Switching to all metric for academics

[sea_ledford]

I'm seriously considering switching all my academics for classes to metric, especially for my scientific diver class. I've come across an issue I can't figure out:

In imperial, I'll designate pressure/density differences between fresh water and salt water (ie 33 feet/atm for salt and 34 feet/atm for fresh), but I'm not finding any consistent references to that in metric, it's just 10m/atm. I thought that all of the easy conversions were based on fresh water (1L of water weighs 1 kg, 1g/1cucm, etc), but I'm definitely seeing 10m of seawater is one atm. Are the differences just ignored?

On the SDI/TDI gas laws equation worksheet it has 10m/atm seawater and 10.3/atm freshwater, but if the standard is based on fresh water shouldn't it be 9.7m/atm seawater and 10m/atm freshwater?

I'm so confused...

I'm asking about this for dive physics calculations, not real world application.

 

[Nick_Radov]

The answer depends on the context. Are you concerned about calculating precise depth, like for surveys, or pressure, like for decompression? Salt water density varies based on salinity (among other factors) so the numbers will be slightly different if you're diving in, let's say, the Baltic Sea versus Red Sea.

 

[tursiops]

if the standard is based on fresh water

I believe it is based on "salt" water, "defined" as 10m/atm, even though it varies by location and temperature.

 

[lizardland]

I imagine it's probably a convenience thing. I vaguely remember from the Encyclopaedia of Diving (metric version) when I was doing my DM course in the mid 90's that the difference between salt and fresh was discussed and an exact figure given.

Is "Scientific Diver" a course aimed at teaching actual academics/researchers how to do research diving? I'm surprised SI units aren't in common use on it as that's what most modern published research uses and most publishers ask for. If it's more of a taster for the public of what scientific diving involves then I can understand why it wouldn't be. When I did my postgrad, using anything but SI units for any of the underwater work we were doing would have been cause for crucifixion.

 

[JMarc]

I thought that all of the easy conversions were based on fresh water (1L of water weighs 1 kg, 1g/1cucm, etc), but I'm definitely seeing 10m of seawater is one atm.

Original (1795) definition of kg was the mass of 1L of pure water at 0 °C, then changed to 4 °C (temperature of maximum density), then (1799) replaced by the mass of a standard, then (2019) replaced by fixing a few physical constant instead of trying to measure them.

So yes, it is for fresh water.

Are the differences just ignored?

Most of the time in my practice, yes as the 3% or so difference doesn't really matter. When it matter, I'm using 1.03 kg/l (I don't remember a time when it wasn't about buoyancy, so I don't think I've ever used 9.7 m/atm and I wonder if it mattered how precise should be the other relevant factors, for instance 1 atm is 101 325 Pa so that shouldn't be 9.7 m/atm, the measured density of the involved water is probably not 1.03 kg/l, and so on).

 

[Duke Dive Medicine]

I'm seriously considering switching all my academics for classes to metric, especially for my scientific diver class. I've come across an issue I can't figure out:

In imperial, I'll designate pressure/density differences between fresh water and salt water (ie 33 feet/atm for salt and 34 feet/atm for fresh), but I'm not finding any consistent references to that in metric, it's just 10m/atm. I thought that all of the easy conversions were based on fresh water (1L of water weighs 1 kg, 1g/1cucm, etc), but I'm definitely seeing 10m of seawater is one atm. Are the differences just ignored?

On the SDI/TDI gas laws equation worksheet it has 10m/atm seawater and 10.3/atm freshwater, but if the standard is based on fresh water shouldn't it be 9.7m/atm seawater and 10m/atm freshwater?

I'm so confused...

I'm asking about this for dive physics calculations, not real world application.

What standard are you referring to? 33 feet converts to 10.06 meters and 34 feet converts to 10.36 meters.

I used to teach the pressure/depth calculation (in English units) as based on the cube weight of water. 1 cubic foot of salt water weighs 64 lbs. Divide that by 144 square inches (area of the bottom of the 1 ft3 container) and you get .445 lbs per square inch per foot of sea water. Divide 14.7 psi/atmosphere by .445 psi/foot and you get 33 feet/atmosphere. Do the same with fresh, 62.4 lbs per cubic foot = .432 lbs per square inch, divided by 14.7 = 34 feet/atmosphere. Kind of a roundabout way of deriving it but it helps students visualize that the pressure is literally from the weight of the water.

Best regards,
DDM

 

[Dominik_E]

The unit [atm] was not based on a column of water, but rather on the pressure of the air column at sea level. This is roughly 1.013bar, equivalent to a salt water column of about 10m, or a fresh water volumn of about 10.3m. The TDI / SDI material has it correct.

 

[MarkA]

EN13319 = 1020kg/m3. Shearwater has a setting for it on their computers.

On earth a mass of 1kg exerts a force of 9.81N.

In practice for most people and probably close enough. 1 bar = approx surface pressure. 1kg/cm2 = approx 1 bar = approx 10m.

 

[elmo]

So combining @Dominik_E and @MarkA 's posts (note gravity varies across the earth - I use g = 9.82 m/s2 where I live), and just working with metric units.
1 atmosphere = 1.013 bar = 101.3 kPa
1 kPa = 1 kN/m2
Density of freshwater = 1000 kg/m3 = 1.00 t/m3
Density of saltwater = 1020 kg/m3 = 1.02 t/m3 (EN13319 but it varies across waterbodies)
Pressure due to 1m depth of freshwater = 1.00 t/m3 * 9.81 = 9.81 kPa/m
Pressure due to 1m depth of saltwater = 1.02 t/m3 * 9.81 = 10.006 kPa/m
Depth resulting in 1 atm (= 1.013 bar = 101.3 kPa) increase in pressure
In freshwater
101.3 kPa = depth * 9.81 kPa/m
depth = 101.3 / 9.81 = 10.33 m (rounded to 10.3 m, which is what SDI/TDI is telling you)

In saltwater
101.3 kPa = depth * 10.006 kPa/m
depth = 101.3 / 10.006 = 10.12 m

If instead you take density of saltwater as 1.03 t/m3 then
Pressure due to 1m depth of saltwater = 1.03 t/m3 * 9.81 = 10.104 kPa/m
depth of 1 atm = 101.3 / 10.104 = 10.03 m (rounded to 10.0 m, which is what SDI/TDI is telling you)

 

[inquis]

Starting with 1 atm = 14.6959 psi or lb/in^2, this equates to 10.354 meters fresh water (from 14.6959 * 1000 / 2.2 / 2.54^2 / 100). Divide by 1.024 (average saltwater density) for 10.11 msw. Divide by .3048 for ft of either.