Metric versus Imperial System for Diving?

[agilis]

Most of my interaction with the metric system is a result of my interest in vintage handguns, which I collect and shoot. I also do some reloading, especially for difficult to obtain calibers, like 8mm Nambu and .455 British cartridges.

The use of imperial and metric designations seems to reflect the age of the caliber. Cartridges like .455 are very old, while 9mm were developed when the metric system was young. When the metric system was adopted in Europe they did not change calibers in wide use, like the .25 and .30 calibers. They simply redesignated them with metric measurements, 6.35mm and 7.62mm instead of the decimal inch original.

Things can get very confusing at times, with all sorts of equivalents: .380 and 9mm Kurtz are the same exact cartridge, but the old .38 was reduced in size when smokeless powder came into use, down to .357, or 9 mm. Modern .38 Special bullets are really .357 inches, as are the lighter less powerful .380.

The ubiquitous .32 is called a 7.65mm in Europe. Its projectile is actually closer to 7.9 mm, or a bit over .31 caliber, measured accurately. History and the logistics involved trumps everything else.

 

[EFX]

I'm sure divemasters around the world would also appreciate it if they didn't have to ask "are you in bar or PSI" every time a new diver arrives. And then have to keep track of it during the dive. Easy to get confused there and I've seen it happen. "How much air do you have left?" 7 fingers go up. What's that again, 70 bar or 700 PSI?

The problem here is not the measuring system but the human (communication) system. How are you showing remaining air? Do you sign each digit? If so, there is NO confusion. If signing 7-0-0, it can't be bar -- must be psi. If signing 7-0, it can't be psi -- must be bar.

 

[Tigerman]

Except 7-0 CAN be psi (although you'd be in somewhat of a big f-ing hurry)...

 

[victorzamora]

Except 7-0 CAN be psi (although you'd be in somewhat of a big f-ing hurry)...

It'd be so much easier for the dive guide to simply say "tell me when you get to Xbar or Ypsi with the fist-on-chest symbol"

I've never seen the need to know the exact pressure of a fellow diver.

 

[TMHeimer]

It'd be so much easier for the dive guide to simply say "tell me when you get to Xbar or Ypsi with the fist-on-chest symbol"

I've never seen the need to know the exact pressure of a fellow diver.

Maybe you're right. There are too many ways to signal exact pressure. When I assist with a class I just try to remember which way this or that instructor prefers. Of course, it's different if you are wearing 3 fingered mitts instead of 5. But maybe one standard method would make sense. Uh oh, now I'm in the "metric" camp.

 

[Tigerman]

Half, quarter and "I'm boned" should do

 

[Storker]

The ubiquitous .32 is called a 7.65mm in Europe. Its projectile is actually closer to 7.9 mm, or a bit over .31 caliber, measured accurately. History and the logistics involved trumps everything else.

Nah, that's just a question of whether you measure the caliber across the grooves or the lands. Just as a .30 cal (7.62mm) rifle uses .308 cal (7.81mm) bullets.


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[agilis]

Nah, that's just a question of whether you measure the caliber across the grooves or the lands. Just as a .30 cal (7.62mm) rifle uses .308 cal (7.81mm) bullets.


--
Sent from my Android phone
Typos are a feature, not a bug

Generally you measure across the lands because that's the original diameter of the barrel before the groves are cut into it. The bullet should be lightly marked or cut by the lands, and the extent by which the bullet is slightly larger than the lands is in part dependent on the material from which the bullet is made and the pressure handling capacity of the weapon. With some older guns I use cast lead bullets slightly fatter than the brass originals and a slow powder. Some weapons, like the late war Nambus, have intentionally oversized bores. They don't explode often, but are wildly inaccurate until you handload 8.4mm soft lead bullets.

In any case, the caliber designation is, by convention, applied to modern manufactured ammunition first and the weapon secondarily, as "This 8mm Mauser fires 7.92x57 cartridges" which are, of course, slightly larger than 8mm in order to ride the lands. the barrel, before being cut with rifling, was 7.92mm. It seems the
the older the weapon the more imprecise the terminology, especially since these details are of limited interest. Metric and imperial size classified weapons are equally casual in applied designations and odd departures from what should be norms, and it is not until you get into the minutia of small scale cartridge manufacturing that any of this matters.

All this applies to standard infantry and hunting weapons. Ultra high velocity stuff is a different category. I sincerely hope this is not off topic.

 

[DaleC]

All this reminds me of a discussion trying to decide which language is better.

There is a big push for english to be the universal language of business but that doesn't make it better. It just makes it easier for people of different nations to simultaneously communicate more effectively.

In diving, people usually dive in smaller, regionalized groups. There is no need to communicate globally in order to do a dive. As long as the members of the group can communicate effectively, their "language" is fine. I can make all the calculations I need to dive safely by using PSI/CuFt. The fact that, in another part of the world, you can so using Bar/L doesn't make you right or better. Just different. If it's ok for you to be different, I hope it's also ok for me to be different.

Hoping one day your group will speak my language solely, when there is no real need, is a little egotistical. Living in Canada we have such an challenge with the French/English dichotomy. Somehow we manage to exist using both, and while some might believe it would be better if the country had one language, I think it adds to our experience and doesn't detract. It teaches us to respect differences. So, while some countries may see the value of their choice of measures, perhaps they could respect the US's choice in using theirs. Unless of course, some peoples choices are considered more deserving of respect than others.

Or we could follow the mature example of some and just boil it down to the claim that the other country sucks. I sooooo want to respect that persons opinion.

 

[EFX]

Except 7-0 CAN be psi (although you'd be in somewhat of a big f-ing hurry)...

Impossible. The output pressure of first stage regulators are set anywhere from 110 to 150 psi ABOVE ambient pressure depending on brand. Using 110 psi, at the surface the first stage output is 110 + 15 or 125 psi. If the tank pressure is 125 psi the pressure drop from tank to 1st stage output is 125 - 125 = 0 psi. Zero pressure drop equals no flow and this is at the surface. This means absolute rock bottom tank pressure cannot drop below 125 psi. At depth to maintain flow the tank pressure must be higher.