US divers using metric?

[tkdgodess]

I buy gasoline in litres, buy food by the gram and kilo, drive by km's, weigh myself by the pound, measure myself by inches and feet, measure things by cm, temp in Celcius, dive in feet and psi....

 

[Kevrumbo]

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.

If compressibility is that important for your applications, then use whatever significant "fudge" factor you need like 10% less the ideal gas calculation calls for (see bottom p.35 of Vance Harlow's Oxygen Hacker's Companion as it applies to adding O2 first in mixing a Trimix Blend). Your "yellow/red" precision warning zones on your SPG is moot; your usable breathing gas should actually be quantitatively based on the vital requirements needed for you and your buddy on the dive with plenty of margin (i.e. Rock Bottom, Modified Thirds Pressure, Lost Buddy Gas Reserves etc as described in earlier posts), IOW, the classic admonishment "be back on the boat with 33bar/500psi" and exactly where it is on your SPG for your breathing gas mix is not a very helpful, objective or practical idea to utilize.

Here are a few examples now with different size tanks and smartly doing the math to derive a useful bar per minute value to use with your SPG:

Given with a 22 litres/min volume SCR and a variety of different tank sizes, your pressure SCR in bar/min obviously varies inversely, depending on the size tank in use, for any fill pressure in bar up to cylinder's max rating:

22 litres per minute -divided-by- 11L/bar tank (AL80): 2 bar/min
22 litres per minute -divided-by- 13L/bar tank (AL100): 1.7 bar/min
22 litres per minute -divided-by- 22L/bar twinset (double AL80's): 1 bar/min
22 litres per minute -divided-by- 34L/bar twinset (double Pressed Steel 104's with a cave fill): 0.7 bar/min

Your volume SCR, 22 litres/min in the example above, is the "arbitrary" constant across all metric tank ratings ("Arbitrary" in this instance because consumption rate depends on environmental conditions, physical fitness, workload etc). In the example above, the goal is to utilize a pressure SCR that is commonly understood to be based on the metric tank rating in use, because your SPG is is in bar pressure units --NOT volumetric liter units. Your depth multiplier factor to convert pressure SCR (Surface Consumption Rate) to pressure DCR (Depth Consumption Rate) is again your dive depth in ATA -easily derived from depth in meters by dividing-by-ten and adding one.

---------- Post added July 2nd, 2014 at 11:58 AM ----------

Now since this is the basic scuba forum, here's an exercise to show why a beginner/novice should not go beyond the 18m/60' depth range:

A Quick Contingency "Rock Bottom" Calculation and Gas Plan Estimate for Basic Open Water to Depth of 30 meters:

For a single 11 litre tank (AL80), a total of 11 litres/bar metric tank rating and a volume Surface Consumption Rate (SCR) of 22 litres/min -same as a pressure SCR of 2 bar/min*ATA (divide 22 litres/min by 11 litres/bar)- using an example NDL air dive to 30m (4 ATA) depth in Open Water.

Emergency Reserve/Rock Bottom pressure calculation, from 30 meters with one minute stops every 3 meters to the surface,
-->Just "tally the ATA's":
4.0
3.7
3.4
3.1
2.8
2.5
2.2
1.9
1.6
1.3

Sum Total: 26.5

Multiplied by 2 bar/min*ATA equals 53 bar Rock Bottom absolute reading remaining on your SPG. --this also happens to be the pressure in bar needed for one person in an emergency contingency to reach the surface with the above minimum very conservative NDL ascent profile.

So ideally for a two person buddy team, multiply 53 by 2 which is 106 bar for both to reach the surface (sharing in a buddy Out-ot-Gas contingency).

But realistically, for two experienced divers stressed: 106 bar plus 30% of 106 bar equals 138 bar Rock Bottom SPG reading.

For two novice divers stressed: 106 bar plus 100% of 106 bar equals 212 bar (!!!)
--->obviously then, two novice divers on single 11L tanks should not be diving to 30m for any significant length of time. . .

 

[DuboisP]

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.

standard time unit is 24h a day
not 12am plus 12pm a day

 

[Storker]

Your "yellow/red" precision warning zones on your SPG is moot; your usable breathing gas should actually be quantitatively based on the vital requirements needed for you and your buddy on the dive with plenty of margin (i.e. Rock Bottom, Modified Thirds Pressure, Lost Buddy Gas Reserves etc as described in earlier posts)

My point exactly...

 

[84CJ7]

standard time unit is 24h a day
not 12am plus 12pm a day

I have all my stuff set to "military" time.
Don't forget we also write the date wrong, there are a laundry list of reasons for year/month/day. I will be the first to admit our system of mixing up the size order of units of time is just ass backwards.
As I recall the military here actually spells out the first three letters of the month to avoid date confusion on documents.
That one would be easy to change if the full year was always written because you would clearly see the 4 digit year first and know what you are looking at.

Also people will always need multiple sets of tools, we have made too damn many things already to just throw them all away.
Though on that topic I just recently discovered the LP inflator hose that came with a used reg somehow takes a US sized tool to remove rather than the metric ones on the rest of my stuff and seriously considered pitching it rather than carrying an extra tool.

 

[Tigerman]

When it comes to dates (as we here in northern europe write as dd/mm/yyyy) my personal opinion is that it should be yyyy/mm/dd. Why? Because it sorts properly

Its easier to browse my stuff by year and month than day..

 

[dmoore19]

Actually were glad Europe stuck with the standard time units instead of 20 minute hours.

standard time unit is 24h a day
not 12am plus 12pm a day

 

[Tigerman]

Well, you should be aware that african time, although APPARENTLY the same as the european and us one is not really the same though...

5 minutes in africa or the middle east is a matter of insha'allah...

 

[drrich2]

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.

Ironically, time is the one parameter where the metric people seem to have given up in favor of historical precedent. Think about it; metric measures are based on transitions of 10's; cm, m, km - at each stage varying by a factor of 10.

Time, though, is 60 sec/min, 60 min/hour, 24 hour/ day, 7 day/week, roughly 4 week/month (but 52 weeks & 12 months/year, and there's the leap year thing). Nothing 'metric' about that, except the metric people seem disinclined to try to 'reinvent the wheel' and push a new time system on us.

So, where time's concerned, even the metric people do it 'imperial style.'

Richard.

 

[Kevrumbo]

Six Minute Ascent to Surface From Oxygen Deco Stop.
. .

Typically, this is the last ascent deco profile after an O2 stop at 20 feet/6 meters; the very slow ascent rate taking 6 minutes to the surface while still breathing O2.

Which is easier & smarter --maintaining a three-and-one-third feet per min ascent rate from a 20 foot depth to the surface for a total ascent time of six minutes?

Or using metric system from 6 meters depth and ascending at a 1 meter per minute rate to reach the surface in six minutes?

This is the way you do this with a Bottom Timer having a stop watch function (i.g. a Liquivision Zen):

For each one minute increment in time, your depth should decrement by one meter --and the sum of the two should always equal six.

Example:
At the one minute mark, your depth should be 5 meters: 1 + 5 = 6;
At the two minute mark, your depth should be 4 meters: 2 + 4 = 6;
At the three minute mark, your depth should be 3 meters: 3 + 3 = 6. . . etc

Therefore you should be reading approximately six minutes total elapsed time when your depth reads zero meters and your Bottom Timer goes into Surface Interval Mode.

For the basic open water novice diver diving Air NDL, this is a good drill to practice ascending in nice slow control to the surface from your Safety Stop. . . ​