Metric or Imperial?

[BoltSnap]

I went from metric to imperial to metric now and I prefer the imperial much more because I have a much better resolution in reading tank pressure, depth and also the tank capacity in imperial (ft3 irrespective of pressure in tank) is much easier to know and understand than in metric. Tank capacity in metric in Liters is misleading since it only gives you the tank volume not the tank capacity. You'll have to multiply the tank volume times pressure to get the actually tank capacity in Liters.
Doing some calculations is probably easier to do in metric but that apparent advantage is lost when one compares with the advantages of the imperial system. When I look at the pressure, it is much better to see the imperial scale and get a finer resolution than in metric. Same for depth and even temperature.

 

[Kevrumbo]

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 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. . .

 

[JohnnyC]

I went from metric to imperial to metric now and I prefer the imperial much more because I have a much better resolution in reading tank pressure, depth and also the tank capacity in imperial (ft3 irrespective of pressure in tank) is much easier to know and understand than in metric. Tank capacity in metric in Liters is misleading since it only gives you the tank volume not the tank capacity. You'll have to multiply the tank volume times pressure to get the actually tank capacity in Liters.
Doing some calculations is probably easier to do in metric but that apparent advantage is lost when one compares with the advantages of the imperial system. When I look at the pressure, it is much better to see the imperial scale and get a finer resolution than in metric. Same for depth and even temperature.

Quite literally none of that is true. Tank capacity is ALWAYS dependent on tank pressure. Guess what, an AL80 at 200PSI certainly doesn't have 80 cuft of gas in it. Seriously, some of the things you've said on this forum are just plain wrong. About the ONLY thing that imperial has ANY advantage is is if you have a 3000psi fill and you're diving 1/3rds with no consideration for actual gas volume.

 

[tbone1004]

yes you have 100psi/7bar vs 10bar/145psi resolution, but that is not that significant.
Your computer should have .1m/.3ft increments on it, which would make it more accurate than in ft
temperature I will yield is better in F than in C, though that depends on the accuracy of your computer and whether it does tenths of degrees. I.e. resolution in .1c is more accurate than in 1F increments

the volume is 100% wrong though, because imperial is measured indirectly you have to do the fraction of current over working pressure by rated capacity instead of just current pressure x volume. What is easier 11l*100bar=1100l of gas, or 1000*80/3000=26cf of gas?

 

[Kevrumbo]

I went from metric to imperial to metric now and I prefer the imperial much more because I have a much better resolution in reading tank pressure, depth and also the tank capacity in imperial (ft3 irrespective of pressure in tank) is much easier to know and understand than in metric. Tank capacity in metric in Liters is misleading since it only gives you the tank volume not the tank capacity. You'll have to multiply the tank volume times pressure to get the actually tank capacity in Liters.
Doing some calculations is probably easier to do in metric but that apparent advantage is lost when one compares with the advantages of the imperial system. When I look at the pressure, it is much better to see the imperial scale and get a finer resolution than in metric. Same for depth and even temperature.

Read this and learn:
http://api.ning.com/files/KHeKOD-fw...pVsh/BattlefieldCalculationsDeliaMilliron.pdf
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Also, what's so hard about arithmetic using base units like "1" or "2" bar per minute in metric versus "14.5" or "29" psi/min in Imperial?

In other words, how about tracking gas consumption for your particular tank from the start in bar per minute, per depth factor in ATA ? -->Your SPG reads in pressure units, not volume units, so it makes sense to use derived quantities in bar pressure units.

Example: 22 liters/min per ATA volume Surface Consumption Rate divided-by a 11 liter/bar tank (an AL80) equals 2 bar/min per ATA pressure Surface Consumption Rate.

So at 21 meters depth which is 3.1 ATA (21 divided-by 10 plus 1 is 3.1 ATA), you stay for 10 minutes. 10 minutes multiplied-by 3.1 ATA multiplied-by 2 bar/min per ATA equals 62 bar consumed.

Confirm with SPG that it shows a delta of 62 bar less in that 10 minute interval, so if you started with a full 200 bar cylinder, the SPG indicates "138 bar" remaining (i.e. 200 minus 62 equals 138 bar).

Doesn't that make better algebraic/arithmetic sense and unit consistency?

 

[agilis]

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

 

[Kevrumbo]

Here's a gas supply time estimate given a Surface Consumption Rate (SCR) of 11 liters/min*ATA, for an open circuit dive to the Oite Destroyer in Truk, at 60 meters average depth (7 ATA) for 60 minutes.

Using an AL80 (11 liters/bar) tank as a stage, divide SCR by the metric tank factor of the AL80:

11 liters/min*ATA divided-by 11 liters/bar equals 1 bar/min*ATA.

At 60m depth (7ATA), this value of 1 bar/min*ATA will increase sevenfold to a depth consumption rate of 7bar/min. Therefore a full tank at 200 bar will yield 28 minutes at depth, that is 200 bar divided-by 7 bar/min equals 28 minutes to empty at 60 meters.

Switching to backgas double AL80's (22 liters/bar total tank factor), divide SCR by the metric tank factor of the double AL80's:

11 liters/min*ATA divided-by 22 liters/bar equals 0.5 bar/min*ATA.

At 60m depth (7ATA), this value will increase to a depth consumption rate of 3.5bar/min (0.5 bar/min*ATA times 7 ATA is 3.5bar/min). Therefore a full twinset at 200 bar will yield 57 minutes at depth, that is 200 bar divided-by 3.5 bar/min equals 57 minutes to empty at 60 meters. However, I want to have a Rock Bottom reserve pressure of 90 bar in my backgas twinset to get me & my buddy to our 21m Eanx50 deco stop, in case of an Emergency Gas Share. So 200 bar full tank minus 90 bar Rock Bottom reserve pressure equals 110 bar usable. Hence 110 bar divided-by 3.5 bar/min equals 31 minutes at 60 meters before reaching Rock Bottom reserve.

So 28 minutes on an AL80 stage plus 31 minutes on backgas AL80's twinset equals nearly an hour at 60 meters depth.

This is the methodology -utilizing the easier less cumbersome Metric System- to estimate how long a gas supply will last at depth, given an arbitrary SCR (i.e. SAC or RMV in SLM), for a particular set of gas cylinders.

 

[agilis]

Here's a gas supply time estimate given a Surface Consumption Rate (SCR) of 11 liters/min*ATA, for an open circuit dive to the Oite Destroyer in Truk, at 60 meters average depth (7 ATA) for 60 minutes.

Using an AL80 (11 liters/bar) tank as a stage, divide SCR by the metric tank factor of the AL80:

11 liters/min*ATA divided-by 11 liters/bar equals 1 bar/min*ATA.

At 60m depth (7ATA), this value of 1 bar/min*ATA will increase sevenfold to a depth consumption rate of 7bar/min. Therefore a full tank at 200 bar will yield 28 minutes at depth, that is 200 bar divided-by 7 bar/min equals 28 minutes to empty at 60 meters.

Switching to backgas double AL80's (22 liters/bar total tank factor), divide SCR by the metric tank factor of the double AL80's:

11 liters/min*ATA divided-by 22 liters/bar equals 0.5 bar/min*ATA.

At 60m depth (7ATA), this value will increase to a depth consumption rate of 3.5bar/min (0.5 times 7 is 3.5bar/min). Therefore a full twinset at 200 bar will yield 57 minutes at depth, that is 200 bar divided-by 3.5 bar/min equals 57 minutes to empty at 60 meters. However, I want to have a Rock Bottom reserve pressure of 90 bar in my backgas twinset to get me & my buddy to our 21m Eanx50 deco stop, in case of an Emergency Gas Share. So 200 bar full tank minus 90 bar Rock Bottom reserve pressure equals 110 bar usable. Hence 110 bar divided-by 3.5 bar/min equals 31 minutes at 60 meters before reaching Rock Bottom reserve.

So 28 minutes on an AL80 stage plus 31 minutes on backgas AL80's twinset equals nearly an hour at 60 meters depth.

This is the methodology -utilizing the easier less cumbersome Metric System- to estimate how long a gas supply will last at depth, given an arbitrary SCR (i.e. SAC or RMV in SLM), for a particular set of gas cylinders.

Sure, as an oversimplified intuitive estimate.

 

[Kevrumbo]

The ambiguity of an SAC expressed in units of pressure over time is the absence of a measure of volume. If a diver says that he breathes at the rate of 2.5 psi/min he has not told us anything useful until we know what size tanks he is breathing from. The math is simple - no more complicated than freshman high school algebra. It is all doable in your head (or at least in mine). I find it simpler to reckon my gas volume in fractions of my total tank volume. For example if I am diving my LP95's, then I have 250 CF to start (at 3500 psi). At my breathing rate of .4 SCFM that is a bit over 600 minutes at 1ATA or 75 minutes at 230 feet. A one third rule gives me 25 minutes at depth. All this is planned beforehand. Checking my spg on the dive should show my gas being depleted at a rate such as to bring me to my 2/3 mark at my ascent time. I do not really need to do any complicated calculations on the dive, I can tell by spot checks if I am on track. For example, 12 minutes (half way) into the dive I should be at about 5/6 of my starting pressure or about 2900 psi. Note that these numbers are all approximations. You can only read an spg to +/- 100 psi and you can only measure your breathing rate to +/- 0.1 SCFM (if that).

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A Better, Clear and Definitive Gas Planning Methodology Utilizing Metric System:

If you understand your pressure per time rate is based on your tank's rating, then there is no ambiguity. And it's easier to do it all pre-planning & on-the-fly using the Metric System. . .

For your example of LP95's at 230 feet/69 meters:

Your SCR (Surface Consumption Rate) of 0.4 SCFM converts to 11 litres/min*ATA in metric.

Your total metric tank rating for double LP95's is 30 litres/bar.

Therefore your pressure SCR rated for your double LP95's, is 11 litres/min*ATA divided-by 30 litres/bar which equals approximately 0.4 bar/min*ATA.

So at a depth of 230' or 69 meters (same as 7.9 ATA):
Your DCR (Depth Consumption Rate) will be 0.4 multiplied-by 7.9 equals 3.2 bar/min. Hence for every minute of elapsed time, I expect the SPG to decrease by 3.2 bar when at a depth of 230'/69m.

Thus in 10 minutes of nominal swimming at 230'/69m, I expect the SPG to be down 32 bar from the previous reading. In 10 more minutes, I expect the SPG to be down another 32 bar; in 5 more minutes after that, I expect the SPG to be down 16 bar from the previous reading --and now you're at your third's-turn pressure of 80 bar total delta consumed, with 160 bar remaining pressure (out of 241 bar full fill) as indicated by the SPG.

Re-Check Arithmetic Thirds Turn Pressure & Time: Your fill of 3500 psi is equivalent to 241 bar in metric. One-third of 241 bar is approx 80 bar delta.

Summing the above expected SPG time check readings at respectively 10min, 20min and 25min elapsed time at 230'/69m: 32 bar plus 32 bar plus 16 bar equals 80 bar total delta consumed. Check -->As you stated above and is confirmed, your planned third's turn pressure in this example would be at the 25 minute mark with 80 bar consumed. Your expected SPG reading at thirds turn pressure should indicate 160 bar remaining (241 bar full fill minus 80 is approx 160 bar remaining).

This illustrates my point using your own example that there is no ambiguity in the use of pressure units per time rate during the dive, provided that you understand that it is mathematically dependent upon your total tank rating of the cylinder(s) in use. . . (And again, since your SPG reads in bar units so it makes better sense to work with pressure units instead of volume units).

 

[JohnN]

I went from metric to imperial to metric now and I prefer the imperial much more because I have a much better resolution in reading tank pressure, depth and also the tank capacity in imperial (ft3 irrespective of pressure in tank) is much easier to know and understand than in metric. Tank capacity in metric in Liters is misleading since it only gives you the tank volume not the tank capacity. You'll have to multiply the tank volume times pressure to get the actually tank capacity in Liters.

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