On Sun, Apr 15, 2018 at 12:07 AM, tormento <turm...@gmail.com> 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. _______________________________________________ subsurface mailing list firstname.lastname@example.org http://lists.subsurface-divelog.org/cgi-bin/mailman/listinfo/subsurface