my SPG. I had sent it in before because of calibration issues for air and the compass. Can atomic verify that the other sensors are functioning properly? I would hate to be diving deeper than I thought because of a sensor issue. Have a lot of the colbalts had issues with sensor calibration or is my unit just "special"?


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[RonR]

What kind of pressures are we talking about, a nearly empty tank or more pressure? The HP sensor on the Cobalt is very easy to calibrate if you have an accurate gauge (most dive shop repair stations or fill stations will have an accurate wall mount gauge). The Cobalt is typically more accurate than "brass & glass" analog gauges. In use we haven't seen the factory calibration need to be redone- which is not to say it couldn't happen. If it is not right I would want to have the Cobalt checked out. There were version changes to how quickly the Cobalt 1 updates changes in tank pressure- are you running the most recent firmware? You can PM me with specific questions or details. The tank pressure and depth sensors are completely separate devices.

If this is a Cobalt 1, the compass is another story. That was an early generation compass chip, one of the first that was small enough and inexpensive enough to go in a handheld device. The same part was used in the iPhone 3. Many of them needed recalibration and could be knocked out of adjustment by exposure to environmental magnetic fields. The factory calibration- which sets the gain and offset for each axis- would probably be what was redone if you sent it back. It is possible to redo that yourself, but it is more involved than calibration for local conditions. The compass chip in the Cobalt 2 is far more accurate, faster, and updates the underlying calibration automatically- it's night and day.

The depth sensor is easy to check, since the Cobalt, on the System Info page, displays actual ambient atmospheric pressure, which comes from the same sensor. You can check on the surface to see if the sensor is reading actual atmospheric pressure accurately. Compare the pressure reading on the Cobalt’s System Info page to the local atmospheric pressure you can get from weather stations or an airport. Important: If you are at an altitude above sea level, make sure that you are using actual local pressure, identified as "station pressure" and not the “sea level compensated” pressure most weather outlets display. If you have trouble finding actual pressure readings, or live on a mountaintop, an online conversion calculator is available here: Above Sea Level Barometer Reading Correction Calculator This will also give you conversion to millibars if your local pressure is given in some other units.


The Cobalt should display a value within plus or minus 20-30 mbar of the actual ambient pressure. For perspective, 20 mbar is about 20 cm, less than 8 inches, of water depth. The pressure should not vary more than a few millibars over several minutes, corresponding to actual atmospheric pressure changes.

The depth sensor in the Cobalt is used by many, perhaps most, dive computer manufacturers and has been around a long time. Some early early production Cobalt 2's had issues with damaged sensors (this was a problem from the sensor manufacturer), these started to read deeper than actual depths once they were in the field. But that was an aberration, overall these parts have been accurate and reliable.

Ron