Metric or Imperial?

[Kevrumbo]

And why would one need to do this instead of just giving the tank capacity as they do in the US by its ft3, a one step process that doesn't need looking up or making any calculations? It isn't a big deal to look up the specs or do the multiplications and convert to ft3 from L (if need be) but if the capacity were available as it is in the US market (you noticed that the vendors readily have the capacity in ft3 in the US even for European made cylinders?), it would be more convenient.

Faber isn't the only mfg . in Europe, there at least two or more mfg. of steel tanks there. Faber may not be the largest one but I am not sure.

You are assuming that the whole world is using these cylinders and buy them from this same vendor in the US AND all of them will have access to the internet?

Then ask where you get your tank or tank rental filled.

So what if you have a 12L HPsteel 100, but the diveboat's compressor cannot fill to 3442 max service pressure, and can only go to 2800psi:

How much volume do you actually have? See, you're gonna have to do a "calculation" before you splash in with it anyway. . .

And it's just as easy in Metric with less significant & superfluous figures:

187bar short fill;

12L/bar multiplied-by 187 bar equals 2244 liters of gas.

(approx 80 cuft instead of a full rated fill of 100 cuft).

Check: In US Imperial, 2800psi/3442psi (100cuft) = 81 cuft

 

[Kevrumbo]

And why would one need to do this instead of just giving the tank capacity as they do in the US by its ft3, a one step process that doesn't need looking up or making any calculations? It isn't a big deal to look up the specs or do the multiplications and convert to ft3 from L (if need be) but if the capacity were available as it is in the US market (you noticed that the vendors readily have the capacity in ft3 in the US even for European made cylinders?), it would be more convenient.

Faber isn't the only mfg . in Europe, there at least two or more mfg. of steel tanks there. Faber may not be the largest one but I am not sure.

So what do you and your buddy do then, if you have dissimilar tank sizes for a rule-thirds turn pressure, with short initial fills?

For a buddy team with dissimilar tank sizes the point is to derive a usable turn pressure for your own tank, AFTER calculating a normalized volume of Rock Bottom and/or Thirds for the teammate with the smallest tank. The arithmetic is fairly easy and straight forward (in metric of course):

Example:
12 litre/bar tank multiplied by an initial fill of 180 bar equals 2160 litres;
15 litre/bar tank multiplied by initial fill 150 bar equals 2250 litres --

Therefore the 12 litre tank has the smaller volume at 2160 litres.

Calculate thirds of smaller volume tank's fill pressure of 180 bar, and you get 60 bar delta turn pressure. 12 litres/bar multiplied by 60 bar equals 720 litres volume (this is the normalized value with respect to the smaller tank).

Hence , the calculated delta turn pressure for the larger volume tank is:
720 litres divided-by 15 litre/bar equals 48 bar.

 

[agilis]

Then ask where you get your tank or tank rental filled.

So if what if you have a 12L HPsteel 100, but the diveboat's compressor cannot fill to 3442 max service pressure, and can only go to 2800psi:

How much volume do you actually have? See, you're gonna have to do a "calculation" before you splash in with it anyway. . .

And it's just as easy in Metric with less significant & superfluous figures:

187bar short fill;

12L/bar multiplied-by 187 bar equals 2244 liters of gas;
(approx 80 cuft instead of a full rated fill of 100 cuft).

Check: In US Imperial, 2800psi/3442psi (100cuft) = 81 cuft

Using fractions, it took me no more than 4 seconds in my head to come up with approximately 4/5 of the HP volume, or about 80 CF. I didn't have to write anything down, or do anything beyond visualize the problem.

Easy peasy, a simple and nearly instantaneous solution, and so close that a more exact calculation differs by a meaningless amount, 1cf, of vanishing real world significance.

Fractions and permutations of fractions can be visualized in an almost fluid state if you are very familiar with these wonderful things of infinite flexibility.

I paid attention in 4th grade. Otherwise the nuns would have made life unpleasant.

 

[TMHeimer]

Some very thorough and somewhat complicated responses to the OP's question. Did anyone think that we could actually avoid the "which is better" thing this time?

 

[Kevrumbo]

You regard the use of a watch, a depth gauge and an SPG while submerged during a dive as a math application? Using the tables is basic and simple, more application, not really calculation.

Serious calculation belongs in pre-dive planning. Most contingencies should be covered by substantial safety margins and common sense.

Using fractions, it took me no more than 4 seconds in my head to come up with approximately 4/5 of the HP volume, or about 80 CF. I didn't have to write anything down, or do anything beyond visualize the problem.

Easy peasy, a simple and nearly instantaneous solution, and so close that a more exact calculation differs by a meaningless amount, 1cf, of vanishing real world significance.

Fractions and permutations of fractions can be visualized in an almost fluid state if you are very familiar with these wonderful things of infinite flexibility.

I paid attention in 4th grade. Otherwise the nuns would have made life unpleasant.

If you can visualize the above in 4 seconds, why can't you comprehend the following hypothetical "serious calculation" during a 20 minute dive time???

[In other words --What don't you see, understand or appreciate about the advantage about working smartly with base unit quantities of 1 or 2 (bar/min), and a timer, a depth gauge and an SPG?]

Let's see . . .I have a full 200bar tank and consume 30 bar in five minutes at 18m depth:

Five minutes elapsed time and my SPG should read 170bar -check! It does.

Another five minutes for total Ten minutes elapsed time and my SPG should read 140bar -check! It does.

Fifteen minutes elapsed time and my SPG should read 110bar -check! It does.

Twenty minutes elapsed time and my SPG should read 80bar -check! It does,
and then start a multi-level ascent to the shallows & eventual safety stop. . .

Is it that hard to perform the simple iterative operation above during the dive, especially after years of dive experience (and that's some 40 years or more for yourself Agilis?). Fractions and permutations of fractions can be visualized in an almost fluid state if you are very familiar with these wonderful things of infinite flexibility --but you can't perform repeated subtraction from an initial total in your head???

 

[Patoux01]

Why would they lose their + capacity??? If you go to a hydro test facility that actually knows the standards and knows what they are doing, they would put the + (if appropriate). The steel tanks made in Europe don't have the + sign BTW, what it says on the tank is what it is and one doesn't worry about the +.

Sorry I'm late.
First: I don't believe someone wondering between metric or imperial lives anywhere else but the US, hence the + tanks exist.
Now, let's grab a + tank, let's say a LP77 will ya? What's its service pressure? 2400psi. What's its capacity? 77cuft AT 2640 PSI. So that LP77 is actually.... 70cuft at service pressure .

So if for any reason you don't get your + fill, which EXCEEDS service pressure, you're not diving an LP77, but an LP70. However, it's still the same tank. So why is it called 77 if it doesn't hold that unless overfilled?

Even worse, look at the LP85! First, it holds 83 cuft when overfilled. Which means it will old about 76 when filled to service pressure.


Are you really gonna argue that "it makes much more sense to not know the internal volume of the tank" then?

 

[Kevrumbo]

Some very thorough and somewhat complicated responses to the OP's question. Did anyone think that we could actually avoid the "which is better" thing this time?

More Metric vs. US Imperial Units (Bar vs PSI)

Here's another more comprehensive example:

Modified Thirds Planning and Calculating On-the-Fly Gas Needed to Do a Lost Buddy Search in a Wreck Penetration:

(The harder to figure but equivalent US Imperial example follows later down below in bold):

Suppose you originally planned a wreck penetration starting with 200 bar at the entrance, with an openwater Rock Bottom minimum gas reserve of 50 bar. So 200 minus 50 bar equals 150 bar usable for the penetration --Modified Thirds of this value is 50 bar (one-third of 150 equals 50), so you would turn-around for egress when you consume 50 bar of gas with an actual SPG reading of 150 bar. If you needed to do a gas-sharing emergency egress with your buddy at this point, you would together need 100 bar to get out of the wreck, with 50 bar Rock Bottom remaining to get both of you to the surface (or your Oxygen deco bottle stop of 6m).

Lets say you used up 30 bar already getting to the entrance of the wreck for a total of 170 bar pressure available --can you quickly recalculate Modified-Thirds?

No problem with bar pressure metrics: 170 bar minus 50 bar Rock Bottom yields 120 bar usable for the penentration; One-Third of 120 bar is 40 bar which is your new Modified Thirds turn pressure value. Therefore you would turn the dive when you consume 40 bar for an actual turn pressure SPG reading of 130 bar (170 bar minus 40 bar equals the actual turn pressure SPG reading of 130 bar).

What if you lose your buddy at this instance, at the farthest distance inside the overhead your Modified Thirds value allows? How do you calculate the amount of gas to do a Lost Buddy Search?

Easy! At your turn around pressure reading of 130 bar on the SPG, simply add your Rock Bottom value to your Modified Thirds value (50 bar Rock Bottom plus 40 bar Modified Thirds equals 90 bar); Put a line-arrow pointing the way out on your mainline that you've laid, and take reference note of where you are inside the overhead at that exact point as well. Now go and search for your buddy with the understanding that you must be back at this line-arrow marker by the time your SPG reads this actual value (90 bar). So you would have from 130 bar down to 90 bar reading on your SPG, or 40 bar delta of gas to search for your buddy --if you were to do a straight line search down a long corridor inside the wreck for example, tactically you should use 20 bar out and 20 bar back to your line-arrow marker for a delta of 40 bar, and an actual end of search SPG reading of 90 bar-- you must start your egress whether you found your buddy or not when you use up this 40 bar delta of gas, at the line arrow marker, with the actual 90 bar final reading on your SPG.

At any point before your Modified Thirds turn pressure, for a lost buddy search, the final egress pressure is figured just by adding your Rock Bottom value to the amount of gas you've consumed on the penetration up to that point --for example you start with 170 bar on your SPG and you lose your buddy with 140 bar SPG reading for a delta consumption of 30 bar. 50 bar Rock Bottom plus delta consumption of 30 bar equals 80 bar. Drop a line arrow, and now you've got from 140 bar down to 80 bar (a tactical delta search pressure of 60 bar) to look for your buddy, and be back to your line arrow to egress smartly when your SPG reads 80 bar.

At any point after your Modified Thirds turn pressure, all you need to do to figure out a final egress pressure for a lost buddy search is to subtract your Modified Thirds value from your actual pressure reading, and place a line-arrow pointing out at this point on your mainline. For example, if you're egressing and you lose your buddy with 120 bar actual reading on your SPG: Subtract the Modified Thirds value of 40 bar from 120 bar -which equals 80 bar- and it is this actual reading that you must have on your SPG when you get back to your line arrow to successfully exit the wreck with all your Rock Bottom still available to reach the surface. Another way of looking at this, at your nominal turn-around point & afterward on egress, the amount of gas tactically available for a lost buddy search is always just your Modified Thirds value --in this case 40 bar.

Remember that on a lost buddy search, you will deliberately encroach and use up the Modified Thirds Reserve Value needed for an emergency gas-sharing egress contingency (and possibly use up Rock Bottom as well) --in other words, if you do find your lost buddy and worst of all worst scenarios he happens to be out-of-gas in a silt-out . . .well dea ex machina. I hope you're in a 3-person Team, somehow make it out and run into other divers on the outside who can donate gas & assist. .

[Note: the above gas plan is taken from wreck penetration dives on the HMCS Yukon (San Diego); USS New York (Subic Bay Philippines); HMAS Perth/USS Houston (Sunda Strait Indonesia); and various wrecks in Truk Lagoon. Depth 30m using twin 11L/bar tanks (double AL80's) and Oxygen deco.]
---------

Same thing as above but now using difficult & cumbersome US Imperial Units in PSI:

Suppose you originally planned a wreck penetration starting with 3000 psi [double AL80’s] at the entrance, with an openwater Rock Bottom minimum gas reserve of 750psi. So 3000 minus 750 equals 2250psi usable for the penetration --Modified Thirds of this value is 750 psi (one-third of 2250 equals 750), so you would turn-around for egress when you consume 750 psi of gas with an actual SPG reading of 2250 psi. If you needed to do a gas-sharing emergency egress with your buddy at this point, you would together need 1500psi to get out of the wreck, with 750psi Rock Bottom remaining to get both of you to the surface (or your Oxygen deco bottle stop at 20').

Let’s say you used up 450psi already getting to the entrance of the wreck for a total of 2550 pressure available --can you quickly recalculate Modified-Thirds?

Well . . . 2550psi minus 750psi Rock Bottom yields 1800psi usable for the penentration; One-Third of 1800psi is 600psi which is your new Modified Thirds turn pressure value. Therefore you would turn the dive when you consume 600psi for an actual turn pressure SPG reading of 1950psi (2550 minus 600 equals the actual turn pressure SPG reading of 1950psi).

What if you lose your buddy at this instance, at the farthest distance inside the overhead your Modified Thirds value allows? How do you calculate the amount of gas to do a Lost Buddy Search?

NOT very easy! At your turn around pressure reading of 1950psi on the SPG, simply add your Rock Bottom value to your Modified Thirds value (750psi Rock Bottom plus 600psi Modified Thirds equals 1350psi); Put a line-arrow pointing the way out on your mainline that you've laid, and take reference note of where you are inside the overhead at that exact point as well. Now go and search for your buddy with the understanding that you must be back at this line-arrow marker by the time your SPG reads this actual value (1350psi). So you would have from 1950psi down to 1350psi reading on your SPG, or 600psi delta of gas to search for your buddy --if you were to do a straight line search down a long corridor inside the wreck for example, tactically you should use 300psi out and 300psi back to your line-arrow marker for a delta of 600psi, and an actual end of search SPG reading of 1350psi-- you must start your egress whether you found your buddy or not when you use up this 600psi delta of gas, at the line arrow marker, with the actual 1350psi final reading on your SPG.

At any point before your Modified Thirds turn pressure, for a lost buddy search, the final egress pressure is figured just by adding your Rock Bottom value to the amount of gas you've consumed on the penetration up to that point --for example you start with 2550psi on your SPG and you lose your buddy with 2100psi SPG reading for a delta consumption of 450psi. 750psi Rock Bottom plus delta consumption of 450psi equals 1200psi. Drop a line arrow, and now you've got from 2100psi down to 1200psi (a tactical delta search pressure of 900psi) to look for your buddy, and be back to your line arrow to egress smartly when your SPG reads 1200psi.

At any point after your Modified Thirds turn pressure, all you need to do to figure out a final egress pressure for a lost buddy search is to subtract your Modified Thirds value from your actual pressure reading, and place a line-arrow pointing out at this point on your mainline. For example, if you're egressing and you lose your buddy with 1800psi actual reading on your SPG: Subtract the Modified Thirds value of 600psi from 1800psi -which equals 1200psi - and it is this actual reading that you must have on your SPG when you get back to your line arrow to successfully exit the wreck with all your Rock Bottom still available to reach the surface. Another way of looking at this, at your nominal turn-around point & afterward on egress, the amount of gas tactically available for a lost buddy search is always just your Modified Thirds value --in this case 600psi. . .

--->The main point is that extra significant digit in the PSI quantities makes the arithmetic less easy to see & calculate realtime on-the-fly for this contingency, compared to the Bar pressure evaluation above. . .
_______________

 

[beester]

Kev you are totally right, and your arguments are perfectly sound. But you are not going to "win" this one...

On on side you are preaching to the choir... and these people all agree with you.

On the other hand you have basically 2 guys:
- One has been diving since the 60s and feels (maybe rightfully so... no judgement) that training since he started diving hasn't moved on, but rather moved backwards. He's doing tropical recreational dives... and has been using imperial all his life. It works... and if it works why should he change (and he's right).
- One is an instructor who uses pressure rating in the metric system to win this argument. Which basically tells me he has no practical experience with metric diving.

Let me explain. I'm planning a recreational dive to 30m (100 ft). I'm diving a double 12L 232b, buddy no 1 dives a 15L 200 bar and my last buddy dives a 10 L 300 bar. In practice this is not likely going to happen because only very very old tanks are rated to 200b.... but I digress.

For a 30m recreational dive I know I'll need about 1200L of gas to return to the surface doing a safe ascend while sharing gas with my buddy(min deco).

So for me in my double 12L = 50bar = mingas/rock bottom
So for buddy no 1 in his 15L=70bar = mingas/rock bottom
So for buddy no 3 in his 10L 300b = 120bar =mingas/rock bottom

The fact that someone is diving a different pressure rated tank is irrelevant for these dives. I also know immediately that our limiting factor is the 10L (because of ideal gas law vs vanderwaals that 300 bar 10L is not holding 3000 liter but more likely 2700 liter). 3rds can be used on this the same.

In technical diving, where I want to know more than just when to ascend, yes total volume of gas comes into the equation, and I'll need to use kev's calculation if we are diving disimilar tanks.

 

[Kevrumbo]

Kev you are totally right, and your arguments are perfectly sound. But you are not going to "win" this one...

On on side you are preaching to the choir... and these people all agree with you. . .

.

Well . . .since ScubaBoard went to the new format, all my "user notes" were deleted. The best ones were from the Metric/Imperial ("Star") Wars, and so going through old posts & threads throughout the years --"reposting and repositing" it all here. . .

"Good objective & practical examples always trump plain & lame rhetoric. . ."

(Ideal Gas vs Vanderwaals and their potential factors in tank matching was a good contribution & thought exercise too).

 

[XS-NRG]

Dive whatever you understand. For me I don't understand meters because I'm so used to using feet as a reference. Doesn't mean meters may not be better (I know if certainly makes more sense and is more logical) but I can't visually reference something using meters. It's greek to me. I also use PSI over BAR. For this one it could go either way I just learned on PSI. As long as you know your working pressure on your tank and can identify 1/2 a tank or the thirds rule then you can use either. The only metric I use actually is temperature because Fahrenheit makes no logical sense to me. I bought a computer that specifically allows me to pick and choose the measures I want for each item vs an all metric display or all imperial display. There is no right or wrong answer to this. It's what works for you.