Metric or Imperial?

[DevonDiver]

Metric is certainly the easiest and most intuitive to work with. On a global scale, it's also, by a vast majority, the most popular system.

None of that matters, of course, if your regular dive team use imperial...

There are dual-system SPGs on the market if you look around. That solves many problems... as you have both sets of information immediately available to you (and most electronic gauges can be easily switched).

When I teach Americans visiting the Philippines, all of them convert to Metric. All it takes is to pose them with a few theoretical gas loss 'on-the-fly' gas-versus-deco obligation type of calculations...

 

[Kevrumbo]

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

If compressibility is important for your applications, then use a "fudge" factor like for example, say 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).

Easy Gas Blending Recipes in Metric System:

Gas Blending in Metric is a lot more intuitive as well. . .

Starting from an empty tank or set of tanks -For example, an AL40 deco cylinder and AL80 twinset backgas Cylinders:

For nitrox 50 (add 37% O2):
That is for every 100 bar of Eanx50 deco mix, you need 37 bar of pure O2 and top off the remainder with [hyper-filtered clean] Air;

Nitrox 32 (add 14% O2):
For every 100 bar of Eanx32 mix, you need 14 bar of pure O2 and top off the remainder with Air;

20/20 Trimix (add 4% O2 & 20% Helium):
For every 100 bar of 20/20 Trimix, you need 4 bar of pure O2, 20 bar of He and top off remainder with Air;

25/25 Triox (add 12% O2 & 25% Helium):
For every 100 bar of 25/25 Triox, you need 12 bar of pure O2, 25 bar of He and top off remainder with Air;

30/30 Triox (add 19% O2 & 30% Helium):
For every 100 bar of 30/30 Triox, you need 19 bar of pure O2, 30 bar of He and top off remainder with Air;

21/35 Trimix (add 9% O2 & 35% Helium):
For every 100 bar of 21/35 Trimix, you need 9 bar of pure O2, 35 bar of He and top off remainder with Air;

18/45 Trimix (add 8% O2 & 45% Helium):
For every 100 bar of 18/45 Trimix, you need 8 bar of pure O2, 45 bar of He and top off remainder with Air;

15/55 Trimix (add 7% O2 & 55% Helium):
For every 100 bar of 15/55 Trimix, you need 7 bar of pure O2, 55 bar of He and top off remainder with Air;

12/60 Trimix (add 5% O2 & 60% Helium):
For every 100 bar of 12/60 Trimix, you need 5 bar of pure O2, 60 bar of He and top off remainder with Air;

10/70 Trimix (add 4.5% O2 & 70% Helium):
For every 100 bar of 10/70 Trimix, you need 4.5 bar of pure O2, 70 bar of He and top off remainder with Air.

Using a 11L (AL80) cylinder, or set of twin 11L doubles (double AL80's) for a total of 22L, a full tank or set of tanks is 200 bar:
-->Therefore, all you need is 2 times the amount in bar for O2 (& He for Trimix) from above recipes for a particular mixture.

Example)
21/35 Trimix requires blending 9% Oxygen and and 35% Helium; Therefore a full 11L (Al80) cylinder(s) at 200 bar total pressure needs 18 bar of O2 (2 times 9 bar equals 18 bar), and 70 bar of Helium (2 times 35 bar equals 70 bar); and top off remainder to 200 bar with [hyper-filtered clean] Air.

Gas Blending in Metric (cont.):

Now how did we get the "cookbook recipes" for blending the particular mixtures above? And why for instance with Nitrox 50, can't we use 50 bar of Oxygen and 50 bar of Nitrogen for every 100 bar of Eanx50 to blend a seemingly real intuitive "half & half" mixture?

The answer is YES! You can blend 50 bar of O2 and 50 bar of N2 -->If you are mixing pure O2 AND PURE N2 TOGETHER!!!
But for practical means, why do you need pure N2 when you can just use Air with its natural constituent contribution of 21% Oxygen & 79% Nitrogen?

Using Air this time, let's blend 100 bar of deco mix Nitrox 50:
We know empirically that for every 100 bar of Eanx50, 50 bar must be O2 and 50 bar must be N2; Quantitatively then, how much Air do we need to add in order to give us a N2 amount of 50 bar?

Algebraically and by Dalton's Law, you divide 50 bar N2 by 79% (the %age of Nitrogen in Air), and this yields approximately 63 bar of Air needed. And of this 63 bar of Air, 21% of it (the %age of Oxygen in Air) contributes to the amount needed for O2: approx 13 bar of Oxygen.

Therefore, instead of initially mixing in a full 50 bar of O2, and since we're using Air instead of pure N2, you only need 50 bar minus the 13 bar Oxygen contribution from Air, which equals 37 bar. So hence the cookbook recipe of for every 100 bar of Nitrox 50, add 37 bar of pure O2, and fill the remainder with Air to 100 bar (100 bar total minus 37 bar of O2 equals 63 bar --the amount of Air needed for N2 which we calculated above).

Now to fill an empty 5.5L/bar deco tank (same as an AL40) with Nitrox 50, we need 200 bar of the above recipe (i.e. "For every 100 bar of Eanx50, add 37 bar of pure O2, and then fill the remainder with Air"). Therefore just multiply 37 bar by 2 which equals 74 bar of O2 needed, and then fill the remainder with Air to the total fill pressure of 200 bar.

Similarly, it all applies also to the Trimix recipes --the only difference is accounting for the exact percentage and amount in bar of the Helium constituent in the total mixture . . .​

 

[DevonDiver]

Doing math calculations while diving is idiocy a contingency capability for the unexpected.

Fixed it for you.

Contingencies are about those variable we try and eliminate, but can never guarantee won't occur.

 

[agilis]

Fixed it for you.

Contingencies are about those variable we try and eliminate, but can never guarantee won't occur.

You are too kind. Thank you.

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?

 

[markmud]

Hi all,

Both systems are easy. The metric system is a little bit easier than imperial.

Use whatever system floats your boat.

If I were diving with bloaks who could not understand imperial, I would switch the setting on my computers and go-along to get-along.

Aint no thang!

markm

 

[Kevrumbo]

Simplicity and instant recognition are always paramount. Doing math calculations while diving is idiocy.

. . .
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?

If your actual SPG reading indicates 30% or more consumption than expected, then there is a leak problem or you're physically exerting/breathing harder than normal and probably should consider aborting the dive. This is why you keep a running total in your head of how much remaining tank pressure you have, and confirm it with a SPG reading reading every 10 minutes of elapsed dive time (or a five minute interval if deep on single tank).

 

[Kevrumbo]

Easy Scuba Maths In Metric
[Or how an American Diver figured out the logical use of a Bar SPG]:

How much easier & intuitive is it to work with "1 bar/min" Metric vs "14.5 psi/min equivalent" in US Imperial Units???

My Surface Consumption Rate (SCR) after drift diving so many years on holiday in Palau's 28deg C tropical water temp is a personal best 11 litres/min per ATA.

Using this SCR value with a 11 litres/bar tank (i.e. an AL80 Cylinder):

Divide 11 litres/min per ATA by 11 litres/bar equals 1 bar/min per ATA .

So again --How much easier, intuitive & advantageous is it to work with "1 bar/min" and the metric system in general for Scuba? Well, the arithmetic can all be figured easily & quickly in your head and on-the-fly:

All my dives were on Nitrox32, averaging 20 meters depth always going with the drift current; 20 meters is 3 ATA (divide 20 by 10 and add 1 gives a depth in atmospheres absolute of 3 ATA).

Therefore at 20 meters, my 1bar/min per ATA gas pressure consumption rate will increase threefold --that is 1bar/min per ATA multiplied by 3 ATA equals a depth consumption rate of 3 bar/min at 20 meters. Hence checking my elapsed bottom time every 10 minutes, I expect to consume 30 bar (3 bar/min multiplied by 10min equals 30 bar), and accordingly I already know my SPG will read 30 bar less in that 10 minute time frame. (If however the actual SPG reading indicates 30% or more consumption than expected, then there is a leak problem or I am physically exerting/breathing harder than normal and probably would consider aborting the dive).

So by the first 10 minutes delta time at 20 meters, I expect to be down 30 bar from a full AL80 tank at 200bar, or 170bar remaining actual SPG reading (3bar/min multiplied by 10min is 30bar consumed; and 30bar consumed from 200bar total full tank is the SPG showing 170bar remaining pressure). At the end of another 10 more minutes delta time drifting along at 20 meters, I've consumed 30bar from 170bar, or 140 bar remaining in tank. And finally after another 10 minute period at the elapsed dive time mark of 30 minutes total, I've consumed 30bar delta from 140bar, or 110bar remaining and nearing half tank.

At 40 minutes elapsed time, I'm ascending off the deep wall into the shallow coral plateau around 9 meters (down 30bar from 110bar, or 80 bar remaining in tank). And finally at the 45 to 50 minute mark, I'm at 6m and my 3-5min safety stop with 60 to 70 bar left. I surface and I know even before looking at my SPG that I have around 50 bar remaining in my tank.

This is how you should actively use your SCR with your particular tank, knowing how much breathing gas you have left not only on pre-planning, but also during the actual dive at depth, real-time-on-the-fly --all with easier to use metric units . . .additionally, you have a SPG that reads in units of pressure: why not convert your SCR to a Depth Consumption Rate (DCR) in pressure units to make use of it???

In summary & recap: divide your volume SCR (or SAC/RMV rate) by your particular tank's cylinder rating factor to get a figure in pressure units per minute since your SPG reads in pressure units -not volume units. Multiply this SCR in pressure units by your planned depth in ATA, and you'll know what your Depth Consumption Rate (DCR) per minute in pressure units at that depth will be. And the Metric System for Scuba diving makes the arithmetic much easier especially if your pressure Surface Consumption Rate (SCR) turns out to be roundable up to convenient integer like 1 or 2bar/min per ATA.

----
Some example pressure SCR values for a variety of common cylinders, given a arbitrary nominal constant volume SCR of 22 litres/min per ATA (a reasonable & achievable breathing RMV for most novice divers):

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.

 

[agilis]

If your actual SPG reading indicates 30% or more consumption than expected, then there is a leak problem or you're physically exerting/breathing harder than normal and probably should consider aborting the dive. This is why you keep a running total in your head of how much remaining tank pressure you have, and confirm it with a SPG reading reading every 10 minutes of elapsed dive time (or a five minute interval if deep on single tank).

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.

 

[dberry]

I've wondered about moving to metric when diving in a largely imperial environment. I'm fairly fluent with metric calculations (not that it's much of a challenge, quite the opposite My SPG is imperial, but my AI computer could be switched to metric. My SPG would let me communicate with my buddies in imperial with no worries about mistakes in on-the-fly conversions (or if they sneak a peak at my SPG) , while metric depth / pressure readings on the computer would be for my planning and normal use while diving. Okay, this violates the KISS principal, but how bad an idea is it really?

Or would it be better to just replace the gauges on my SPG and go all metric, and just make sure buddy and I are clear on turn and reserve pressures in both PSI and bar? Pressure conversions at 15 psi per bar (3000psi ~ 200 bar) and depth at 33 ft per 10 m are close enough, and converting between ft3 to liters doesn't really come up between buddies under water (but the magic number is 28 L per ft3, if it did.)

Does trying to be sane inside the asylum just mean I'm insane?

-Don
----
"There are 10 kinds of people in the world; those who understand binary and those who don't."

 

[Kevrumbo]

Just get a dual unit SPG: ScubaPro Dual Scuba Pressure Gauge - SPG in Metric and Imperial | SPG

I've wondered about moving to metric when diving in a largely imperial environment. I'm fairly fluent with metric calculations (not that it's much of a challenge, quite the opposite My SPG is imperial, but my AI computer could be switched to metric. My SPG would let me communicate with my buddies in imperial with no worries about mistakes in on-the-fly conversions (or if they sneak a peak at my SPG) , while metric depth / pressure readings on the computer would be for my planning and normal use while diving. Okay, this violates the KISS principal, but how bad an idea is it really?

Or would it be better to just replace the gauges on my SPG and go all metric, and just make sure buddy and I are clear on turn and reserve pressures in both PSI and bar? Pressure conversions at 15 psi per bar (3000psi ~ 200 bar) and depth at 33 ft per 10 m are close enough, and converting between ft3 to liters doesn't really come up between buddies under water (but the magic number is 28 L per ft3, if it did.)

Does trying to be sane inside the asylum just mean I'm insane?

-Don
----
"There are 10 kinds of people in the world; those who understand binary and those who don't."

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 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. . .