Metric versus Imperial System for Diving?

[cdemith]

I think you're on to something! Forget metric vs imperial... how about a BINARY SPG?

Needle stays at "1" as long as there's any air whatsoever in the tank, and doesn't move until you are completely out of air... at which point the needle suddenly swings to 0. What could be simpler?

I have seen many divers using exactly that. In their case, green is good and red is bad. Or, maybe it is really a ternary SPG, green=good, yellow=time for safety stop, red=bad and I may die.

On a number of trips on the boat after a dive I have heard people talk about how they were still in the green and could have stayed down longer. Or, they had to surface quickly because they were in the red.

I don't care if you use metric or imperial, just use something.

 

[Aguablanco1]

Answer the question objectively then without all that crap rhetoric above: what is so hard about understanding how to work with the Multiplicative Identity Unit of 1 bar/min* ATA in Metric???

Anything multiplied-by-one is itself . . .can you understand how much easier AND SMARTER that is to do arithmetically with regard to Gas Management Calculations, both pre-planning and on-the-fly???

Kev,
You are missing my point entirely. I have no obligation to do anything, by any means, that I do not want to do. No matter how easy you feel it is. You are also free to do anything you want, any way you want to do it. You have absolutely no right to demand any answers or justifications from me or anyone else. Our choices do not need your approval. As I said earlier, this whole thread has devolved into one that purports to show the "perfect way". If your system works for you I am glad. What I can't comprehend is why you feel you have the right to either impose your beliefs on others, or challenge their intelligence if they choose a way other than yours.
RichH

 

[mrdre]

Um... no.

Time to dig out your Open Water manual, where buoyancy is covered in Chapter 1.

If displacement is not a factor in determining buoyancy please explain why THIS object - which weighs 2oz - sinks... Yet THIS object - which weighs 22oz - floats...

Unless you're telling me that the basketball will sink if I weigh it in grams...

Again, these object do not weight, 2 and 22 oz underwater. it's their MASS. The weight is the force. The question was "weighting 100 pounds". "weighting" - applying force. The object itself could be 1000 pounds, what we care is how much it was weighting at 60 feet. It's a common mistake to mix weight and mass. Mass is property - what you mentioned 2 and 22 oz - is mass. The weight - is force. The basketball will have less weight underwater. But the question was about something with unknown mass weighting 100 pounds underwater. Just elementary Newtonian physics.

 

[dfx]

Again, these object do not weight, 2 and 22 oz underwater. it's their MASS. The weight is the force. The question was "weighting 100 pounds". "weighting" - applying force. The object itself could be 1000 pounds, what we care is how much it was weighting at 60 feet. It's a common mistake to mix weight and mass. Mass is property - what you mentioned 2 and 22 oz - is mass. The weight - is force. The basketball will have less weight underwater. But the question was about something with unknown mass weighting 100 pounds underwater. Just elementary Newtonian physics.

Unfortunately the imperial system does not make this distinction clear at all, as they use the pound for two completely different things.

Pound (mass) - Wikipedia, the free encyclopedia
Pound (force) - Wikipedia, the free encyclopedia

 

[mrdre]

Unfortunately the imperial system does not make this distinction clear at all, as they use the pound for two completely different things.

Pound (mass) - Wikipedia, the free encyclopedia
Pound (force) - Wikipedia, the free encyclopedia

Kilograms sometimes used as force too, but it's not part of standard and there is a clear distinction. From the other side we can't really expect people starting using "newtons" and remembering Earth's gravitational field strength. But you are right, in imperial system "pound" IS officially used as force adding even more confusion.

 

[EFX]

B = VD - W

According to the equation the buoyant force B is the difference between the weight of the water displaced by the object (VD) and the dry weight (W) of the object. If B is zero the object is neutrally buoyant and will neither sink nor float. If B is negative (-) the object will sink and if B is positive (+) the object will float.

Weight is a bad term to use for objects in the water. Like the pound it can have two meanings. It can refer to bouyant force (B) or the force exerted by the acceleration of gravity which is the W in the equation above. (W cannot be pounds-mass because VD is in pounds-force). And yes, objects have a W when emersed in water. It's gets particularly confusing when people use weight without context as in, "A 7mm wetsuit makes me lighter". On land or in the water it makes you heavier. The buoyant force will make you lighter. I prefer the term force to weight when refering to objects in the water. The two terms (VD and W) are always positive but when combined in the equation produce a vector which can be positive or negative.

I use the imperial system only because I'm use to it and everyone I dive with uses it. However, I would prefer the metric system. It's a neat coincidence that 1 atm = 10 msw (33 fsw). It's easier to do calculations in factors of 10 vs. 33.

 

[victorzamora]

Again, these object do not weight, 2 and 22 oz underwater. it's their MASS. The weight is the force. The question was "weighting 100 pounds". "weighting" - applying force. The object itself could be 1000 pounds, what we care is how much it was weighting at 60 feet. It's a common mistake to mix weight and mass. Mass is property - what you mentioned 2 and 22 oz - is mass. The weight - is force. The basketball will have less weight underwater. But the question was about something with unknown mass weighting 100 pounds underwater. Just elementary Newtonian physics.

Actually, this post is inaccurate. Ounces are also a weight, not a mass. Slugs are mass, as are pounds-mass. Pounds are assumed to be force unless otherwise defined. Pounds-force and pounds-mass are different units. Period.

The most important distinction is that proper buoyancy calculations are done in force units....not mass, as buoyancy is an upwards force exerted on an object immersed in a fluid. Weight is the force resultant of the acceleration of gravity acting upon the mass of an object. Regardless of what fluid it is immersed in, an object under constant gravity of constant mass has a constant weight.

 

[Diver0001]

You know... I grew up in Canada and learned to dive in 1984. Everything was imperial and that's what I used for a long time and a lot of dives.

When I moved to the Netherlands everything here was metric. It took me exactly 1 dive to make the adjustment from PSI to BAR.

Depth felt a bit weird for a while because 30 metres doesn't sound as deep as 100 ft. I think it must have taken me about 4 dives to adjust to that.

Will both systems work? Sure, why not? However, the metric system has some distinct advantages because 10m = 1 ATA. For technical diving it simplifies all the calculations down to stuff you can do easily in your head, which is nice.

I didn't read the whole thread but what I did see through the posts I read was that people really feel a need to hold on to what they've done in the past because they think the transition would be difficult. If you don't want to change, then don't, it's a non-issue since both systems work..... However, the transition is not difficult. I think the transition from metric to imperial would be more difficult in some ways because the imperial numbers are not nice and round. But the transition from imperial to metric.... 4 dives and it's mastered.

R..

 

[victorzamora]

It's a neat coincidence that 1 atm = 10 msw (33 fsw). It's easier to do calculations in factors of 10 vs. 33.

It's actually not a "neat coincidence" at all. In a really goofy, roundabout way (and I can't remember the proof for it)....it was defined that way, kinda-sorta.

 

[Diver0001]

Yes it was, because measurements of distance and weight were connected. It was chosen in such a way that 1 cubic centilitre (a cube of 1x1x1 centimetre) of pure fresh water weighs 1 gram. One cubic liter is therefore 1 kilogram and so forth. As a nice side effect of this connection between units of distance and the weight of water, 10 metres is spot on 1 ATA.

R..