Talk:Metre sea water

Revision of values for conversion
US Navy Diving Manual gives values which do not agree with some of the existing conversion factors. This is mainly because of the definition of equivalence between 10 msw and 1 bar. I am assuming that the USN know what they are talking about, and will be adjusting conversion factors accordingly. If anyone has a more authoritative and reliable source that contradicts the USNDM, I am open to discussion, but it agrees with my experience in the industry. (The USNDM does not give a reference for their claim that 10 msw = 1.00 bar in the metric system.) &bull; &bull; &bull; Peter (Southwood) (talk): 13:21, 15 March 2017 (UTC)

I have not used the old references, and list the links here in case they are still useful:
 * http://www.scuba-doc.com/physics.htm
 * http://www.divetable.de/workshop/V1_e.htm

Changes have been made to fit in with the USNDM definitions and list of conversions. I would appreciate someone checking in case I have made any errors. &bull; &bull; &bull; Peter (Southwood) (talk): 14:12, 15 March 2017 (UTC)
 * I don't have any sources but I will note that the "Hyperbaric chamber pressure gauge calibrated in bar and msw" indicates that 60 msw is just slightly more than 6 bar. Kendall-K1 (talk) 23:36, 21 March 2017 (UTC)
 * That does seem to be the case. Gauges can be expected to be accurately marked. So there is some discrepancy between this gauge and the US Navy statement. &bull; &bull; &bull; Peter (Southwood) (talk): 14:14, 22 March 2017 (UTC)
 * Part of the problem is that salinity of sea water varies by a significant amount, which then alters the density and hence the pressure–depth relationship. The other definition of msw that I find online is 1 msw = 0.100693064 bar, which is only 0.7% different from the USN definition. One possibility is that the USN's definition is just a convenient approximation, bearing in mind that the variations in seawater salinity are likely to be of the same order as the approximation. In the real world, there won't be many applications that require better levels of accuracy, so there's unlikely to be much of a problem with slightly fuzzy pressure units. --RexxS (talk) 13:48, 23 March 2017 (UTC)
 * As msw is a unit of pressure, it makes sense to disconnect it from variable salinity and temperature which would also affect density. I am fairly confident that the USN definition is convenient, but is it an approximation, as they claim it is defined as 10msw = 1 bar, which is not the way I would describe an approximation, but is actually a very logical and practical way to go considering the use of the unit. Where is the online definition you mention? Cheers, &bull; &bull; &bull; Peter (Southwood) (talk): 20:44, 23 March 2017 (UTC)
 * The point is that if you choose the obvious definition "the pressure exerted by a column of 1 metre of seawater" you have to first define "seawater" in terms of its salinity (and also state the temperature). The previous version of this article used such a definition, but relied for the actual conversion factor on Scuba Doc or divetable.de, both of which give 1 atmosphere (i.e. 1.01325 bar) as 10.08 meters sea water (implying 1 msw = 0.10052 bar). For what it's worth, I found TranslatorsCafe using 1 msw = 0.100693064 bar. None of these have the same reputation for reliability that USN has, but I find it hard to believe that if 1 msw was actually defined as exactly 0.1 bar, there would be multiple sites quoting slightly different conversion factors. I'm sure the same doubt must cross your mind. My guess is that the unit never had an exact definition because of the variability of what "seawater" means, and the USN decided to cut the Gordian Knot by deciding that for their purposes 1 msw shall be 0.1 bar. And why shouldn't they? Cheers --RexxS (talk) 00:35, 24 March 2017 (UTC)
 * That exactly describes my suspicions, There may be as many "definitions" as websites providing conversion factors, based on what the writer found specified as a density for seawater. I suspect that several have not bothered to research the matter in detail. It is logical and plausible, but so far unreferenced, that the USN, being the organisation that probably does the most actual research, would make an arbitrary definition to suit themselves, so any conversion of their tables to msw is precisely defined. Revision 5 included msw tables, since then they have not bothered. I would like to get this settled if possible, but am having difficulty finding out who to contact at the USN to ask about it. I will persevere as I think it is worth getting a definitive answer. I will try to find out whether rev 5 also uses this definition, as it may go back quite a long time. From a functional point of view we can reasonably assume that any USN tables converted to msw will use 10msw=1bar. It would be interesting to know what other algorithms use, as that is really where it matters from a theoretical point of view. From a practical point of view they are all much the same and statistically unlikely to have any effect on diver safety. The deeper diving is mostly commercial, and IMCA seem to agree with the USN definition. Cheers, &bull; &bull; &bull; Peter (Southwood) (talk): 06:29, 24 March 2017 (UTC)
 * Update: US Navy Diving Manual R5 of 15 Aug 2005 provides identical information to R6 and R7. No surprises there. I have not been able to access earlier revisions. I have not yet received useful feedback from Translators Cafe. Scuba-Doc cannot remember where the information came from, and divetable.de says he is upgrading the site, but has not mentioned where the data came from. &bull; &bull; &bull; Peter (Southwood) (talk): 08:05, 29 March 2017 (UTC)
 * Update: Translators Cafe do not know where they got it from, and as a response to my query are changing to the USNDM definition. Now getting more important to get the provenance of the US definition, as it may be all that is left soon. &bull; &bull; &bull; Peter (Southwood) (talk): 17:05, 29 March 2017 (UTC)