Is this true for dive computers?

Please register or login

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. Log in or Register now!

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.

I'm convinced that at least half of the things the Soviet Union did in the 80's were solely to screw with the West. Not out of any real desire to cause issues, just more so to make people go, "huh? Effing weirdos....."

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

I use station pressure when shooting, Trying to explain it to people at the range gets me lots of confused looks. It's not difficult to understand, but if you've never been exposed to the differences and their uses, it's like trying to get a house cat to fly a kite.
 
"Measure it with a micrometer, mark it with a chalk, cut it with an axe."
--- "Foundation design", by Donald P. Coduto

I love the (probably apocryphal) story about the architect going over blueprints with a contractor, blueprints in which he had specified the lengths of some steel beams to three decimal places. The contractor, without missing a beat, asked “At what temperature?”
 
I love the (probably apocryphal) story about the architect going over blueprints with a contractor, blueprints in which he had specified the lengths of some steel beams to three decimal places

I was in an AutoCAD user group and this was a common problem since three decimal places was the default dimension style. A few new members were architects and used AutoCAD for years before joining. They never missed a meeting after discovering the program defaults could be changed to Feet, Inches, and fractional Inches. Some of them even drew in scaled dimensions like on a drafting board instead of in real world dimensions and telling AutoCAD to scale the plot.

This was back in the days of MS-DOS and 640KB of RAM.
 
I love the (probably apocryphal) story about the architect going over blueprints with a contractor, blueprints in which he had specified the lengths of some steel beams to three decimal places. The contractor, without missing a beat, asked “At what temperature?”

My dad's one is when builders ignored the "this side up" writ on the blueprints and welded a fairly big i-beam in place upside-down. The beam wasn't perfectly straight, by design, so one of the ends ended up an inch away from its weld point. He was called in on an urgent "can this be salvaged" business trip. He remembered his granddad telling the story of burning bonfires on metal beams to get thermal expansion to pop them in place, and it worked.

I'm convinced that at least half of the things the Soviet Union did in the 80's were solely to screw with the West. Not out of any real desire to cause issues, just more so to make people go, "huh? Effing weirdos....."

Think they did the above to mess with the West?
 
I was reading Wikipedia's article on Pressure and came across this (bold is mine):

Underwater divers use the metre sea water (msw or MSW) and foot sea water (fsw or FSW) units of pressure, and these are the standard units for pressure gauges used to measure pressure exposure in diving chambers and personal decompression computers. A msw is defined as 0.1 bar (= 100000 Pa = 10000 Pa), is not the same as a linear metre of depth. 33.066 fsw = 1 atm[4] (1 atm = 101325 Pa / 33.066 = 3064.326 Pa). Note that the pressure conversion from msw to fsw is different from the length conversion: 10 msw = 32.6336 fsw, while 10 m = 32.8083 ft.[5]

[5] is from the US Navy Diving Manual:
3. In the metric system, 10 MSW is defined as 1 BAR. Note that pressure conversion from MSW to FSW is different than length conversion; i.e., 10 MSW = 32.6336 FSW and 10 M = 32.8083 feet.
. . .
. . .
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:
Definitions:
1 msw is defined as 1/10 bar;
1 fsw is defined as 1/33 atm (US Navy uses 1/33.066 atm).

SI Conversions:
SI Unit of Pressure is the Pascal (Pa) = Newtons per square meter.
1 bar is 100 kPa;
1 atm is 1.01325E+5 Pa;
Therefore 1 atm is 1.01325 bar.

Hence conversion formula from msw to fsw as a derived & consistent unit of Pressure:

1 msw = 0.1 bar X (1 atm / 1.01325 bar) X (33 fsw / 1 atm ) = 3.25685 fsw.

[Using 33.066 fsw / 1 atm instead yields the US Navy value of 1 msw = 3.26336 fsw].
 
Last edited:
. . .
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.

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.
The millibar is still often used in weather reports and forecasts for the public, but the term hectopascal is increasingly being used. Hectopascals and Millibars are one-to-one corresponding pressure units; they're essentially the same thing: 1 hPa = 100 Pascals = 1 mbar.

1000 hPa are equal to 1000 mbar, which is equal to 750 mm of mercury in a barometric column, which is 0.987 of the average atmospheric pressure, which on global average is 1013 millibars or 1013 hectopascals.

1 atm = 1.01325 bar = 1013.25 mbar = 1013.25 hPa.
 
Last edited:
. . .
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). . .
For base reference and derived units of cylinder pressure, what's easier and more intuitive (and less "awkward") to work with?

Multiple factors of 14.7 psi which itself is equivalent to 1 atm (1 ATA) ?
-->Or multiples of 1 atm (1 ATA) which itself is approx 1 bar of pressure?

Also, it's easier to do the equivalent arithmetic with two significant figures in bar pressure units versus three sig figs in psi: Again, what's faster to calculate and easier to SEE and THINK in your head -->

200 minus 80 equals 120 bar ?
-or-
3000 minus 1160 equals 1840 psi ???

The motivation to use Bar Metric in Scuba is to work smarter -not harder. The extra digit in arithmetic with PSI units actually makes it harder and less intuitive to work with, and the reason why divers who use US Imperial may need the extra expense of a computer with Air Integration, Air Time Remaining & other digital logging features, versus a simple analog Bar SPG & BT/Computer with understanding practical manual arithmetic using the Metric System.

Also, it's easier to conceptualize the rated volume and service pressure of a scuba cylinder based on a surface atmosphere reference of approx 1 bar: The common AL80 tank has a metric cylinder rating factor of 11 liters/bar (look it up online if unfamiliar, for any particular tank), or in other words, at the surface of 1 bar, if you pour water into the cylinder, the measured volume it can contain is 11 liters. (It's easier to work with Metric Cylinder Ratings like 11L/bar, rather than cf/psi like 0.025 cf/psi for the AL80 tank)

However when pressurized to any value up to its recommended Service Rating (207 bar for the AL80 tank/11L per bar cylinder in this example ), a cylinder carries an equivalent volume of free gas much greater than its water capacity, because the gas is compressed to several hundred times atmospheric pressure (while water is incompressible). So if you have a gas pressure reading of 200 bar in your AL80 tank, you have a total available free gas volume of 200 bar multiplied-by 11 liters/bar or 2200 liters.

. . . 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.
Naw, it would be less pretentious -but more importantly less confusing- by keeping the convention of relating units of distance to the surface (i.e. depth) as meters; units of cylinder pressure as bar; and units of total ambient pressure at a particular depth as ATA.
 
Last edited:
For base reference and derived units of cylinder pressure, what's easier and more intuitive (and less "awkward") to work with?

Multiple factors of 14.7 psi which itself is equivalent to 1 atm (1 ATA) ?
-->Or multiples of 1 atm (1 ATA) which itself is approx 1 bar of pressure?

Also, it's easier to do the equivalent arithmetic with two significant figures in bar pressure units versus three sig figs in psi: Again, what's faster to calculate and easier to SEE and THINK in your head -->

200 minus 80 equals 120 bar ?
-or-
3000 minus 1160 equals 1840 psi ???

The motivation to use Bar Metric in Scuba is to work smarter -not harder. The extra digit in arithmetic with PSI units actually makes it harder and less intuitive to work with, and the reason why divers who use US Imperial may need the extra expense of a computer with Air Integration, Air Time Remaining & other digital logging features, versus a simple analog Bar SPG & BT/Computer with understanding practical manual arithmetic using the Metric System.

Also, it's easier to conceptualize the rated volume and service pressure of a scuba cylinder based on a surface atmosphere reference of approx 1 bar: The common AL80 tank has a metric cylinder rating factor of 11 liters/bar (look it up online if unfamiliar, for any particular tank), or in other words, at the surface of 1 bar, if you pour water into the cylinder, the measured volume it can contain is 11 liters. (It's easier to work with Metric Cylinder Ratings like 11L/bar, rather than cf/psi like 0.025 cf/psi for the AL80 tank)

However when pressurized to any value up to its recommended Service Rating (207 bar for the AL80 tank/11L per bar cylinder in this example ), a cylinder carries an equivalent volume of free gas much greater than its water capacity, because the gas is compressed to several hundred times atmospheric pressure (while water is incompressible). So if you have a gas pressure reading of 200 bar in your AL80 tank, you have a total available free gas volume of 200 bar multiplied-by 11 liters/bar or 2200 liters.

Naw, it would be less pretentious -but more importantly less confusing- by keeping the convention of relating units of distance to the surface (i.e. depth) as meters; units of cylinder pressure as bar; and units of total ambient pressure at a particular depth as ATA.
Yes, I would prefer bars, ATA, and decibars. Depth is the awkward one.
 
Definitions:
1 msw is defined as 1/10 bar;...

There's the fallacy, or poorly phrased term. They are approximations for the convenience of mental calculations, not definitions. The General Conference on Weights and Measures (CGPM), as reflected in the Système international d'unités (SI), defines them, regardless of what you are measuring. The USN's rough approximations are fairly close but not exact. I know the US Navy Diving Manual uses the word "defined" but it is not technically or universally correct. I am in the process of contacting the editor of the manual to point out the imprecise phrasing.

1 Inch = 25.4mm (exactly)
1 Foot = 304.8 or 12 x 25.4 (exactly)
1 Meter = 3.2808399 Feet (rounded)
1 International Standard Atmosphere = 1.01325 Bar or 14.69594878 PSI (at sea level and 15°C)

Seawater density varies but the US Navy has traditionally used 0.445 PSI/Foot of Sea Water when specifying the conversion factor for precision pressure gauges scales used on all of their chambers and pneumpfathometers. In practice, that is the baseline pressure that virtually all of the US Navy's decompression testing data is based on.

Edit: 14.7 PSI (1 ATM rounded) divided by 33 FSW = 0.445454 (rounded) PSI/FSW
 
Last edited:
Definitions:
1 msw is defined as 1/10 bar. . .
There's the fallacy, or poorly phrased term. They are approximations for the convenience of mental calculations, not definitions. The General Conference on Weights and Measures (CGPM), as reflected in the Système international d'unités (SI), defines them, regardless of what you are measuring. The USN's rough approximations are fairly close but not exact. I know the US Navy Diving Manual uses the word "defined" but it is not technically or universally correct. I am in the process of contacting the editor of the manual to point out the imprecise phrasing.

1 Inch = 25.4mm (exactly)
1 Foot = 304.8 or 12 x 25.4 (exactly)
1 Meter = 3.2808399 Feet (rounded)
1 International Standard Atmosphere = 1.01325 Bar or 14.69594878 PSI (at sea level and 15°C)

Seawater density varies but the US Navy has traditionally used 0.445 PSI/Foot of Sea Water when specifying the conversion factor for precision pressure gauges scales used on all of their chambers and pneumpfathometers. In practice, that is the baseline pressure that virtually all of the US Navy's decompression testing data is based on.

Edit: 14.7 PSI (1 ATM rounded) divided by 33 FSW = 0.445454 (rounded) PSI/FSW

1 atm ÷ 10 msw = 0.1 atm/msw;
1 bar (rounded) ÷ 10 msw = 0.1 (rounded) bar/msw.

In Europe/Asia, sea level by convention is 1.0 bar; Standard Atmospheric Pressure in US is defined as 760 mmHg or 14.69595 psi -which is 101.325 kPa in SI Units and derived equivalent to 1.01325 bar, a difference of 13.25 mbar (1.3%).

Are you really going to obsess over semantics based on a trivial 1.3% difference that is mostly outweighed by the daily fluctuations of barometric pressure (i.e. weather)???

If so. . . Knock yourself out @Akimbo :
When a federal agency determines that there is a need for a revision of this standard, a written request for revision should be submitted to the General Services Administration, Federal Supply Service, Environmental and Engineering Policy Division (FCRE), Washington, DC 20406. The request shall include data that support the proposed change. The Metric Program, National Institute of Standards and Technology, as custodian of this standard, will coordinate all proposed changes with the Metrication Operating Committee.
National Institute of Standards and Technology | NIST
 
Last edited:

Back
Top Bottom