Awesome explanation and great diving computer knowledge, thanks! I asked as I have interest in physiology and I do my best to know how things work under the water and inside body.
Your reasons are more than good for the average user. Il giorno dom 15 apr 2018 alle 20:55 Linus Torvalds < [email protected]> ha scritto: > On Sun, Apr 15, 2018 at 12:07 AM, tormento <[email protected]> wrote: > > > > Henry's Law is based on (partial) pressures and not depths. I dunno how > > diving computer talks to subsurface. If giving depths instead of > pressures > > it would be a pity. > > Almost all dive computers only give depth. > > Some give salinity, some don't. And some only give the flag (sweet vs > salt) rather than the salinity value they actually use to calculate > depth. > > In other words: do not EVER play games with salinity. You don't know > what it was, and you will get it wrong. > > The whole notion of "user should set salinity" is broken garbage. A > dive computer shouldn't even have that setting. Because that setting > cannot possibly matter, because the only thing that matters - and the > only thing the dive computer measures - is actual pressure, and all > you can do with salinity is make a "correction" to the depth > calculation that you can get wrong. > > So in my not-so-humble opinion, a dive computer should always just > show depth as "salt water equivalent depth", with no way to screw up, > and no complications. If you dive in a lake, you'll get the depth > wrong by a few percent, but nobody cares since it's not a really > meaningful value anyway, it's just a user-friendly approximation for > the value that matters: pressure. > > Giving the users just the absolute ambient water pressure migth be > *technically* the right thing to do, but it's such a user-hostile > datapoint that it's completely wrongheaded. It only moves the > possibility for error into another place (ie the UI during diving, or > the UI during later logging). > > So what you should do is: > > - set your dive computer to salt water (if it has a setting), and > forget about it. Don't ever touch the setting, all it can do is cause > confusion. > > - think of "depth" as "equivalent depth in salt water" and be happy > > - mark your lake dives as such in the dive log tags if you care, the > same way you mark boat dives and buddy names. > > The sweet-vs-salt water thing has absolutely zero meaning outside of > informing yourself, and has exactly the same relevance as the name of > your dive buddy: nice to know, but not relevant for any other meaning. > Don't give it any relevance that will only confuse you and get things > wrong. > > Because anything else is just a disaster waiting to happen. You *will* > get it wrong, and you will only confuse yourself. Trying to correct > things later is just going to make things worse. > > Linus > > PS. Yes, there are dive computers that don't report depth at all, and > report the actual hydrostatic pressure that their sensor gives. Before > you say "that's what everybody should do", let me just say that (a) > they are a tiny minority and (b) even they aren't consistent (ie is it > absolute pressure, or relative to surface pressure?) > > So subsurface logs what the vast majority of dive computers give you: > depth. In fact, we don't even see the pressure, because the conversion > will have been done by libdivecomputer. So as far as we're concerned, > no dive computer gives us pressure, but technically you can get the > water pressure from the Atomic Aquatics Cobatl, from the > Heinrichs-Weicamp OSTC and from the Reefnet Sensus Ultra. (That last > one isn't actually a dive computer, it's just a data logging device. >
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