US divers using metric?

[Kevrumbo]

[h=2]Example Gas Planning for the Basic OW 18m/60' Limit [/h]

Given: Emergency "Stressed" 28 litres/min SCR (Surface Consumption Rate, also referred to as SAC), with 18 meters (2.8 ATA) depth NDL, and with one minute stops every 3 meters to surface:

2.8 x 28 x 1 = 78.4
2.5 x 28 x 1 = 70
2.2 x 28 x 1 = 61.6
1.9 x 28 x 1 = 53.2
1.6 x 28 x 1 = 44.8
1.3 x 28 x 1 = 36.4
1.0 x 28 x 1 = 28

Sum Total: 372.4 litres 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 litres/bar. So 372/11 = approx 35 bar.

That's 35 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: 35 x 2 = 70 bar. Therefore your Rock Bottom Reserve is 70 bar showing on your SPG --if there is no emergency air sharing contingency at that instant, 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 70 bar equals 130 bar usable. Let's use 20 bar of this 130 to get squared away in good trim & buoyancy on the descent to 18m which leaves 110 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 110 bar or 55 bar consumed.

So for a nominal dive with an SCR of 22 litres/min on a 11L (AL80) tank, your resulting pressure SCR will be 2 bar/min [22 divided-by 11 equals 2bar/min]. 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 pressure SCR.

18 meters depth is 2.8 ATA (divide 18 by 10 and add 1 equals 2.8); your 2bar/min 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 down of 56bar. . .

Okay, you splash in to start the dive with 200bar and a 70bar rock bottom. You use up 20bar on descent --initial exertion, inflating your wing etc. -so you now have 180bar with 70bar rock bottom at present depth 18m, a net usable of 110bar (180 minus 70 is 110). 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 that you've used up 56bar (see previous paragraph above), so you decide to turn the dive. You should now know you have roughly 50bar of usable gas left before encroaching on your Rock Bottom Reserve, and realize that you must start a nominal ascent within the next 10 minute interval. 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; you immediately check your SPG and it reads 70bar --bingo! Rock bottom has arrived so start ascending to the shallower depths . . .or if your Buddy just happens to blow his tank neck O-ring at that instant --you know you've got plenty of 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, 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!)
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Notes for American Divers needing US Imperial Units equivalent conversions:
Stressed SCR of 28 litres/min is equivalent to 1 cuft/min;
Nominal SCR of 22 litres/min is approx 0.75 cuft/min.
2 bar/min is same as 29psi/min;
5.6 bar/min is 81psi/min

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.
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Your common counting numbers, or reference cardinal numbers, for depth on a Dive Table 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

So hence the 3/10 or 10/3 conversion factors between the two number sequences above. . .
​

 

[Diving Dubai]

One could argue that because metric is actually the common term for the International System of Units or SI then if you are diving abroad then you should use SI units. but that would be a very short sighted view.

I am a child of both systems - growing up in the UK I was taught metric but of course almost everything we did was imperial - I do have a mix of systems in my head. For instance I measure using millimeters and meters ( although some areas of the world prefer centimeters and meters) I still think of aircraft altitude in feet, and until recently distance in miles. Now I deal with kilometers but thats no problem as if a road sign say 100km to the destination and your speedo is in KM you can do the mental maths. I still do a conversion to mph for speed in my head. PSI is still used for my car tyres but tank pressure is in Bar. I measure body weight in Stones and pounds, but everything else in Kilograms. I still do rough measures in feet as some people can't visualise 1.220m but can visualise 4'

I find fractions of an inch and never have- just dumb to measure with - but was brought up using spanners of the AF size - of course in the UK there were 5/16 AF, whitworth and BSP - all of which were differing spanner sizes, thank got a 10mm spanner is 10mm!

Volume is in litres except I've never got to grips with litres per 100km and think in mpg (but then is that a US gallon or a imperial gallon?)

I guess the SI system is easiest to mentally work with as it's all base 10, however the problem is always what people can relate too from previous experience.

As others have said - for recreational diving you could just have coloured (or is that colored) areas on your SPG (green yellow red) - in fact I suggest that unless someone is actually asked their tank contents, when they check their spg they just register the gauge clock position. Do you actually care how many gallons or litres you have in your car fuel tank? I bet not - you go by the gauge position...

When I had a buddy with a PSI SPG we simply showed each other our gauges - if teh needle is less than the 10 O clock position time to ascent and if its at the 8 O clock position then you should be at you safety stop (rule of thumb) - Where there is a will there is a way....

 

[Redshift]

As others have said - for recreational diving you could just have coloured (or is that colored) areas on your SPG (green yellow red)

Well, they are already. Usually SI pressure gauges will be red below 50 bar. I imagine Imperial gauges will be similar below an equivalent pressure?
Recreational divers can easily use the half tank and reserve signs (if using similar tanks) without the need to actually say how much gas they have at all time. It's easy to understand and there are no maths involved.

 

[Tigerman]

The whole "people cant visualise 1.2m but they can visualize 4'" argument keep coming up.. and it keep being wrong... If you can visualize 4' you ARE visualising 1.2m, the only difference is what you call it...

 

[tomfcrist]

If you were consistent (and smarter), you would utilize the world standard all the time --the best uses the best. . .

Unfortuantely, I can't give you a new & better brain as a trinket either. . .

Are you implying that because he and I use Imperial we are not as smart as you? And FYI....the most common is NOT always the best.

 

[XJJoe01]

I am an imperial diver living abroad and my DM uses metric. We do the normal half tank and low air signs that are discussed during dive planning. Never an issue. Just know 1 meter is 3 feet and 200 bar is 3000 psi which makes dive planning easy.

 

[alewar]

I think GUE should do the first step and switch to metric. They love standardizing things and this is the huge thing still missing.

I'm happy the Americans at least use standard time units though. Imagine if they had an imperial calendar with 73 seconds per minute and 49 minutes per hour.

 

[FinnMom]

Everyone has to learn metric for science, many use it for work, calculations are so much easier, wouldn't it be nice if at least 2o yrs from now you didn't need to buy 2 sets of spanner wrenches, when baking: wouldn't it be nice not to have to try to calculate cups are in 200 ounces, etc. etc. etc.

It would be the best upgrade since indoor plumbing.

 

[Storker]

for recreational diving you could just have coloured (or is that colored) areas on your SPG (green yellow red)

If every tank had the same water volume and pressure rating, that might not be a bad idea. However, they haven't. Some (metric) tanks are 200 bar, some are 232 bar, some are 300 bar. Some are 10L, some are 12L, some are 15L, and some are even larger.

For a 200 bar tank, half pressure is 100 bar, for a 232 bar tank it's 115 bar and for a 300 bar tank it's 150 bar. And due to compressibility, half capacity for a 300 bar tank isn't 150 bar, it's about 130 bar. Where should the yellow zone start?

If I rent or borrow a 15L 200 bar tank, I have about the same amount of air left at 35 bar as I have in my standard 10L 300 bar tank at 50 bar. Where should the red zone start?

Give me the numbers, please. I'll do the math myself.

 

[PfcAJ]

If you were consistent (and smarter), you would utilize the world standard all the time --the best uses the best. . .

Unfortuantely, I can't give you a new & better brain as a trinket either. . .

/snipped the novel​

That's quite alright. Your interpretation of "new and better" has always been a bit suspect, and my old and inferior brain has got me along just fine.