Why isn't DIR universally metric?

[laurin]

The fundamental difference in rating systems isn't that one is in metric and the other imperial units, it's that one (metric) volume number is empty (i.e. at one atmosphere) while the other (imperial) is full (i.e. at 'rated pressure').

Yup. The difference might not be that big in non-diving use, where you just buy a volume of gas and use it until it's empty. For diving, the US tank rating system is convoluted. A diver used to the system may be fluent at using it, but that doesn't change the fact, at all.

For any meaningful gas calculations you need to know or figure out both 1ATM volume (AKA water capacity) and tank pressure. In the european system you know both straight away; in imperial you need to know the rated pressure regardless of current tank pressure; and usually also completely unnecessary (from euro view) tank factors. Both are needed just because you don't know the water capacity.

From one diver's viewpoint the difference might not matter much; you use the system you're used to. However, from communications (online discussions, for example) viewpoint the US system is a major pain and probably makes gas planning a much bigger hurdle than it needs to be for the "mathematically illiterate" AKA "dumb" folks.

Just taking a look at some of the gas planning threads at TDS (the "1/2 + 200" stuff) etc is a good example of a page after page of stuff that would be pretty much crystal clear in metric.

I think I've mentioned this before, but making up some kind of "metric math tricks" is just approaching the metric system with imperial mindset - another kind of "tank factors". The whole point of the simplicity and elegance of the metric system is that you don't need this kind extra complication.

I don't think there is much hope of US diving (DIR or otherwise) moving to metric, not because of any merits of the imperial system rather than simple resistance to change.

//LN

 

[Kern]

"Who cares" ???

Your buddy cares if you've gotta get him out from an overhead, in an out-of-gas contingency: Imperial or Metric, you better have figured the correct amount to get you both out & up to the surface (or at least to your deco gas switch). . .

I think your being a little disingenuous here. Obviously we all want to get out & bring our buddy with us. Some of us just don't care too much about the system of measurements used to do the planning.

As far as I can see, planing before the dive, whether imperial or metric, & management during the dive will get you both out. Histrionics aside, I don't really need to figure the cubic capacity of gas I have left when it hits the fan, I did all that on the surface. In fact, if my planned run time & ascent pressure ever coincide, I'v either done the dive in a total panic or I screwed the plan. As everyone I know plans in metric, I can assure you, use of the metric system, is no guaranty mistakes won't be made.

 

[Blackwood]

and usually also completely unnecessary (from euro view) tank factors.

They aren't un-necessary, they're just stamped on the cylinder.

The 'tank factor' for a 15L cylinder is 15L/bar.

(though given the resolution on BAR SPGs, I think maybe 150L/10bar would be easier to use).

 

[Blackwood]

"Who cares" ???

Your buddy cares if you've gotta get him out from an overhead, in an out-of-gas contingency: Imperial or Metric, you better have figured the correct amount to get you both out & up to the surface (or at least to your deco gas switch). . .

For some reason you and I always go back and forth on this issue, on multiple forums.

I'm not anti-metric. In most cases I prefer using the metric system. As you're probably aware, here at (legacy) TRW, most manufacturing capital equipment is imperial, and it's thus best to design in imperial rather than introduce un-necessary rounding errors on the floor.

I'd be in total favor of an international unit system (though I'd lobby for an international language first...).

All's I'm saying is that understanding how volume (what you breathe) relates to pressure (how much you stuff in the cylinder) and being able to apply that to diving is much more important than that units we use to display volume or pressure.

I do like that BAR cylinders are rated at one atmosphere (i.e. water capacity). As has been stated, that takes one step out of the math. Doing it in advance and rounding to a legible resolution on the gauges (100PSI) eliminates that step on-the-fly.

I do my gas planning on the surface (for dives I'm not familiar with... regular profiles I just know the numbers). I am capable of doing calcs in the water it either unit system, so if we find we are using less gas than anticipated we can figure out how to safely extend it.

It's the understanding of how to manipulate these figures easily and expeditiously in problem solving, to your advantage . . .for whatever unit system. That's what matters...

We finally agree.

 

[Blackwood]

double post - please delete

 

[AndyNZ]

I'd be in total favor of an international unit system (though I'd lobby for an international language first...).

Ye Gods, please no.... that's one things that us Europeans got completely wrong, the "Euro-language" of Esperanto. I can only hope it stays buried with a stake through it's heart forever.

All's I'm saying is that understanding how volume (what you breathe) relates to pressure (how much you stuff in the cylinder) and being able to apply that to diving is much more important than that units we use to display volume or pressure.

I do like that BAR cylinders are rated at one atmosphere (i.e. water capacity). As has been stated, that takes one step out of the math. Doing it in advance and rounding to a legible resolution on the gauges (100PSI) eliminates that step on-the-fly.

It seems to me, that once you are working in either bar or psi (as opposed to litres of cubic feet) that there is little difference in complexity of the maths that a bit of appropriate rounding doesn't fix.

 

[lamont]

Just taking a look at some of the gas planning threads at TDS (the "1/2 + 200" stuff) etc is a good example of a page after page of stuff that would be pretty much crystal clear in metric.

don't go to TDS to get gas planning advice.

 

[Kevrumbo]

You can get good gas planning advice right here on Scubaboard

Before I went to the "dark side" of utilizing the Metric System, I was using lamont's rule-of-thumb which served me very well:

0.75 SAC: 300 psi per 10 mins per ATA for Al80/HP100/LP72, 200 psi per 10 mins per ATA for LP104/LP95/HP120s/HP130. http://www.scubaboard.com/forums/1640508-post61.html.

Now simply converting from psi to bar, look how easy the arithmetic becomes:
22 litres/min SAC: 20bar per 10mins per ATA for Al80/HP100/LP72; 13bar per 10mins per ATA for LP104/LP95/HP120s/HP130.

Easy because of that reduction in an order of magnitude using bar units makes the pre-dive and on-the-fly gas calculations more suitable to do quickly in your head. And since you know your depth in meters, --it's a cinch to convert to depth in ATA and simply mutiply depth in ATA by the bar per minute SAC rates above for your particular tank.

Example: 30m depth, how much will I consume in 10min using a single AL80 (11L Tank)?
Solution: 30m is 4.0ATA;
(4ATA)(20bar/10min*ATA)(10min) is 80bar of gas consumed.
So if I read on my SPG 190bar ten minutes ago, I would expect it now to read 110bar.

 

[Gombessa]

Example: 30m depth, how much will I consume in 10min using a single AL80 (11L Tank)?
Solution: 30m is 4.0ATA;
(4ATA)(20bar/10min*ATA)(10min) is 80bar of gas consumed.
So if I read on my SPG 190bar ten minutes ago, I would expect it now to read 110bar.

I'm struggling to see how different this is in imperial. Maybe someone can explain for me? Taking the same example, I get:

Example: 99ft depth, how much will I consume in 10min using a single AL80?
Solution: 99ft is 4.0ATA;
(4ATA)(300psi/10min*ATA)(10min) is 1200psi of gas consumed.
So if I read on my SPG 2800psi ten minutes ago, I would expect it now to read 1600psi.

 

[Kevrumbo]

If you can form a one-to-one correspondence between all the cardinal numbers in Imperial Units (i.e. depth in feet) to their equivalents in ATA, then good for you Gombessa.

(For comparison, do the same for the metric equivalents and explain to us what you notice. . .)