Advantages to Imperial units

[billt4sf]

I am from one of the three countries in the world that use Imperial units (feet, pounds, etc.) and I have found one advantage to this (apologies to Dr. Neil deGrasse Tyson).

My depth reminds me about how much air I should have to start my ascent, to wit:

When I have 1000 lbs, I start ascending if I am 100 ft deep,
When I have 800 lbs, I start ascending if I am 80 ft deep,
...etc... (I do not go deeper than 30m as a rule).

Now before you can say "depending on conditions", I totally agree this is assuming benign conditions below *and* above.

Now before you say "WHAT?!? Way too early!" -- I say: this is what works for me -- as a guideline, because I am a heavy breather.

Now before you say: "You gotta work on your air consumption, buddy", I say: Tell me something I *don't* know!

Now let the compliments, condolences, and criticisms roll in.

Have a happy.

- Bill

 

[runsongas]

degrees of arc is way easier to deal with than radians

technically, bar is not a SI unit. nobody is going to quote tank pressure in megapascals though

 

[UWZen]

Telling the fill station what pressure you need in PSI results in fewer strange looks!

To the OP: What cylinder type does the rule apply to? One of my favorite things about metric: calculating remaining gas volume is as simple as (bar)x(tank volume e.g. 11L). If you know min gas (adjusted for your own SAC) this is an infinitely more scalable way to determine your turn/ascent pressure.

 

[tbone1004]

you're listed as a DIR practitioner, if you are, then you'll know rock bottom and ascents are based on volume not pressure so that "rule" only works with certain tanks, particularly al80's and larger. If you use say an AL63, it goes all sideways really fast...

 

[billt4sf]

Telling the fill station what pressure you need in PSI results in fewer strange looks!

To the OP: What cylinder type does the rule apply to? One of my favorite things about metric: calculating remaining gas volume is as simple as (bar)x(tank volume e.g. 11L). If you know min gas (adjusted for your own SAC) this is an infinitely more scalable way to determine your turn/ascent pressure.

Great question: 80's. Oh, that's 15L tanks for the ROW.

 

[JohnN]

Telling the fill station what pressure you need in PSI results in fewer strange looks!

To the OP: What cylinder type does the rule apply to? One of my favorite things about metric: calculating remaining gas volume is as simple as (bar)x(tank volume e.g. 11L). If you know min gas (adjusted for your own SAC) this is an infinitely more scalable way to determine your turn/ascent pressure.

Ahh but your scheme disregards air not behaving as an ideal gas. IIRC the deviation is ~6% at 3500 PSI, an al80 holding about 77 cuft is pretty close to bang-on

 

[billt4sf]

you're listed as a DIR practitioner, if you are, then you'll know rock bottom and ascents are based on volume not pressure so that "rule" only works with certain tanks, particularly al80's and larger. If you use say an AL63, it goes all sideways really fast...

You are right. I should change that. Now of I could just figure out how...

Although I got a lot out of it, we did not finish Fundies, so I am not a true DIR practitioner.

- Bill

 

[dberry]

Title of thread: "Advantages to Imperial units"; OP listed only one (questionable) advantage. I'm also from a country cursed with legacy imperial units, but am totally conversant with metric / SI due to my line of work. I'm also a hobbiest woodworker and have no excuse for decades of dealing with the stupidity of imperial units, when I could have just traded in my rules and measures for metric versions. No excuse whatsoever.

 

[gr8jab]

I'm going to invent my own scuba units. I'm going to call them Jeffs. Feel free to join the craze if you find them useful. I'm sure they'll catch on.

1 tank = 5 Jeffs. Don't care about tank size. All you tech divers, and GUI, and whatever other complicated and fancy rigs some can go suck eggs. Most of us dive on 80CF tanks, and the other 1% of you can go do your own thing!

Now.. the big advantage...

When my buddy asks how much gas I have, I don't have to do some crazy hand flashing, counting, cross handed sign language. I just hold up one hand, and show the number of Jeffs. There's a reason humans invented a base-10 number system. Now divers can use the even more handy 5 finger salute!!

Cattle Boat guidelines:

1. Turn around the group at 3 Jeffs
2. Start your ascent at 2 Jeffs
3. Back on the boat with no less than 1 Jeff!

You suck down your air and feed bad you made the group turn around early? No prob, just get a 100CF tank. Keep the units as is... Full = 5 Jeffs

You've heard of Hogarhtian. I just invented Jeffarthian!!

 

[UWZen]

Great question: 80's. Oh, that's 15L tanks for the ROW.

Standard AL80 is 11.1L

Ahh but your scheme disregards air not behaving as an ideal gas. IIRC the deviation is ~6% at 3500 PSI, an al80 holding about 77 cuft is pretty close to bang-on

For scientific accuracy, very fair point. An AL80 holds 77cuft at 3000psi, 80cuft is nominal.

And that's where actual internal volume of a cylinder and pressure expressed in bar make gas planning something you can do on a napkin. 11L cylinder at 200(ish) bar gives 2200L for gas planning purposes.

Now let's say SAC of 20L/min for example, that 2200L is going to last you 110 minutes at the surface (1ATA). At 2 ATA you get 55min. At 3 ATA you get ~37minutes.

How many ATA are we at depth? 1 standard ATA + 1ATA per 10m. 10m is 2 ATA. 20m is 3ATA. Yay bar/L/meters!

Although personally I just make sure I have enough reserve Jeff in my tank to get me and my buddy up safely, then you never have to do any math, just keep an eye on your jeffometer