OP
Switching to all metric for academics
- Thread starter sea_ledford
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Herein lies the problem many are having: not only is "salt water" an undefined term, 1 kg/cm**2 is not quite 1 bar (it is 0.98 bar), nor is it quite 1 atm (it is 0.97 atm). So 10m of fresh water is neither 1 bar nor 1 atm.
However, 10.33m of fresh water IS 1 atm, and that is about the same weight as 10m of "normal" saltwater. So, it just kindo works out, that you can either ignore all the little 2 and 3% factors, or you can say that 10m of salt water is 1 atm.
It's not almost. A column of SI standard water (that's sort of a joke, but not really) 1cm x 1cm x 10m high exerts exactly 1 bar of force.
The "almost" comes into play if you are talking about atmospheres rather than bar.
Divers tend to use them interchangeably, but they are not the same thing. An atmosphere is not a pure SI unit that can be derived from the meter. Instead, it's a shorthand created for convenience and is "intended to represent the average atmospheric pressure at the average sea level at the latitude of Paris, France". The fact that 1 bar is so close to 1 atmosphere is a pure coincidence.
But again it doesn't matter one bit when it comes to using these measures in diving. In the real world, the precision of any measurement is limited by the accuracy of the tools you use to measure it. Our pressure gauges, both the SPG and in our computers, are only accurate to within a few percent, so the 1.3% difference between bar and atm is immaterial.
Even if you had a perfect gauge, your accuracy would still be limited because our bodies extend into three dimensions and so are always exposed to a range of pressures that cannot be distilled down to a single number.
I'd keep telling them it is pressure we care about, and that referring to it as "depth", as we commonly do, is where the "almost" comes from. I think really the only time "depth" comes up in any of the deco stuff is in Workman's M-value formula, and my impression is, it's because he used pressure relative to sea level and "pressure-relative-to-sea-level" is a mouthful. Everything else: Schreiner's formula, Buhlmann's formula, etc., is all P.
Again, the only time as a diver when I care about the difference between salt and fresh water is for buoyancy. And when rinsing my stuff.
For the transformation between a pressure and a height of column of water, I don't. If you do, you probably have to consider the relevance of the temperature, the variation of g around the globe, the variation of salinity between seas, the changes in atmospheric pressure, the accuracy of your instruments, the nuances between accuracy, precision and resolution. The only interest of doing a careful computation once is to show that it isn't relevant.
For the transformation between a pressure and a height of column of water, I don't. If you do, you probably have to consider the relevance of the temperature, the variation of g around the globe, the variation of salinity between seas, the changes in atmospheric pressure, the accuracy of your instruments, the nuances between accuracy, precision and resolution. The only interest of doing a careful computation once is to show that it isn't relevant.
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