Switching to all metric for academics

[sea_ledford]

Very similar in the metric world.
1 litre = 1000cm3.
Stack these 1cm3’s on top of each other makes a column 10 metres high.
As 1 litre of water has a mass of 1kg, makes 10 metres of water = 1kg/cm2 that is almost 1 bar

It's that "almost" that is killing me.

I'm probably over thinking it all and I should just use 10m/atm, 1kg/L, etc and tell them it is just as accurate as a steel "72" or an aluminum "80"

 

[tursiops]

As 1 litre of water has a mass of 1kg, makes 10 metres of water = 1kg/cm2 that is almost 1 bar

Herein lies the problem many are having: not only is "salt water" an undefined term, 1 kg/cm**2 is not quite 1 bar (it is 0.98 bar), nor is it quite 1 atm (it is 0.97 atm). So 10m of fresh water is neither 1 bar nor 1 atm.
However, 10.33m of fresh water IS 1 atm, and that is about the same weight as 10m of "normal" saltwater. So, it just kindo works out, that you can either ignore all the little 2 and 3% factors, or you can say that 10m of salt water is 1 atm.

 

[lowwall]

Very similar in the metric world.
1 litre = 1000cm3.
Stack these 1cm3’s on top of each other makes a column 10 metres high.
As 1 litre of water has a mass of 1kg, makes 10 metres of water = 1kg/cm2 that is almost 1 bar

It's that "almost" that is killing me.

I'm probably over thinking it all and I should just use 10m/atm, 1kg/L, etc and tell them it is just as accurate as a steel "72" or an aluminum "80"

It's not almost. A column of SI standard water (that's sort of a joke, but not really) 1cm x 1cm x 10m high exerts exactly 1 bar of force.

[Edit - Oops, it doesn't, because the force comes from the gravity we experience on Earth which isn't an even SI number. See discussion on next page.]

The "almost" comes into play if you are talking about atmospheres rather than bar.

Divers tend to use them interchangeably, but they are not the same thing. An atmosphere is not a pure SI unit that can be derived from the meter. Instead, it's a shorthand created for convenience and is "intended to represent the average atmospheric pressure at the average sea level at the latitude of Paris, France". The fact that 1 bar is so close to 1 atmosphere is a pure coincidence.

But again it doesn't matter one bit when it comes to using these measures in diving. In the real world, the precision of any measurement is limited by the accuracy of the tools you use to measure it. Our pressure gauges, both the SPG and in our computers, are only accurate to within a few percent, so the 1.3% difference between bar and atm is immaterial.

Even if you had a perfect gauge, your accuracy would still be limited because our bodies extend into three dimensions and so are always exposed to a range of pressures that cannot be distilled down to a single number.

 

[tursiops]

It's not almost. A column of SI standard water (that's sort of a joke, but not really) 1cm x 1cm x 10m high exerts exactly 1 bar of force.

Only true at 4DegC, when water is its densest. At our usual diving temperatures, it is less dense, so that 10m of pure exerts less than 1 bar.

 

[dmaziuk]

It's that "almost" that is killing me.

I'm probably over thinking it all and I should just use 10m/atm, 1kg/L, etc and tell them it is just as accurate as a steel "72" or an aluminum "80"

I'd keep telling them it is pressure we care about, and that referring to it as "depth", as we commonly do, is where the "almost" comes from. I think really the only time "depth" comes up in any of the deco stuff is in Workman's M-value formula, and my impression is, it's because he used pressure relative to sea level and "pressure-relative-to-sea-level" is a mouthful. Everything else: Schreiner's formula, Buhlmann's formula, etc., is all P.

 

[JMarc]

Again, the only time as a diver when I care about the difference between salt and fresh water is for buoyancy. And when rinsing my stuff.

For the transformation between a pressure and a height of column of water, I don't. If you do, you probably have to consider the relevance of the temperature, the variation of g around the globe, the variation of salinity between seas, the changes in atmospheric pressure, the accuracy of your instruments, the nuances between accuracy, precision and resolution. The only interest of doing a careful computation once is to show that it isn't relevant.

 

[JMarc]

It's not almost. A column of SI standard water (that's sort of a joke, but not really) 1cm x 1cm x 10m high exerts exactly 1 bar of force.

g isn't 10.

 

[inquis]

A column of SI standard water (that's sort of a joke, but not really) 1cm x 1cm x 10m high exerts exactly 1 bar of force.

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

 

[Dominik_E]

The EN13319 salinity (precise: density at a fixed temperature) is so that 10m column result in 1bar pressure. However, 1 physical atmosphere is also a bit more than 1bar, and the salinity of ocean water often enough is a bit higher than in the EN.

In the SI, actually, neither is the [kg] any longer connected to a volume of water of any sort, nor is the [bar] even the unit of pressure (would be Pascal, 10^-5 vs bar). But it is all close together, in part for that historical reason. I would argue in a lot of cases, 10m water column adding one bar (and let the dive start at 1bar) is close enough for physicist work…

 

[sinidive]

Btw why are you preferring atm instead of bar?

With bars and fresh water the mathematics is really simple.

So for practical math just use bars with surface = 1, and 10m = 2bar. Then all the math will be simple.

And then you can add a footnote that e.g. salinity and temperature will affect density and how many percent of variation they cause.

However diving related things tend to depend on pressure - not depth. And depth is also measured (in diving) by pressure...