Switching to all metric for academics

[Dominik_E]

First, let me say that what we are discussing at the moment, percent-level inequalities, does not even depend on metric units. Those issues also are there when using imperial units. Just often ignored. Which is OK. It is actually not even a given that pressure sensors measure to percent precision.

This being said: using bar instead of atm also does not eliminate the issue. The bar is fixed to be 10^5 Pascals. And the Pascal is a force (measured in Newtons) per area. If one liter of freshwater weighs 1kg, a 10m column exerts 0.98bar. Not 1bar.
Having the pressure double after the initial 10m is convenient, this is the attraction of the unit atm. But it will never be perfect any way.

In practice? Let the dive start at 1bar and add 1bar per 10m. Happy dive, happy life.

 

[lowwall]

g isn't 10.

I think I get it now. I thought g didn't matter because the units that define a bar are just kilograms, meters, and seconds and kilogram is a mass unit rather than a weight unit.

But on second thought, in this particular scenario of the force of a column of water (or air) above you, the force that is acting on the column, the meters per second squared, is gravity.

It's just another coincidence that the g force we experience on Earth (9.80665 metres per second squared) is close to a round number.

I do want to point out that the force required to move water in order to swim under the surface is not affected by gravity. Swimming underwater a length of a pool on the moon would take as long as doing it on Earth.

Correct if it's seawater, specifically with a density of 1.020 g/mL, which is the value assumed by EN13319.

I'm not familiar with EN13319, but that's weird. A kilogram was actually defined as the weight of a litre or 1 cubic decimeter of pure water (first at 0⁰ and later 4⁰ C) for most of its existence. In other words exactly 1 g/ml. The current definition of the kilogram differs by only 1 part in 30 million from the original water kilo.

 

[JMarc]

'm not familiar with EN13319, but that's weird

Again mass and weight difference and 1.02 ~= 1/g

Another coincidence is 1 atm ~= 1 bar

 

[inquis]

I'm not familiar with EN13319, but that's weird

I suspect the point was to eliminate these various "almosts" from common use. 10.0 m of depth increasing by 1.0 bar pressure is convenient. They fudged the density of "seawater" to make that happen. The surface pressure already varies (rarely is it exactly 1.0 atm), so assuming it's 1 bar (0.987 atm) was another bit of rug-sweeping to enable the nice numbers in that standard.

 

[lowwall]

Correct if it's seawater, specifically with a density of 1.020 g/mL, which is the value assumed by EN13319.

Wait, you said "assumed". Does this mean the standards group decided to define 1 bar as 10m of water and then let the water adjust?

How on earth did they get the Germans to agree with that?

 

[inquis]

How on earth did they get the Germans to agree with that?

98.1% majority?

 

[Jai Bar]

In real life, science included, one has to determine the Least Significant Digit (LSD, not to be confused with the deep purple stuff ). Now, quoted values of water salinity (including fresh) may and can vary on different locations, depths, temperatures and even gravity (which contrary to popular belief it varies at different locations and altitudes).

When it comes to scubadiving, the rule of thumb of 1 atm = 10 meters of water column is perfectly practical and valid as any of the quoted values in this thread with varying number of decimal digits, which are not of significant value.

Moreover, these values go into calculating pressure for things like nitrogen intake in some tissue compartment models (or bubble permeabilty models or whatever)- all of which were derived from generalized data from some studies (goats, navy divers, doppler studies, dry recompression chamber simulated dives etc.) and none are universally accurate for everyone, as each of us has different physiology, weight, tissue properties, physical conditions etc. etc.

We are all different yet using the same average model.

Moral of the story, it is more important to understand the last sentence, so that students understand that when we teach to always add safety margins it is because of that. Computers and decompression models are not accurate at all, they were developed using some studies and they fit everyone differently. It was easier when teaching tables, telling to always round down the depths etc. Nowadays the vast majority use computers and think they are very accurate but they are not: pressure sensors' accuracy varies (most computers 0.1m resolution- not to be mistaken with precision...), the computer models assume some salinity which can differ from actual water at dive site, temperature is usually unaccounted for and so on, and the whole thing is anyways fed into some model that is inherently approximate.

TLDR 10 meters is 1 atmosphere, good in real life of scuba diving, makes easier calcuting. Always add safety margins to cover for many unknown factors.

If we are discussing real academic study (i.e not teaching OW classes) then we should definitely go metric SI units: it is the only viable option (see post #11 for one of the reasons why ). Accuracy should be based on the instrumentation used and the nature of the calculations. Always perform a proper error analysis and round results accordingly.

 

[lowwall]

I suspect the point was to eliminate these various "almosts" from common use. 10.0 m of depth increasing by 1.0 bar pressure is convenient. They fudged the density of "seawater" to make that happen. The surface pressure already varies (rarely is it exactly 1.0 atm), so assuming it's 1 bar (0.987 atm) was another bit of rug-sweeping to enable the nice numbers in that standard.

I apologize for bumping a dormant thread, but I found a copy of EN 13319 for another thread and I think it should also be included here. FWIW, the phrase "geometric depth" is far more elegant than "the depth you'd get if you were to attach a plumb bob to a tape measure"

4.1 Depth measurement

4.1.1 Gauge factor for the transformation from pressure to depth

The gauge factor shall be such that an increase of pressure of 1 bar would cause an increase in the depth displayedof 10 m.

NOTE: This rule assumes a water density of 1,0197 kg/l, i.e., in fresh water of 1,00 kg/l the geometric depth is 102 % of the display while in sea water of a density of 1,03 kg/l the geometric depth is 99 % of the display. Since the physiological relevant figure is the environmental pressure only, the geometric depth is of much inferior relevance for the diver.

 

[Dominik_E]

Just to add: what does 1% of 10m really mean? This is 10cm. This is even challenging for the calibration of dive computer pressure sensors. And on top of that, even in very good trim, the vertical extent of most divers will be several 10s of cm So where to measure?

Decompression anyhow is not precision science on such a level. Models really have larger unknowns than a 1%-level deviation in measurement. What I find important is to decide on a thing, and then to stick with it. To me (physicist by training) a case of consistency beating absolute precision.

 

[lowwall]

Just to add: what does 1% of 10m really mean? This is 10cm. This is even challenging for the calibration of dive computer pressure sensors. And on top of that, even in very good trim, the vertical extent of most divers will be several 10s of cm So where to measure?

Decompression anyhow is not precision science on such a level. Models really have larger unknowns than a 1%-level deviation in measurement. What I find important is to decide on a thing, and then to stick with it. To me (physicist by training) a case of consistency beating absolute precision.

To add to the abovementioned issues with precision, we take the solution from our models, modify it with a 15-80% fudge factor - sorry Gradient Factor - and then round up to the next minute and meter. And then the diver adds a couple of minutes on top just in case