Some interesting replies.
I would like to give this thread a little time before I respond.
And then you can critique my response.
---------- Post Merged at 09:36 PM ---------- Previous Post was at 12:29 PM ----------
All my key points were hit quickly;
Depth = pressure; although my wording would be 'a depth gauge is a pressure gauge calibrated to display as depth in feet of sea water'. I am assuming that the dive computer pressure transducer is similarly interpreted.
I heard an instructor teach that there is a decompression safety factor diving in fresh water, as if the depth gauge were actually a tape measure.
Accuracy = e.g. is the depth 100 feet or 100 feet +/- 2 feet ?
Offset = if you are diving in fresh water, i.e. at altitude, what is the 0 (zero feet or surface level) offset ?
I was surprised to find there are dive computers that claim to be able to automatically detect and compensate for altitude and fresh water.
So, now it's your turn.
Disclaimer: I'm one of the designers of the Atomic Cobalt, so this is my day job.
Well, the main point is that decompression is based on pressure, so depth is really immaterial. If you were able to dive in a very dense liquid- mercury, say, you would get into deco trouble at very shallow depths (this would be secondary to more serious problems, of course, but we're being theoretical here).
It would certainly be possible to detect fresh vs. salt water with a computer, we already detect water presence and salt water is a lot more conductive. If you did so the depth would be more accurate (by a pretty trivial amount), but it would make zero difference to the decompression calculations. Pressure is pressure. Altitude compensation is something else entirely, and has real value. Lower surface ambient pressure makes a very significant difference in the decompression calculations.
In the Cobalt (and in quite a few other computers- any that are automatically altitude compensated), the pressure sensor is an
absolute sensor- it measures pressure relative to a perfect vacuum- so it registers the ambient pressure at the start of the dive and adjusts the decompression algorithm appropriately for the actual surface ambient pressure. It is essentially functioning as an altimeter or barometer as well as a depth/ pressure gauge. In the Cobalt, we display an info screen with the barometric pressure in millibars as a diagnostic check of instrument accuracy, and you can see how the no-stop times adjust as you drive over a mountain pass. The sensor is quite accurate (to within a few millibars- a couple of inches of water), but only displays in whole feet or tenth of a meter increments as a depth gauge.
Offset for depth at altitude wouldn't matter for decompression, since you are (presumably) returning to the same surface pressure. But roughly, 1000 mbar = 10m / 33 feet of water, so of your surface pressure at your current altitude is 900 mbar you would not get to 1 atm until you were about 1m / 3' under water. A computer with a "gauge" sensor (one that measures relative to atmospheric pressure, like a tire pressure gauge) can register depth as accurately, but doesn't "know" what surface ambient pressure actually is. So the real issue is that the gauge sensor relies on user input to set the altitude compensation for the algorithm's decompression calculations, to provide a range for the presumed surface ambient pressure. Absolute sensors will measure this directly.
Ron
