Is this true for dive computers?

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Akimbo

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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.

Is it true that dive computers are calibrated to this standard rather than an actual 10 Meters under seawater? Same question for computers set to Feet of Seawater.

I know that when precision pressure gauges for saturation diving systems were ordered in feet or meters of seawater based on the actual linear distance.
 
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I think the only answer you'll get is from @Shearwater since the other manufacturers are pretty tight lipped.
I believe the conversion is 1bar=10m, and then you get the three salinity settings. That said at .2ft/atmosphere difference between pressure depth and linear depth, I don't think it really matters unless you are trying to get dead accurate depths, and in that case you'd need something with more sig figs than a dive computer.

For deco from the computer, pressure is the only thing that matters anyway which is why the salinity settings don't actually matter on the computers. If you're running the computer as a bottom timer for tables or ratio deco then it may throw you into the next bracket, but it shouldn't matter that much
 
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Looks like anything following the EN13319 standard would use 1 bar = 1 msw.

Did you miss a zero? IE, 1 Bar = 10 MSW?.

EN13319 looks like it is above freshwater and below seawater:
  • Fresh Water = 1000kg/m³
  • EN13319 = 1020 kg/m³
  • Salt Water = 1030 kg/m³
For reference: One Standard Atmosphere Equals
  • 1.01325 1 Bar
  • 10.06275861 Meters of Sea Water
 
Guys, careful with your numbers!
It is 10m=1bar, more or less, not 1m=1bar. (Posts 1 and 3 both have this error.)

Keep your Perdix set on EN13319 and you are on a good compromise between fresh and salt water, with the added advantage of 10m=1bar, almost exactly.
If you really want to know how deep you are, you can reset the Perdix to fresh or salt, but the difference is only 1-2%, and your pressure gauge isn't that good, so you'd have to calibrate it first.
 
It is 10m=1bar, more or less, not 1m=1bar. (Posts 1 and 3 both have this error.)

Good catch, corrected: There was a copy and paste error from the USN Manual in .pdf format. For some weird reason it dropped a few characters when copied into Scubaboard's reply box but not when pasted into MS Word first. That's a little disconcerting.
 
The equations all work from pressure, so for computer's primary function it's irrelevant as @tbone1004 says. Some of them have it user-settable, some of them have it in the manual in small print, and some just don't mention it. Mine for instance has in "technical specifications": Depth sensor: sea water setting( fresh water depths are about 3% lower).
 
I can only speak directly for us (Atomic), but I'm quite sure that every dive computer is going to handle decompression calculations based only on actual pressure, not depth. Converting pressure to depth is always going to be an approximation because the density of water will vary somewhat with salinity and to a lesser extent with temperature. Even different sections of the ocean can vary in salinity quite a bit. Or maybe you are diving in some other fluid (liquid mercury? liquid helium?)- your computer should still work, in theory if not physical reality, though depths would not be displayed accurately. Back in the day we had some commercial diving done inside leaking wine tanks at around 14% alcohol...:)
The actual depth is irrelevant to decompression issues, those are determined only by pressure.

-Ron
 
One of the requirements for getting an EN13319 approval on a dive computer, is the depth reading testing must comply with the EN13319 standards for a depth pressure. This is designed on 1020 kg/m³, which is typical of salt water in the temperate zones.

Some tech dive computers let you set the depth reading more accurately for the water type: fresh and various levels of salt concentrations. That makes the dive instrument more honest as a depth measurement meter.

The deco calculations in a good tech computer, internally it uses sensor pressures and absolute pressure dimensions, and the deco algorithm is also in absolute pressure. The limits are derived from that information. The dive computer internally knows what the diver wants for deco steps ..for example every 307mb. The ascent limits are made accordingly.

The deco calculations are indifferent to what dimension the diver wishes to use, be it FSW, EN13319, or fathoms.

.
 

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