Why isn't DIR universally metric?

[Blackwood]

I think in terms of pressure as well, and utilize a favorable one-to-one metric pressure to depth relationship.

It just so conveniently turns out that my gas consumed in bar is directly related to the depth I'm at in meters:

That's unequivocally not unique to your unit system.

Metric / Imperial is a "who cares" type of a deal.

It's inconsequential.

 

[AndyNZ]

Metric / Imperial is a "who cares" type of a deal.

It's inconsequential.

I guess for me, the challenge with Imperial is knowing exactly how much gas you have in your tank - this whole "80 cu ft tank only contains 77.4 cu ft" thing.

Would be useful to me, if someone can briefly explain how you work out how much gas you actually have at the start of the dive.

Metric is straightforward: tank volume * pressure = litres

How much gas is in an 80 cu ft tank filled to 2000psi?

 

[PfcAJ]

For Imperial, we use "tank factors," which is cubic ft per 100psi.
Rated volume/(Rated pressure/100)= tank factor.

So for an al80, you have 77/30 = 2.566, or a TF of 2.5 for conservatism

To go back to the cubic feet in a given tank (in your example 2000psi in an al80:

TF x (pressure/100)= Cubic feet of gas

2.5x20=50cuft

It all becomes quite fluid with a bit of practice. Double al80s have a TF of 5, double 104's is 8, and so on.

 

[mikemill]

I guess for me, the challenge with Imperial is knowing exactly how much gas you have in your tank - this whole "80 cu ft tank only contains 77.4 cu ft" thing.

That isn't really the fault of the system but the fault of those who named it. That be the case of a tank called a "Y liter" tank really only containing x liters.

Would be useful to me, if someone can briefly explain how you work out how much gas you actually have at the start of the dive.

Metric is straightforward: tank volume * pressure = litres

How much gas is in an 80 cu ft tank filled to 2000psi?

Depends on the service pressure.

I think most people know their tank and just do some basic divisions. Really if I have 123 cuft or 113 cuft is of little real consequence at the beginning of my dive.

I generally do things like: I need 40 cuft for rock bottom. My tank is 123 cuft (though it is called a 119) which is about 1/3 of the tank. 1/3 of my tank is about 1150 psi (3442 is close to 3300 which would give 1100 with about 150 left so add 50 more).

 

[rjack321]

Tank factor = cf/rated pressure/100

Tank factor table in cf per 100psi
common imperial single tanks
al80 = 2.5
hp100/lp85 = 3
hp119/lp95 = 3.5
hp130/lp104/lp108 = 4

doubles are obviously 2x

 

[Blackwood]

I guess for me, the challenge with Imperial is knowing exactly how much gas you have in your tank - this whole "80 cu ft tank only contains 77.4 cu ft" thing.

Yah, that is annoying.

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').

There aren't a whole lot of cylinders I'm aware of whose capacities are substantially different than what the tank is called (as in the case of an aluminum '80').

 

[Guy Alcala]

If I am unconcious please just keep a reg in my mouth and screw looking at my SPG. Unconcious people aren't stressed and don't hoover.

I said injured/incapacitated (probably in pain), not unconscious. People in a world of hurt tend to pant.

you've probably got a lot of more important things to worry about first before worrying about metric vs. imperial.

Ummmm yeah I agree with Lamont, get back to us once you're through Tech1, I have a funny feeling it won't seem so important then.

Sure, but since I'll be changing to doubles it seems like a convenient time to make the switch, as well as providing the benefits I've mentioned.

In any case, it seems we've (long since) reached the point at which we're repeating the same arguments, so let's agree to disagree and

argue about something really critical to DIR: the 'proper' way to attach the bungie to our secondary:

Extra zip tie! :whack: Double overhand knots!!
Zip Tie!! :comeandgetsome: Overhand knots!!!:biggun:

Tie!!! Knots!!!! :nuke:





Oh, let's just dive :dancingsnoopy:



Guy

 

[Guy Alcala]

Yah, that is annoying.

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').
<snip>

That and the fact that imperial tank pressure is in PSI, when to make it similar to the metric method you would need an SPG marked in ATA. And probably change to some unit smaller than a cubic foot, as most people have an RMV smaller than 1. It's so much easier for most people to multiply integers rather than decimal fractions. Having tank volume in quarts and tank pressure in ATA would give the same method as liters/bar, and the numbers would be very close to the same: 237 bar is 234 ATA, and a liter is 1.06 qts, pretty negligible differences. Gal./ATA would also work (and would keep the numbers smaller), but I suppose you might run into the whole US vs. Imp.gal thing.

Guy

 

[Blackwood]

That and the fact that imperial tank pressure is in PSI, when to make it similar to the metric method you would need an SPG marked in ATA. And probably change to some unit smaller than a cubic foot, as most people have an RMV smaller than 1. It's so much easier for most people to multiply integers rather than decimal fractions. Having tank volume in quarts and tank pressure in ATA would give the same method as liters/bar, and the numbers would be very close to the same: 237 bar is 234 ATA, and a liter is 1.06 qts, pretty negligible differences. Gal./ATA would also work (and would keep the numbers smaller), but I suppose you might run into the whole US vs. Imp.gal thing.

Guy

"Tank factors" make it more like the 'metric' tank system and address your concern with decimal fractions, though it's CF per about 7ATM rather than 1 since 100PSI is reasonable to read on an analog SPG where as 14.7PSI, or 1BAR, is not.

I still feel that most of the "volume" fuss (quarts, gallons, liters, cubes) is a "gee whiz" discussion. Know what your averages are so you can go from tank to tank, but work in the system your gauge displays (pressure).

Screw the PSI/BAR gauges, PADI should sell SVG (submersible volume gauges) which match specifically to tanks. Think of all the sales they could push if new students thought they needed a different gauge for each variety of tank.

 

[Guy Alcala]

"Tank factors" make it more like the 'metric' tank system and address your concern with decimal fractions, though it's CF per about 7ATM rather than 1 since 100PSI is reasonable to read on an analog SPG where as 14.7PSI, or 1BAR, is not.

I still feel that most of the "volume" fuss (quarts, gallons, liters, cubes) is a "gee whiz" discussion. Know what your averages are so you can go from tank to tank, but work in the system your gauge displays (pressure).

I have no problem working in either system, it's just a question of choosing the easiest system to use, which is also understood around the world.

Screw the PSI/BAR gauges, PADI should sell SVG (submersible volume gauges) which match specifically to tanks. Think of all the sales they could push if new students thought they needed a different gauge for each variety of tank.

Please delete that last section of your post before PADI sees it! Let's not give them any ideas

Guy