Should I go Full Metric from the Start ?

[Kevrumbo]

Only narcosis quiets the voices.

Nitrox 21% (Air) at 10 ATA for example, will quiet all consciousness. . . Period. (Gas Density 10 times that of Air at the surface, and a ppO2 of 2.1).

What depth is 10 ATA?

To easily convert to depth in meters from ATA, subtract 1 and multiply by 10 -->so 10 ATA is 90 meters.

OTOH, convert to feet from ATA, subtract 1 and multiply by 33 -->so 10 ATA equals 9 atm gauge pressure times 33 feet per atm which is . . . 297, round it approx to 300 feet.

 

[Roger Hobden]

Nitrox 21% (Air) at 10 ATA for example, will quiet all consciousness. . . Period. (Gas Density 10 times that of Air at the surface, and a ppO2 of 2.1).

Not to mention convulsions due to Oxygen Toxicity ...

 

[Kevrumbo]

Nitrox 21% (Air) at 10 ATA for example, will quiet all consciousness. . . Period. (Gas Density 10 times that of Air at the surface, and a ppO2 of 2.1). . .

Not to mention convulsions due to Oxygen Toxicity ...

At that depth on Air, you would probably succumb outright to metabolic CO2 poisoning and extreme anesthetic narcosis first along with later concurrent onset of OxTox symptoms/convulsions.

Because of an Air Gas Density at 10 ATA that is ten times more dense than at the surface, you cannot effectively eliminate metabolic build-up of Carbon Dioxide through breathing/respiration anymore, especially with physical exertion at that depth.

@Wookie and @TMHeimer mentioned a depth/pressure of 40meters/5 ATA which happens to be the long established recreational depth limit. With regard to CO2 retention, there finally appears to be scientific validation for this: Gas Density of Air at the surface is 1.3 grams/liter; at 5 ATA this value is five times as much or 6.5 g/L. A gas density over 6 g/L has been found to increase susceptibility to CO2 retention along with increased risk of Oxygen Toxicity. (For reference read this good article and being comfortable in the Metric System to start makes it easier to understand: Advanced Knowledge Series: The Gas Density Conundrum).

For Air at 5 ATA, this critical depth in meters is at 39 to 40m, for a gas density of 6.5 g/L. Compare to a depth of 90m or 10 ATA, where the density is a gross 13 g/L . . . !

 

[stuartv]

(Do you have a clue now @stuartv ?
If not, then you really ought to get two Perdix wireless AI units, one for each wrist, along with a redundant Data Mask and a NERD heads-up-display. . . all telling you what you need to know.)

I guess I don't, because so far, 1 SPG (a wireless one on my wrist) seems very adequate for my needs.

At that depth on Air, you would probably succumb outright to metabolic CO2 poisoning and extreme anesthetic narcosis first along with later concurrent onset of OxTox symptoms/convulsions.

Because of an Air Gas Density at 10 ATA that is ten times more dense than at the surface, you cannot effectively eliminate metabolic build-up of Carbon Dioxide through breathing/respiration anymore, especially with physical exertion at that depth.

Right. Because nobody has ever dived to that depth on air and lived. Much less done it multiple times...

 

[tbone1004]

@Kevrumbo call up Brett Gilliam and ask him about his almost 500ft dive on air, or Hal Watts to near 400, or Tom Mount to near 400 as well. All three quite alive and well

 

[RayfromTX]

Breathing air maximum terminal depth
reposting this relevant thread from the recent past about co2 retention and maximum terminal depth on deep air.

 

[Kevrumbo]

@Kevrumbo call up Brett Gilliam and ask him about his almost 500ft dive on air, or Hal Watts to near 400, or Tom Mount to near 400 as well. All three quite alive and well

What do you think they did at 13 to 16 ATA in terms of physical activity @tbone1004??? Swim circles and somersaults around the descent line in celebration???

Even floating there at that depth neutral and static with no exertion, you can go into acute Hypercapnia and unconsciousness just because of the dyspnea in the work of breathing -if not frankly incapacitated due to extreme anesthetic narcosis- by staying down there only a few minutes on Air. All they had time for was to grab their record depth marker, allegedly do some math and task problems on a slate, and quickly start their ascent to deco stops.

With a gas density of near 20 g/L, that's literally like sucking molasses through a straw, trying to breath Air at 150m: there's no way to expel the excess metabolic CO2 build-up fast enough after a few minutes at that depth before succumbing to Carbon Dioxide poisoning. They're all lucky to be alive and will admit as much, compared to those who went before and after but passed away. . .

. . .Right. Because nobody has ever dived to that depth on air and lived. Much less done it multiple times...

First to die on an Aqua-Lung after reportedly reaching 120 meters:
Maurice Fargues - Wikipedia

 

[Kevrumbo]

I was certified OW and Advanced twenty years and did about 30 dives but then stopped diving. I am starting back again and have projects to do a GUE Fundamental Skills course very soon.

I downloaded the course materiel and started doing the gas management exercices. I have been doing a lot of supplementary reading also. Comparing the Metric and Imperial systems, it appears the Metric is much more simple and intuitive for all aspects of dive planning.

I am thinking of going Full Metric and for instance buying a SPG that reads in Bars instead of PSI.

I live in Montreal, Quebec, Canada, and I realize that probably most potential buddies around me will still be using the Imperial system.

Any thoughts and comments about me taking this route ?

Below are some example pressure Surface Consumption Rate (SCR) values for a variety of common cylinders with an arbitrary volume SCR (also known as volume SAC rate or RMV):

Given a arbitrary nominal volume SCR of 22 liters/min per ATA (that's 0.78 cuft/min per ATA in US Imperial Units, a reasonable & achievable volume SCR for most novice divers, and exercise level for fit advanced divers with sustained active finning):

Cylinder Size | Pressure SCR
11L/bar tank (AL80): 2bar/min per ATA;
12L/bar tank (Steel HP100): 1.8bar/min per ATA;
13L/bar tank (AL100): 1.7bar/min per ATA;
15L/bar tank (Steel HP119): 1.5bar/min per ATA;
16L/bar tank (Steel HP130): 1.4bar/min per ATA;
11L Twins (Double AL80's): 1bar/min per ATA;
12L Twins (Double HP100's): 0.9bar/min per ATA;
16L Twins (Double HP130's): 0.7bar/min per ATA.

Given a arbitrary nominal volume SCR of 15 liters/min per ATA (0.53 cuft/min per ATA in US Imperial Units, relaxed with minimal finning for advanced divers):

11L/bar tank (AL80): 1.4bar/min per ATA;
12L/bar tank (Steel HP100): 1.3bar/min per ATA;
13L/bar tank (AL100): 1.2bar/min per ATA;
15L/bar tank (Steel HP119): 1bar/min per ATA;
16L/bar tank (Steel HP130): 0.9bar/min per ATA;
11L Twins (Double AL80's): 0.7bar/min per ATA;
12L Twins (Double HP100's): 0.6bar/min per ATA;
16L Twins (Double HP130's): 0.5bar/min per ATA.

Given a arbitrary nominal volume SCR of 11 liters/min per ATA (0.39 cuft/min per ATA in US Imperial Units, drift diving floating neutrally buoyant & going with the current):

11L/bar tank (AL80): 1bar/min per ATA;
12L/bar tank (Steel HP100): 0.9bar/min per ATA;
13L/bar tank (AL100): 0.8bar/min per ATA;
15L/bar tank (Steel HP119): 0.73bar/min per ATA;
16L/bar tank (Steel HP130): 0.68bar/min per ATA;
11L Twins (Double AL80's): 0.5bar/min per ATA;
12L Twins (Double HP100's): 0.45bar/min per ATA;
16L Twins (Double HP130's): 0.3bar/min per ATA.

The point is that SAC rate for most nominal activity on single tank is going to fall in between 1 bar/min per ATA and 2 bar/min per ATA. And how easy is it to figure factors of numbers like 1 and 2? --All you need now is a convenient time interval like 10 minutes and your metric depth converted to ATA as multiplying factors, and you will then know what your Depth Consumption in bar will be over that time interval, with a particular tank, and level of physical activity. . .

You can get SPG's that read in both, however in North America in general it's easier to work in PSI and cubic feet for most people as those are the units our tanks are sold.
I have bar spg's on some of my regulators, but PSI is easier for most people to work with so I usually end up having to convert back

So to recap, it's actually a lot easier working with multiplying factors of 1 bar/min to 2 bar/min SAC rates in Metric, versus handling factors of 14.5 to 29 psi/min in US Imperial units.

 

[beester]

20 points just seems like such a small number to describe such a large shift in temperature. Same for measuring depth in meters. I guess you have to get used to doing everything as fractions/decimals.. e.g. "It's 22.22 degrees outside right now" or "I came up to 6.096 meters and did my safety stop". Just seems odd. I guess you have the option to reduce precision, but personally I think that sounds distasteful.

The moment you can hold a stop in blue water to 6.096m give or take 0.001 m I'll accept that you need those kind of small numbers. Same for temperature... the moment your body will feel the difference between 0.01 degrees celcius I'll accept that you need those small numbers.

To the OP... check what your regular buddys are using?! Specially when you are going to do fundies (team oriented diving) it makes sense to talk the same "language".

I was brought up in metric and use it in all calculations because it's very easy, but I can talk imperial (feet, yard, fahrenheit, psi for the regular volume tanks ).

 

[kelemvor]

The moment you can hold a stop in blue water to 6.096m give or take 0.001 m I'll accept that you need those kind of small numbers. Same for temperature... the moment your body will feel the difference between 0.01 degrees celcius I'll accept that you need those small numbers.

To the OP... check what your regular buddys are using?! Specially when you are going to do fundies (team oriented diving) it makes sense to talk the same "language".

I was brought up in metric and use it in all calculations because it's very easy, but I can talk imperial (feet, yard, fahrenheit, psi for the regular volume tanks ).

I don't know about .001m but I can definitely maintain buoyancy without bouncing around 3' (1 meter). Same with the temperature measurement; look back at my post. To get any level of precision in the ranges I'm used to in metric your only option is to use fractions/decimals to some degree. I don't know about "in blue water". That doesn't seem relevant to my point. I can still count on one finger the time I've had to hold a stop or other depth without a visual reference point. I guess if those are the dives you do, then maybe maintaining tight buoyancy control might be something you have to loosen up on.

It's not that difficult to feel a change in water temperature when moving from 72f to 73f water.

In the end, it doesn't matter other than to illustrate part of the reason I'd be uncomfortable switching to metric for distance or temperature when diving. YMMV.