Units of Measure in Diving?

[peterbj7]

I'm perfectly happy with both, and diving with mixed groups here (Belize) I often have to use both systems concurrently.

One aspect that concerns me is tank capacity measurement. Here what changes between the US-dominated sphere and the rest of the world is not particularly the units used, but what actually is being measured.

 

[vladimir]

Then there are also US ex-patriots who may or may not have made the transition.

Hey, I'm an expatriate, but I'm still a patriot.

 

[Quero]

All my gauges are in metric and I learned in metric, so it comes more "naturally" to me to use metric, and I prefer it for that reason when I'm doing my own diving. I can make the conversions, no problem, but as I rarely dive in N. America, imperial seems rather backwards/quaint to me.

When I'm teaching I generally find metric easier since it's just factors of ten (except when converting between fresh and salt water). I can and do teach both, especially if my students prefer imperial. For math-phobes, though, imperial just makes it more complicated having to remember seemingly random numbers for calculations. It really makes a difference for calculating lift/displacement, RMV/SAC rates etc.

I don't think it has anything to do with patriotism, but I do think it shows very little confidence in the intelligence of our citizenry that the country reneged on the plan to convert to metric back when I was a school girl.

 

[peterbj7]

There are actually strong reasons against metric as we have it. One is that the units bear no relationship to the physical world, whereas the older system grew up precisely because of such relationships. Secondly the metric system itself is of far less value than it could have been, being to the base of 10 which is not divisible by 3. A far more useful system would have been a base of 12, but that didn't come about because we only have 10 fingers (the binary system used by computers is of even less value to us in our everyday lives). And lastly, any metric system is prone to errors due to misplacement of the "decimal" point - such an error is easy to make and often not at all obvious.

 

[Akimbo]

Hey, I'm an expatriate, but I'm still a patriot.

Oops, I never noticed it was spelled that way. I originally drafted "X-Pats" but figured not everyone would get it. I stand corrected… any icons around with a sheepish grin???

 

[JanK]

If you're just counting a small number of sheep and dividing them between yourself and your two brothers, I agree that divisibility by 12 is convenient (but what if you have 4 brothers).

As soon as you go to larger numbers and fractions, which mean the use of a positional system (which, BTW, has nothing to do with metric), i.e., number 523.23 means 5*100 + 2*10 + 3*1 + 2*0.1 + 3*0.01, and do mathematical operations, you have the same "decimal point problem", regardless of the base you're working in. In base 12 you might have a number like 3AB2.A9, divide it by 100 (i.e., 144 in decimal), mess up the decimal point position and be off by a factor of 12 - so you'd actually make a bigger error.

But with metric you have a great advantage that you can easily convert within a great range of numbers with very simple operations, e.g., metric tons/cubic metre = kilograms/litre = grams/cubic centimetre. Try converting between tons/cubic yard, pounds/cubic foot and ounces/cubic inch. More of less *any* conversion in the metric system is trivial, with only simple decimal point shifts.

Not to mention that as soon as you go above around 10,000 the scientific notation comes into play, so I can instantly convert 12.3 kilometres to 123*10^6 millimetres. Compare that with 12 inches in a foot, 3 feet in a yard, and 1760 yards in a mile... So, how many inches in 12.3 mile?

And BTW, binary is not as useless as you think. I can count to 1024 on my 10 fingers and to 32 with just one hand using binary. The latter I actually do sometimes, the former not.

Of course, one gets conditioned to whatever system they have been using since childhood and any system has its flaws, but, if someone tries telling me that having to know all the conversion factors listed on United States customary units - Wikipedia, the free encyclopedia is easier and more convenient for calculations than the metric system, I will just laugh.

 

[Herk_Man]

Stoichiometry rules! If it's a unit, I can convert it!

 

[Akimbo]

There are actually strong reasons against metric as we have it. One is that the units bear no relationship to the physical world, whereas the older system grew up precisely because of such relationships…

I am not promoting one over the other but in fairness, that is not entirely accurate. Just to name a few:

• The meter was originally intended to equal one ten-millionth of the length of the meridian through Paris from pole to the equator. For convenience, it was represented by a metal rod measured at a certain temperature stored in France. The definition was updated in 1983 to equal the distance a certain light frequency travels through a vacuum in one second. You are right though, using the King's right foot and part of his finger also is a physical relationship.
• 1 Cubic Meter of fresh water weighs 1 metric ton or 1000 Kg
• 1/1000th of a cubic meter is a Liter, and weighs 1 Kg
• 0° C is the freezing temperature of fresh water and 100° C is the boiling point.

One of the few non-metric measurements that continues to dominate world-wide is the Nautical Mile's use in navigation, not to be confused with the Imperial Statute Mile, because of it's relationship to the earth — though that will become less relevant as GPS and computer time-to-target calculations take over.

…Secondly the metric system itself is of far less value than it could have been, being to the base of 10 which is not divisible by 3. A far more useful system would have been a base of 12, but that didn't come about because we only have 10 fingers (the binary system used by computers is of even less value to us in our everyday lives)…

The first official measurement system I am aware of using powers of 10 is the US Dollar. The Metric or SI system followed. I think powers of 10 was chosen because Base-10 numeric systems dominated the world rather than the number of human digits on both hands — though that may well have influenced the creation of the Base-10 system. I would agree with you if the world used a Base-12 number system. Binary is used by computers because there are only two states to semiconductors, off or on represented by 0 and 1.

Days, Hours, Minutes, and Seconds somehow slipped past both.

…And lastly, any metric system is prone to errors due to misplacement of the "decimal" point - such an error is easy to make and often not at all obvious.

In my experience, far more errors are made calculating Feet, Inches, and fractional Inches than any from an improperly placed decimal. Example: Multiply 1' 7-5/8" (which is 19.625") by two. How about Gallons, quarts, cups, and teaspoons? When it comes to precision work where the SI system is not used, the primary linear unit is Inches with decimal fractions where the same problem exists.

In the end, the most compelling argument may be that the USA and Liberia are the only two countries in the world where the Imperial System is dominant or official — the justification being the value of being on the same page for trade and communications rather than the majority rules.

 

[Herk_Man]

I do all my dive calculations in binary.

 

[GrumpyOldGuy]

I dive whatever units my buddy is using. It is easy enough to switch the computer to metric/imperial and I use SPG's labeled in both PSI/BAR. No conversion needed.

In the end, they are only numbers. By matching my buddy, I reduce the chance of misunderstanding each other. This is far more important the which system I use.