H2Andy:
yeah, i know... but my sentence goes on to explain why it makes a difference
i'll try again:
if you are diving in 33 feet of
fresh water using the standard model (where 1 atm = 33 feet of SALT water) then you are actually diving one foot "shallower" than your profile will indicate.
not sure if this is worth much, but i guess every bit counts
Right. But no matter which conversion you use, you haven't changed position in the water column, so the pressure on you is exactly the same even if the linear distance from the surface may be a foot different than you expect. The key to being as accurate as possible, I guess, is to make sure that whatever you're doing matches whatever the developer of the decompression model used.
How about a thought experiment....
Let's suppose you're using a gauge that has been calibrated as 1 atm = 33 fsw. You take a marked rope straight down from the surface with you in a
freshwater environment & stop at the 170ft marking. I believe that if you looked at your gauge at that point, you would see 165ft (6ata), since your gauge was calibrated for saltwater.
Now, your gauge can't obviously be at both 165ft & 170ft at the same time, but no one can argue that the pressure on you isn't 6ata. So the question then becomes, in your dive planning, did you consider this to be a 165ft or a 170ft dive? The answer, I believe, would depend on the model that you're using & whether the modeler associated the physiological effects of diving at 6ata with a depth of 165 or 170ft. I would assume that most models are based on depths in seawater, so the gauge reading of 165 ft is the depth that should be used.
Next, say you repeat the above but have the ability to switch your gauge such that 1atm=34ft. You take your 170ft rope down with you again & this time, when you reach the same pressure of 6ata, your gauge reads the appropriate linear depth of 170ft. So, if you go back to the same model you used on the first dive, you're now calling it a 170ft dive, and would likely have planned accordingly....
See the contradiction? The pressure on the diver both times was exactly the same, but the gauge in the two scenarios read out two different depths. So, it becomes important to know how the model was developed. I would argue that if a model is developed with depths in seawater associated with the pressures that exert the physiological effects, there is no need to change the gauge's conversion to anything but 1atm=33ft....unless the computer, on switching to 1atm=34ft, brings up a whole new "freshwater algorithm."
Jim
(I'll copy this over to the Deco forum with a link to this thread for those interested....I think it's an interesting topic. I may be completely wrong, but it'll be fun getting sorted out.
)
