• Welcome to ScubaBoard


  1. Welcome to ScubaBoard, the world's largest scuba diving community. Registration is not required to read the forums, but we encourage you to join. Joining has its benefits and enables you to participate in the discussions.

    Benefits of registering include

    • Ability to post and comment on topics and discussions.
    • A Free photo gallery to share your dive photos with the world.
    • You can make this box go away

    Joining is quick and easy. Login or Register now by clicking on the button

Is this true for dive computers?

Discussion in 'Computers, Gauges, Watches and Analyzers' started by Akimbo, Dec 28, 2017.

  1. Akimbo

    Akimbo Just a diver Staff Member ScubaBoard Supporter

    10,173
    8,251
    113
    OK, it sounds like the statement in the US Navy Diving Manual does not apply to dive computers but relates to the mental shortcuts that metric users take in their calculations. IE: 10M = 1 ATM = 1 Bar, which is pretty close but not exact. I see nothing wrong with that for recreational divers estimating gas consumption or even decompression off tables but I found hard to believe that dive computers would be programmed that way.

    Thanks all, I learned a bit. The Navy manual is not entirely incorrect but could be phrased more precisely given the almost universal prevalence of dive computers now. Example:

    10MSW is not defined by the Metric system as 1 Bar, even the EU for dive computers. Divers using Metric tend to use it that way for rough calculations, computers don't. I think I will send an E-mail to NEDU.

    It is interesting that Bars are no longer the proper (preferred?) Metric unit for pressure. I suspect it will be around a very long time for diving and weather since Pascals do convert evenly (by a factor of 10) to Bars but requires a lot more shifting of the decimal. The part that I always thought was strange is that one Bar is not an accurate conversion to the Standard Atmosphere, even though it was invented to measure atmospheric pressure -- IE the Millibar.
     
  2. tbzep

    tbzep Public Safety Diver

    182
    53
    28
    I immediately pictured a Daffy Duck cartoon with him crawling out of a wine tank hammered!
    unnamed.0.png
     
    peterak likes this.
  3. Akimbo

    Akimbo Just a diver Staff Member ScubaBoard Supporter

    10,173
    8,251
    113
    The story I heard was the wine industry stopped hiring commercial divers because they lost less inventory by letting their tanks leak then allowing divers anywhere near them. :facepalm:
     
    Bob DBF likes this.
  4. tursiops

    tursiops Marine Scientist and Master Instructor ScubaBoard Supporter

    # of Dives: 2,500 - 4,999
    Location: U.S. East Coast
    10,768
    8,549
    113
    No, 1020 kg/m^3 is quite atypical. It is an extreme surface value, corresponding to 35 g/l of salt (average ocean salinity) and 35 Deg C (very high surface temperature). Or, 30 g/l (very low salinty) and 23 deg C. Or, 28 g/l (even lower salinity) and 17 deg C (average ocean surface temperature). See Seawater Density Online Calculator.
     
  5. tursiops

    tursiops Marine Scientist and Master Instructor ScubaBoard Supporter

    # of Dives: 2,500 - 4,999
    Location: U.S. East Coast
    10,768
    8,549
    113
    That would be a horrible definition, because 1msw depends on what kind of sea water.
    Yeah, bars are only tolerated by the purists. Again, since a Standard Atmosphere is just another definition, it is awkward to have one definition (bar) depend on another (atmosphere). The whole SI system is designed to get one back to fundamentals, not to derivatives of fundamentals. So, we use Pascals, which are 1 N/m^2, = 1 kg/m-s^2, which are all fundamental units. With the bar defined as 100,000 Pascal, we can get to fundamental units...with a lot of messy zeros. Note that oceanographers refer to depth in decibars (almost equal to a meter), since it is instrumentally measured with a pressure sensor. I have to snicker a little bit at the imperial-unit haters who insist their SPG be in bars, but do not stay consistent by talking about their depth in decibars. They really ought to refer to a 30db dive, not a 30m dive.
     
    RonR likes this.
  6. DiveNav

    DiveNav ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 200 - 499
    Location: Southern California
    3,889
    489
    83
    I agree with Ron.
    We (DiveNav) do the same in our datalogger and our Smartphone Dive Computer
     
  7. Akimbo

    Akimbo Just a diver Staff Member ScubaBoard Supporter

    10,173
    8,251
    113
    "Measure it with a micrometer, mark it with a chalk, cut it with an axe."
    --- "Foundation design", by Donald P. Coduto

    I'm not so sure it is purists so much as those that work more from the engineering side of the process. A neighbor was a project manager for Boeing on some of their Mars landers projects. Pascals makes a lot more sense for his work.

    For other readers:
    • 1 Bar = 100 kilopascals
    • 1 Standard Atmosphere = 760 mm (29.92 inches) of mercury (like in a manometer), 14.70 PSI, 1,013.25 × 10 3 dynes per square centimetre, 1,013.25 millibars, or 101.325 kilopascals
    • 1 Pascal = 1 Newton/M²
    • 1 Newton = force required to accelerate 1 Kg 1 Meter/Second/Second or 100,000 Dynes
    Aren't all of you glad we cleared that up? :rolleyes:
     
  8. tursiops

    tursiops Marine Scientist and Master Instructor ScubaBoard Supporter

    # of Dives: 2,500 - 4,999
    Location: U.S. East Coast
    10,768
    8,549
    113
    LOL. I'm just glad I haven't had to use slugs since I was in mechanical engineering school.
     
  9. dmaziuk

    dmaziuk Regular of the Pub

    6,791
    2,982
    113
    There was a point back when, when USSR TV weather people started reading out conditions and forecasts in hecto-pascals. Every time was country-wide FTW moment.
     
  10. RonR

    RonR Dive Equipment Manufacturer

    # of Dives: I just don't log dives
    Location: Washington State
    1,271
    489
    83
    And if you want to add a layer of confusion, try putting an atmospheric pressure reading into your dive computer as a means of checking sensor accuracy. You then need to explain to users who are not at sea level that most local weather sites one uses for reference don’t publish the actual atmospheric pressure, but a “sea level corrected” pressure, i.e. what the actual pressure would be if they were at sea level. That is unless the weather station is catering to aviation, when actual “station pressure” is used.

    -Ron
     
    Akimbo likes this.

Share This Page