Metric or Imperial?

[dberry]

Just get a dual unit SPG: ScubaPro Dual Scuba Pressure Gauge - SPG in Metric and Imperial | SPG
Easy Imperial US/Metric Conversions for depth & pressure, that you can do in your head:
<snip>

Like I said in my post, I'm personally not concerned about the conversion (I work with metric units daily and teach this stuff, in a slightly different context.) I was asking whether using metric (with dual gauges, etc.) here in the U.S. with insta-buddies on imperial would be asking for trouble. Thoughts?

 

[Littlerayray]

I'm from Canada you tell me the temp of water in centigrade and I'm lost tell me the air temp on farenheight I'm lost ask me depth of water in meters I can figure it out with some simple math tell me the depth in feet and I understand immediately volume of tanks give it to me in c.f. air pressure give it to me in psi

Farenheight Temps are more sensitive without using decimals

 

[KevinNM]

What is easier 11l*100bar=1100l of gas, or 1000*80/3000=26cf of gas?

Not to mention that an AL80 doesn't actually have 80 cubic feet at service pressure, So I hope you didn't need that extra minute or two of air that you don't actually have but are including in your plans.

 

[Kevrumbo]

Like I said in my post, I'm personally not concerned about the conversion (I work with metric units daily and teach this stuff, in a slightly different context.) I was asking whether using metric (with dual gauges, etc.) here in the U.S. with insta-buddies on imperial would be asking for trouble. Thoughts?

"You teach best what you need most to learn. . ." (Richard Bach).

Read and apply some of the ideas implied for mixed imperial/metric communication in this great pdf monograph on Gas Planning:

http://api.ning.com/files/KHeKOD-fw...pVsh/BattlefieldCalculationsDeliaMilliron.pdf

Also give your regular buddies a primer lecture on the Metric System, starting with these conversions (and motivated by the pdf above):

Easy Imperial US/Metric Conversions for depth & pressure, that you can do in your head:

Depth in Meters multiplied by 10/3 gives Depth in Feet;
Feet multiplied by 3/10 gives Meters.
Example: 18m(10/3) = 60' ; 60'(3/10) = 18m

Pressure Bar multiplied by 3/2, and multiplied again by 10 gives Pressure PSI;
Pressure PSI multiplied by 2/3, and divided by 10 gives Pressure Bar.
Ex): 200bar(3/2)(10) = 3000psi ; 3000psi(2/3)/10 = 200bar.

Your common counting numbers, or Reference Cardinal Numbers, for depth as seen in most Dive Tables are:

Imperial US (feet) by 10's:
Ex): 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110 etc

Metric System goes by 3's:
Ex): 3, 6, 9, 12, 15, 18, 21, 24, 27, 30, 33 etc

Practice the depth conversion factors above ("3/10" and "10/3") between the two number sequences. . .

Let your regular buddies know that eventually with rote practice & time, they'll learn to accept using pure metric unit quantities rather the translating them into Imperial units, and vice versa converting back to metric.

If you're anticipating an unknown insta-buddy, try writing out & having ready ahead of time on a slate or wetnotes, a conversion table as well as the dive profile time, depth and/or gas pressure equivalents that you plan to act on during the dive. Go over the plan together and then give it to the buddy before splashing in. Another solution is to offer the insta-buddy the use of a dual unit SPG (i.e. swap out his SPG and loan a spare dual unit SPG), along with the slate/wetnotes conversion plan.

Finally If they can't or don't want to swap out (i.e. he has wireless or hose connected Air Integration Computer), or don't have the aptitude to learn Metric, then you're just gonna have to do the conversions yourself and communicate in Imperial.

 

[DevonDiver]

I more or less follow the rule of thirds, and can't offhand think of a situation where any math calculations would be useful to me during a dive. Planning a dive is another matter, but I did write "while diving", yes?

As a tech diver, I might need to calculate my gas-time in the event of a gas loss. Can I still do my planned deco? If not, what can I accomplish? etc etc

But this is the 'advanced diving', not tech diving, forum... so I'll fall back to the "I have reason to return to my start point/shot-line etc" scenario. Rule of Thirds gets you there and back with a reserve; but there are contingencies that could threaten that. Likewise, we have scenarios involving emergency/unplanned deco... can you do it with the gas you have?

An ability to do some simple SAC based math and decision making is a good tool to have in the box...

 

[Kevrumbo]

I agree completely. This is exactly what I do, except that an arbitrary 30% does not enter into it. If my SPG needle is dropping faster than expected I either modify or abort the dive, depending on the degree of unexpectedly high gas use and what the circumstances are. Fighting a stronger than expected current during a shallow inlet dive is one thing, unexpectedly rapid PSI drop on a deep dive quite another.

As a tech diver, I might need to calculate my gas-time in the event of a gas loss. Can I still do my planned deco? If not, what can I accomplish? etc etc

But this is the 'advanced diving', not tech diving, forum... so I'll fall back to the "I have reason to return to my start point/shot-line etc" scenario. Rule of Thirds gets you there and back with a reserve; but there are contingencies that could threaten that. Likewise, we have scenarios involving emergency/unplanned deco... can you do it with the gas you have?

An ability to do some simple SAC based math and decision making is a good tool to have in the box...

Here's another manual gas planning example & exercise for a novice OW Diver Buddy Team to the Basic Open Water NDL Limit of 18m/60':

Pre-Dive Plan: Given an Emergency "Stressed" 30 liters/min per ATA Surface Consumption Rate (SCR), with 18 meters depth NDL, and with an arbitrary conservative controlled and slow 0.5 minute stops (30sec) every 3 meters ascent rate to a Safety Stop --let's calculate the "Rock Bottom Reserve" value to get to the surface starting from 18meters (same as 2.8 ATA):

Depth x SCR x Minutes = Liters
2.8 x 30 x 1 = 84
2.5 x 30 x 0.5 = 37.4
2.2 x 30 x 0.5 = 33
1.9 x 30 x 0.5 = 28.5
1.6 x 30 x 2 = 96 [2 min Safety Stop]
1.3 x 30 x 0.5 = 19.5
1.0 x 30 x 0.5 = 15

Sum Total: 313.4 liters gas needed to ascend to surface for an emergency contingency.

Divide the above total by the metric tank rating of the Scuba cylinder in use; for this example let's use the AL80 tank again which has a metric rating of 11 liters/bar. So 313 divided-by 11 = round up to approx 30 bar.

That's 30 bar to get you to the surface --to get yourself and sharing gas with an Out-of-Air Buddy you will need at least twice this amount: 30 x 2 = 60 bar. Therefore your Rock Bottom Reserve pressure is 60 bar showing on your SPG --if there is no emergency air sharing contingency at that instant with your SPG reading 60 bar actual, just continue the dive but start a easy nominal ascent to the shallower depths between 5m and 9m. Finally, be at your 5m safety stop with your buddy with no less than 50 bar showing on the SPG.

Your usable gas for the dive is your starting pressure subtracted by the Rock Bottom Reserve --so 200 bar minus 60 bar equals 140 bar usable. Let's use 20 bar of this 140 to get squared away in good trim & buoyancy on the descent to 18m which leaves 120 bar usable. Now if your dive plan calls for returning to near the vicinity of your original point of entry (like a beach dive for instance), then turn the dive back around when you use half of 120 bar or 60 bar consumed.

So for a dive with a nominal volume SCR of 22 liters/min per ATA on a 11L per bar (AL80) tank, your resulting pressure SCR will be 2 bar/min per ATA [22 divided-by 11 equals 2bar/min per ATA]. Your depth in meters, which converts easily to ATA (simply divide-by-10 and add 1) becomes your multiplier depth factor for your 2bar/min per ATA pressure SCR.

18 meters depth is 2.8 ATA (divide 18 by 10 and add 1 equals 2.8 ATA); your 2bar/min per ATA pressure SCR at depth -or Depth Consumption Rate (DCR)- now becomes 5.6 bar/min. [2.8 times 2bar/min equals 5.6 bar/min]. So 10 minutes at depth 18m on an AL80 (11L/bar) tank in nominal conditions, you would expect to consume 56bar of gas (10min times 5.6 bar/min equals 56bar), and your SPG reading to show a delta of 56bar less for that 10 minute elapsed time interval. . .

Okay, you splash in to start the dive with 200bar and a 60 bar rock bottom. You use up 20bar on descent --initial exertion, inflating your BCD/wing etc. -so you now have 180bar with 60 bar rock bottom at present depth 18m, a net usable of 120 bar (180 minus 60 is 120 bar).

Finning out for 10 minutes, an easy swim looking at all the cool marine life around -- you already know by the end of this 10 minute interval and confirmed with a SPG reading that you've used up 56bar, close to half of a net usable of 120 bar, so you decide to turn the dive coming back around on a reciprocal course. You should now know you have roughly 60 bar of usable gas left before encroaching on your Rock Bottom Reserve, and realize that you must start a nominal ascent from 18m at the end of the next 10 minute interval. (Note: 20 minutes elapsed bottom time at 18 meters depth is well within NDL).

Alright, so while turning back, you see a turtle and decide to chase it for a few minutes, huffing & puffing on your reg, until it dives down below your operational depth of 18m; because of this unplanned turtle excursion & physical exertion, you immediately check your SPG and it reads 60 bar --bingo! Rock bottom has arrived so start ascending to the shallower depths & eventual Safety Stop . . .or if your Buddy just happens to blow his tank neck O-ring at that instant with a catastrophic loss of his remaining gas supply --you know you've got enough breathing gas margin for a controlled air-sharing Emergency ascent profile as described above. . .

Be aware that depending on environmental conditions and physical exertion (cold water, stiff current, long surface swim, thick surface kelp forest, heavy workload at depth etc), you may have to reserve a greater Rock Bottom Reserve --perhaps as high or even over 100 bar on the 11L/AL80 tank to be conservative. (Even better just abort the dive, wait another day or find another easier site to dive!)

 

[Storker]

And that calculation is only valid if you believe compressed air behaves as an ideal gas (hint: it doesn't)

Up to some 230-250 bar (3350-3500 psi), the nonideal behavior is small enough to be insignificant. It's only in the range from 250 to 300 bar (3500-4350 psi) that it becomes an issue. That's the reason that while a 15L 200 bar tank holds about 3000 surface liters, a 10L 300 bar tank holds some 10% less than that, even if the two tanks have the same nominal capacity.

So up to your normal fill pressures, compressed air does behave like an ideal gas. At least within the limits of measurement and significance. When I use my 300 bar tanks, I take the nonideality into account, but I don't if I'm using a 200 bar or 232 bar tank.

 

[Peter69_56]

Metric in a heart beat and DIN. Metric is easy and simple. DIN is reliable.

 

[Bennno]

As a tech diver, I might need to calculate my gas-time in the event of a gas loss. Can I still do my planned deco? If not, what can I accomplish? etc etc

Likewise, we have scenarios involving emergency/unplanned deco... can you do it with the gas you have?

An ability to do some simple SAC based math and decision making is a good tool to have in the box...

What would be a situation were you have to calculate gas-time? If it's an NDL dive you just abort the dive and even with a short unexpected deco you'd still have your buddy to give you gas.
If you do a tec dive you planned for lost gas/longer deco anyways, right? If you plan to deco on back gas than you have an extra gas as a cushion.

I literally, never once calculated anything while in the water.

Metric really only has an upside when you have to calculate stuff... if you're mostly diving with other Americans it's probably more practical do use the imperial system.

 

[tbone1004]

recalculating thirds, you can plan to recalc but you can't do it until you're there.
Calculating how much time you have left in a room if you got there before thirds and decided to poke around. A lot easier to quickly glance at the clock because you're already racking up deco, etc etc. It's not all that common, but it's not a bad thing to be able to do.
ratio deco is also easier in metric than in imperial....