@Linus, Thank you for taking the time to educate me instead of leaving me in ignorance, I honestly do appreciate it. While i figured things wouldn't be exact, I thought it would be as near as no matter and i would never have thought that the margin of error could get quite so large.
@Willem, I realize they don't happen exactly as intended, i was just thinking given the intention of the diver we could replicate that intention based on the dive time & overall SAC rate for the purpose of visualization. On Tue, May 9, 2017 at 12:35 AM, Willem Ferguson < [email protected]> wrote: > The schedule for changing sidemount cylinders depends on your training. > Many would agree that keeping them within 30 bar is a good policy to > maintain redundancy of gas. Cylinder changes are almost never at exact > multiples of pressure, so one needs to enter the cylinder changes by hand. > If you do not have air integration, that means writing down the *times* of > cylinder changes on a slate or similar. A computer program cannot > automatically recreate the schedule of changes. > > On 09 May 2017 02:30, "Linus Torvalds" <[email protected]> > wrote: > >> On Mon, May 8, 2017 at 3:23 PM, Ryan McLean <[email protected]> wrote: >> > Attached log. >> > >> > Would a semi-correct model be to allow a the user to specify the 1st >> switch >> > in bar so to take 35bar as an example on 2 cylinder charged to 200 >> > 1st switch is at -35bar (165bar remaining) >> > 2nd cylinder then at 2x35bar (-70bar) from full (130bar remaining) >> > 1st cylinder then at -70bar from last use (95bar remaining) >> > ... >> > continue until end of dive. >> >> I suspect we should aim to have a model where we can just mark both >> cylinders as used at the same time (this is not just a sidemount thing >> where people alternate regulators - it's also true of oxygen vs >> diluent for example). >> >> But right now that's now how we track gas usage, and it would probably >> be pretty painful to implement. >> >> Right now you are realistically forced to either consider it a single >> dual cylinder manifold, or you'll have to manually add a fake gas >> switch event somewhere in the middle of the dive or so. >> >> > I've probably not understood gas compressibility correctly but I took a >> 10L >> > cylinder to hold 2000L at 200bar. So if I had used 80 bar in each >> cylinder >> > then I had used 1600L total... >> >> Yeah, no. >> >> It's an approximation, but it's really not a particularly good one. >> >> First off, 200 bar is not 200 atm, although it's close. It's about 197.4 >> atm. >> >> So there's a 1.3% error there. >> >> But more noticeably, the compressibility factor of air at 200 bar is >> 1.036 - it's not an ideal gas. So that's an additional error of the >> magnitude 3.6%, and it's in the same direction as the bar-vs-atm one >> (ie both errors are in the direction of less real air at STP). >> >> So 10L at 200 bar is actually only 10*197.4/1.036 L of air at STP. So >> about 1906 L. >> >> And 120 bar is 118.4 atm, and at that point air still acts pretty much >> like an ideal gas, so a compressibility factor of 1.0. >> >> So 10L at 120 bar is 10*118.4 L at STP, so 1185 L. >> >> So the amount of gas you used (in one cylinder) is 1906-1185=721L. >> >> Note that the small percentage errors all ended up being bigger due to >> (a) being in the same direction, and (b) mainly being noticeable at >> the higher pressure, so even though we're talking about low >> single-digit percentages for the compressibility factor (and even less >> for the bar-vs-atm factor), you ended up with about a 10% error >> between the rough approximation (800 L used per cylinder) and the more >> accurate one (721 L used per cylinder). >> >> Linus >> _______________________________________________ >> subsurface mailing list >> [email protected] >> http://lists.subsurface-divelog.org/cgi-bin/mailman/listinfo/subsurface >> >
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