Switching to all metric for academics

[MarkA]

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.

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

 

[JMarc]

I've trouble with mentions of EN13319's density. My understanding is that that value of density is not a standard density value for salt water, but an intermediate density value to be used to convert a pressure to a water depth when one desired not to take into account if that water is salted or not. I.E. it's a good value when you switch often between fresh and salt water and don't want to have to change your computer setting (or if you are an instrument maker and you don't want to provide a setting and your instrument can be used in both salt and fresh water). It's not a good value if you want to show the difference between fresh and salt water.

The standard is obviously too costly to buy just to confirm if my understanding is correct or not.

 

[Duke Dive Medicine]

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

That's so much easier. I remember in the early/mid-70s there was a push to switch the USA to metric. America was having none of it

 

[MarkA]

I've trouble with mentions of EN13319's density. My understanding is that that value of density is not a standard density value for salt water, but an intermediate density value to be used to convert a pressure to a water depth when one desired not to take into account if that water is salted or not. I.E. it's a good value when you switch often between fresh and salt water and don't want to have to change your computer setting (or if you are an instrument maker and you don't want to provide a setting and your instrument can be used in both salt and fresh water). It's not a good value if you want to show the difference between fresh and salt water.

The standard is obviously too costly to buy just to confirm if my understanding is correct or not.

My understanding is, it is where the density of 10m of water equals 1bar. So more than fresh water but generally less than salt water.

In practice it’s not relevant to me.

 

[MarkA]

That's so much easier. I remember in the early/mid-70s there was a push to switch the USA to metric. America was having none of it

The term going metric inch by inch was used when I worked for the US part of an engineering company.

 

[lowwall]

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.

It doesn't matter. Outside of specific and limited survey scenarios, divers never use depth. We may talk about depth, but we are actually using an approximation of the true depth that is based on pressure. For example, our depth gauges/dive computers measure pressure rather than depth, our decompression algorithms are based on pressure rather than depth, gas consumption varies with pressure rather than depth.

So, go ahead and use 10m/atm for everything. The actual depth will vary with salinity, temperature, local gravity and local barometric pressure, but it is irrelevant to us. It's the atmospheres (pressure) that matters for everything we do.

 

[boqurant]

Using imperial hogsheads per fathom per yard-pound is completely bananas. Science (and all those who value their sanity) uses SI units

 

[sea_ledford]

Using imperial hogsheads per fathom per yard-pound is completely bananas. Science (and all those who value their sanity) uses SI units

Agreed, which is why I'm switching. Today I went over plus rated LP tanks... talk about bananas.

 

[sea_ledford]

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.

This is to teach students how to do the calculations, and to comprehend how the different gas laws are affecting divers.

I spend about 3 days on physics, the divers that are good at math are bored after the first day, the divers that are bad at math are lost the entire time, and there is like two that I get some comprehension growth out of. I'm working on updating my curriculum to focus more on comprehension and less on math. I was thinking that metric would make life easier since it is WAY easier in actual use. What I'm finding is all the different rounding in standard use makes this more challenging than I was expecting.

 

[sea_ledford]

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.

Yes...sort of. Mostly undergraduates that are marine science majors, but haven't done much in the way of actual research yet. And they are mostly American, so...

If we would have switched in the 70's like we were planning on, I wouldn't be in this situation!