Metric versus Imperial System for Diving?

[RJP]

Here's another, completely different scenario that I stumbled upon a while back.

Let's say you have an object weighing 100 pounds at 60 feet depth. You want to raise it to the surface with a lift bag. How much air from your tank do you need for the lift bag and how much change in pressure does that translate to?

While this is very easy and straight forward to work out in metric units (you can do it in your head), in imperial units the answer comes back as "go !@#$% yourself."

Well, Archimedes, it could rocket to the surface all by itself, without the lift bag, if the object displaces 10cf of water.

Metric system ain't worth *** if you are missing a key piece of information to do the math.

:d

 

[Kevrumbo]

And following the rule of thirds should make those precise calculations strictly academic. Encountering stronger currents or going deeper than originally anticipated modifies my dive plan instantly, with an intuitive accuracy, no exact calculations needed.

I've been scuba diving for more than 50 years, and am rather good at it by most measures, but I still follow those very conservative safety guidelines, perhaps more strictly now than when I was a young man. I do dive solo most of the time, but I think, all things considered, it's the safer option for me.

I never push the envelope. Maybe that's part of the reason why I've never had a serious incident over the course of several thousand dives. Something's going to get me, sooner or later. It might as well happen while I'm having fun.

Precision is not that necessary, and if the accuracy of your gas consumption rate upon checking your SPG is more than 30% than what you arithmetically calculated, you are working much harder than you expected or you have a major leak somewhere --and probably should abort the dive.

Here's another metric gas planning example & exercise, for a non-solo buddy dive to the Basic Open Water Limit of 18m/60':

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 just to get you to the surface if you were solo --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!)

Is this academic AND practical example "schedule" above really too much for you Jersey Boys to get??? Maybe this thread should be renamed "Jersey People get no respect because they just don't get it". . .

 

[dfx]

Well, Archimedes, it could rocket to the surface all by itself, without the lift bag, if the object displaces 10cf of water.

Metric system ain't worth *** if you are missing a key piece of information to do the math.

:d

Ah, someone's paying attention! But you see, 10 cu ft is already way too much, in fact 2 cu ft would be enough. But how does one know? Again this is something you can't easily work out in imperial, but in metric the answer would be staring you in the face.

 

[RJP]

but in metric the answer would be staring you in the face.

So, in the metric system you DON'T need to know the displacement of the object in order to do the calculation? Hey, that really IS easier!

:d

 

[Freewillow]

I don't see the relevance, unless you do SAC/RMV calculations while actually diving. I think these kinds of things are generally done afterward, which was a central element in all my remarks. As far as jumping through several hoops, converting bar to psi is not especially complicated. The dual reading gauge shown in post 126 does it for you in a rough manner, and there are equivalency tables of great exactitude. In any event, the safest thing while diving is, in my opinion, using a direct mechanical gauge with measuring units that are instantly understood by the user.

I read your different answers and I understand that we cannot agree, mostly because we dive very differently. You are diving solo and not very deep, while I dive with my wife and , once in awhile we are planning deco dives. We are planning them of course but in practice, we leave the bottom and start our ascent at the latest when we have reached the following airpresssure in our 15 Liters tanks. The formula is the following: when our TTS ( time to surface) X 3 + 50 bars ( our defined reserve ) = the pressure in our tank. This means that if we have a TTS of 10 minutes ( that we can read on our computor ), 10X3 +40 = 80 bars. This pressure is our safety limit for our ascent.

Planning and calculation after the dive may be nice but of no real utility when you are at 40 meters with some deco time:kiss2:

 

[Kevrumbo]

And following the rule of thirds should make those precise calculations strictly academic. ...

Originally Posted by tomfcrist
I really don't see the argument of gas planning being easier in metric. I do just fine using imperial. To me, I believe you should use what is intuitive to you, and don't dive with a guide....you then won't have to worry about if they understand your preferred system( imperial/metric).

Here's another more comprehensive example:

Modified Thirds Planning and Calculating On-the-Fly Gas Needed to Do a Lost Buddy Search in a Wreck Penetration:

Using difficult & cumbersome US Imperial Units in PSI (the easier Metric calculation in comparison follows later down below in bold):

Suppose you originally planned a wreck penetration starting with 3000 psi [double AL80’s] at the entrance, with an openwater Rock Bottom of 750psi; So 3000 minus 750 equals 2250psi usable for the penetration --Modified Thirds of this value is 750 psi (one-third of 2250 equals 750), so you would turn-around for egress when you consume 750 psi of gas with an actual SPG reading of 2250 psi. If you needed to do a gas-sharing emergency egress with your buddy at this point, you would both need 1500psi to get out of the wreck, with 750psi Rock Bottom remaining to get both of you to the surface (or your Oxygen deco bottle stop of 21’).

Let’s say you used up 450psi already getting to the entrance of the wreck for a total of 2550 pressure available --can you quickly recalculate Modified-Thirds?

Well . . . 2550psi minus 750psi Rock Bottom yields 1800psi usable for the penentration; One-Third of 1800psi is 600psi which is your new Modified Thirds turn pressure value. Therefore you would turn the dive when you consume 600psi for an actual turn pressure SPG reading of 1950psi (2550 minus 600 equals the actual turn pressure SPG reading of 1950psi).

What if you lose your buddy at this instance, at the farthest distance inside the overhead your Modified Thirds value allows? How do you calculate the amount of gas to do a Lost Buddy Search?

[NOT] easy! At your turn around pressure reading of 1950psi on the SPG, simply add your Rock Bottom value to your Modified Thirds value (750psi Rock Bottom plus 600psi Modified Thirds equals 1350psi); Put a line-arrow pointing the way out on your mainline that you've laid, and take reference note of where you are inside the overhead at that exact point as well. Now go and search for your buddy with the understanding that you must be back at this line-arrow marker by the time your SPG reads this actual value (1350psi). So you would have from 1950psi down to 1350psi reading on your SPG, or 600psi delta of gas to search for your buddy --if you were to do a straight line search down a long corridor inside the wreck for example, tactically you should use 300psi out and 300psi back to your line-arrow marker for a delta of 600psi, and an actual end of search SPG reading of 1350psi-- you must start your egress whether you found your buddy or not when you use up this 600psi delta of gas, at the line arrow marker, with the actual 1350psi final reading on your SPG.

At any point before your Modified Thirds turn pressure, for a lost buddy search, the final egress pressure is figured just by adding your Rock Bottom value to the amount of gas you've consumed on the penetration up to that point --for example you start with 2550psi on your SPG and you lose your buddy with 2100psi SPG reading for a delta consumption of 450psi. 750psi Rock Bottom plus delta consumption of 450psi equals 1200psi. Drop a line arrow, and now you've got from 2100psi down to 1200psi (a tactical delta search pressure of 900psi) to look for your buddy, and be back to your line arrow to egress smartly when your SPG reads 1200psi.

At any point after your Modified Thirds turn pressure, all you need to do to figure out a final egress pressure for a lost buddy search is to subtract your Modified Thirds value from your actual pressure reading, and place a line-arrow pointing out at this point on your mainline. For example, if you're egressing and you lose your buddy with 1800psi actual reading on your SPG: Subtract the Modified Thirds value of 600psi from 1800psi -which equals 1200psi - and it is this actual reading that you must have on your SPG when you get back to your line arrow to successfully exit the wreck with all your Rock Bottom still available to reach the surface. Another way of looking at this, at your nominal turn-around point & afterward on egress, the amount of gas tactically available for a lost buddy search is always just your Modified Thirds value --in this case 600psi.

Remember that on a lost buddy search, you will deliberately encroach and use up the Modified Thirds Reserve Value needed for an emergency gas-sharing egress contingency (and possibly use up Rock Bottom as well) --in other words, if you do find your lost buddy and worst of all worst scenarios he happens to be out-of-gas in a silt-out . . .well dea ex machina. I hope you're in a 3-person Team, somehow make it out and run into other divers on the outside who can donate gas & assist. . .
_______________
Same thing as above but now using Metric Units:

Modified Thirds,Turn-Around Pressure and Lost Buddy Search Gas Availability Calculations are easier & more intuitive with a bar SPG too. . .

Suppose you originally planned a wreck penetration starting with 200 bar at the entrance, with an openwater Rock Bottom of 50 bar. 200 minus 50 bar equals 150 bar usable for the penetration --Modified Thirds of this value is 50 bar (one-third of 150 equals 50), so you would turn-around for egress when you consume 50 bar of gas with an actual SPG reading of 150 bar. If you needed to do a gas-sharing emergency egress with your buddy at this point, you would both need 100 bar to get out of the wreck, with 50 bar Rock Bottom remaining to get both of you to the surface (or your Oxygen deco bottle stop of 6m).

Lets say you used up 30 bar already getting to the entrance of the wreck for a total of 170 bar pressure available --can you quickly recalculate Modified-Thirds?

No problem with bar pressure metrics: 170 bar minus 50 bar Rock Bottom yields 120 bar usable for the penentration; One-Third of 120 bar is 40 bar which is your new Modified Thirds turn pressure value. Therefore you would turn the dive when you consume 40 bar for an actual turn pressure SPG reading of 130 bar (170 bar minus 40 bar equals the actual turn pressure SPG reading of 130 bar).

What if you lose your buddy at this instance, at the farthest distance inside the overhead your Modified Thirds value allows? How do you calculate the amount of gas to do a Lost Buddy Search?

Easy! At your turn around pressure reading of 130 bar on the SPG, simply add your Rock Bottom value to your Modified Thirds value (50 bar Rock Bottom plus 40 bar Modified Thirds equals 90 bar); Put a line-arrow pointing the way out on your mainline that you've laid, and take reference note of where you are inside the overhead at that exact point as well. Now go and search for your buddy with the understanding that you must be back at this line-arrow marker by the time your SPG reads this actual value (90 bar). So you would have from 130 bar down to 90 bar reading on your SPG, or 40 bar delta of gas to search for your buddy --if you were to do a straight line search down a long corridor inside the wreck for example, tactically you should use 20 bar out and 20 bar back to your line-arrow marker for a delta of 40 bar, and an actual end of search SPG reading of 90 bar-- you must start your egress whether you found your buddy or not when you use up this 40 bar delta of gas, at the line arrow marker, with the actual 90 bar final reading on your SPG.

At any point before your Modified Thirds turn pressure, for a lost buddy search, the final egress pressure is figured just by adding your Rock Bottom value to the amount of gas you've consumed on the penetration up to that point --for example you start with 170 bar on your SPG and you lose your buddy with 140 bar SPG reading for a delta consumption of 30 bar. 50 bar Rock Bottom plus delta consumption of 30 bar equals 80 bar. Drop a line arrow, and now you've got from 140 bar down to 80 bar (a tactical delta search pressure of 60 bar) to look for your buddy, and be back to your line arrow to egress smartly when your SPG reads 80 bar.

At any point after your Modified Thirds turn pressure, all you need to do to figure out a final egress pressure for a lost buddy search is to subtract your Modified Thirds value from your actual pressure reading, and place a line-arrow pointing out at this point on your mainline. For example, if you're egressing and you lose your buddy with 120 bar actual reading on your SPG: Subtract the Modified Thirds value of 40 bar from 120 bar -which equals 80 bar- and it is this actual reading that you must have on your SPG when you get back to your line arrow to successfully exit the wreck with all your Rock Bottom still available to reach the surface. Another way of looking at this, at your nominal turn-around point & afterward on egress, the amount of gas tactically available for a lost buddy search is always just your Modified Thirds value --in this case 40 bar.

Remember that on a lost buddy search, you will deliberately encroach and use up the Modified Thirds Reserve Value needed for an emergency gas-sharing egress contingency (and possibly use up Rock Bottom as well) --in other words, if you do find your lost buddy and worst of all worst scenarios he happens to be out-of-gas in a silt-out . . .well dea ex machina. I hope you're in a 3-person Team, somehow make it out and run into other divers on the outside who can donate gas & assist. .

[Note: the above gas plan is taken from wreck penetration dives on the Yukon (San Diego); USS New York (Subic Bay Philippines); HMAS Perth/USS Houston (Sunda Strait Indonesia); and various wrecks in Truk Lagoon. Depth 30m using twin 11L/bar tanks (double AL80's) and Oxygen deco.

 

[RJP]

Iambic pentameter... that was metric, right?

Here's another more comprehensive example:

Modified Thirds Planning and Calculating On-the-Fly Gas Needed to Do a Lost Buddy Search in a Wreck Penetration:

Using difficult & cumbersome US Imperial Units in PSI (the easier Metric calculation in comparison follows later down below in bold):

Suppose you originally planned a wreck penetration starting with 3000 psi [double AL80’s] at the entrance, with an openwater Rock Bottom of 750psi; So 3000 minus 750 equals 2250psi usable for the penetration --Modified Thirds of this value is 750 psi (one-third of 2250 equals 750), so you would turn-around for egress when you consume 750 psi of gas with an actual SPG reading of 2250 psi. If you needed to do a gas-sharing emergency egress with your buddy at this point, you would both need 1500psi to get out of the wreck, with 750psi Rock Bottom remaining to get both of you to the surface (or your Oxygen deco bottle stop of 21&#8217.

Let’s say you used up 450psi already getting to the entrance of the wreck for a total of 2550 pressure available --can you quickly recalculate Modified-Thirds?

Well . . . 2550psi minus 750psi Rock Bottom yields 1800psi usable for the penentration; One-Third of 1800psi is 600psi which is your new Modified Thirds turn pressure value. Therefore you would turn the dive when you consume 600psi for an actual turn pressure SPG reading of 1950psi (2550 minus 600 equals the actual turn pressure SPG reading of 1950psi).

What if you lose your buddy at this instance, at the farthest distance inside the overhead your Modified Thirds value allows? How do you calculate the amount of gas to do a Lost Buddy Search?

[NOT] easy! At your turn around pressure reading of 1950psi on the SPG, simply add your Rock Bottom value to your Modified Thirds value (750psi Rock Bottom plus 600psi Modified Thirds equals 1350psi); Put a line-arrow pointing the way out on your mainline that you've laid, and take reference note of where you are inside the overhead at that exact point as well. Now go and search for your buddy with the understanding that you must be back at this line-arrow marker by the time your SPG reads this actual value (1350psi). So you would have from 1950psi down to 1350psi reading on your SPG, or 600psi delta of gas to search for your buddy --if you were to do a straight line search down a long corridor inside the wreck for example, tactically you should use 300psi out and 300psi back to your line-arrow marker for a delta of 600psi, and an actual end of search SPG reading of 1350psi-- you must start your egress whether you found your buddy or not when you use up this 600psi delta of gas, at the line arrow marker, with the actual 1350psi final reading on your SPG.

At any point before your Modified Thirds turn pressure, for a lost buddy search, the final egress pressure is figured just by adding your Rock Bottom value to the amount of gas you've consumed on the penetration up to that point --for example you start with 2550psi on your SPG and you lose your buddy with 2100psi SPG reading for a delta consumption of 450psi. 750psi Rock Bottom plus delta consumption of 450psi equals 1200psi. Drop a line arrow, and now you've got from 2100psi down to 1200psi (a tactical delta search pressure of 900psi) to look for your buddy, and be back to your line arrow to egress smartly when your SPG reads 1200psi.

At any point after your Modified Thirds turn pressure, all you need to do to figure out a final egress pressure for a lost buddy search is to subtract your Modified Thirds value from your actual pressure reading, and place a line-arrow pointing out at this point on your mainline. For example, if you're egressing and you lose your buddy with 1800psi actual reading on your SPG: Subtract the Modified Thirds value of 600psi from 1800psi -which equals 1200psi - and it is this actual reading that you must have on your SPG when you get back to your line arrow to successfully exit the wreck with all your Rock Bottom still available to reach the surface. Another way of looking at this, at your nominal turn-around point & afterward on egress, the amount of gas tactically available for a lost buddy search is always just your Modified Thirds value --in this case 600psi.

Remember that on a lost buddy search, you will deliberately encroach and use up the Modified Thirds Reserve Value needed for an emergency gas-sharing egress contingency (and possibly use up Rock Bottom as well) --in other words, if you do find your lost buddy and worst of all worst scenarios he happens to be out-of-gas in a silt-out . . .well dea ex machina. I hope you're in a 3-person Team, somehow make it out and run into other divers on the outside who can donate gas & assist. . .
_______________
Same thing as above but now using Metric Units:

Modified Thirds,Turn-Around Pressure and Lost Buddy Search Gas Availability Calculations are easier & more intuitive with a bar SPG too. . .

Suppose you originally planned a wreck penetration starting with 200 bar at the entrance, with an openwater Rock Bottom of 50 bar. 200 minus 50 bar equals 150 bar usable for the penetration --Modified Thirds of this value is 50 bar (one-third of 150 equals 50), so you would turn-around for egress when you consume 50 bar of gas with an actual SPG reading of 150 bar. If you needed to do a gas-sharing emergency egress with your buddy at this point, you would both need 100 bar to get out of the wreck, with 50 bar Rock Bottom remaining to get both of you to the surface (or your Oxygen deco bottle stop of 6m).

Lets say you used up 30 bar already getting to the entrance of the wreck for a total of 170 bar pressure available --can you quickly recalculate Modified-Thirds?

No problem with bar pressure metrics: 170 bar minus 50 bar Rock Bottom yields 120 bar usable for the penentration; One-Third of 120 bar is 40 bar which is your new Modified Thirds turn pressure value. Therefore you would turn the dive when you consume 40 bar for an actual turn pressure SPG reading of 130 bar (170 bar minus 40 bar equals the actual turn pressure SPG reading of 130 bar).

What if you lose your buddy at this instance, at the farthest distance inside the overhead your Modified Thirds value allows? How do you calculate the amount of gas to do a Lost Buddy Search?

Easy! At your turn around pressure reading of 130 bar on the SPG, simply add your Rock Bottom value to your Modified Thirds value (50 bar Rock Bottom plus 40 bar Modified Thirds equals 90 bar); Put a line-arrow pointing the way out on your mainline that you've laid, and take reference note of where you are inside the overhead at that exact point as well. Now go and search for your buddy with the understanding that you must be back at this line-arrow marker by the time your SPG reads this actual value (90 bar). So you would have from 130 bar down to 90 bar reading on your SPG, or 40 bar delta of gas to search for your buddy --if you were to do a straight line search down a long corridor inside the wreck for example, tactically you should use 20 bar out and 20 bar back to your line-arrow marker for a delta of 40 bar, and an actual end of search SPG reading of 90 bar-- you must start your egress whether you found your buddy or not when you use up this 40 bar delta of gas, at the line arrow marker, with the actual 90 bar final reading on your SPG.

At any point before your Modified Thirds turn pressure, for a lost buddy search, the final egress pressure is figured just by adding your Rock Bottom value to the amount of gas you've consumed on the penetration up to that point --for example you start with 170 bar on your SPG and you lose your buddy with 140 bar SPG reading for a delta consumption of 30 bar. 50 bar Rock Bottom plus delta consumption of 30 bar equals 80 bar. Drop a line arrow, and now you've got from 140 bar down to 80 bar (a tactical delta search pressure of 60 bar) to look for your buddy, and be back to your line arrow to egress smartly when your SPG reads 80 bar.

At any point after your Modified Thirds turn pressure, all you need to do to figure out a final egress pressure for a lost buddy search is to subtract your Modified Thirds value from your actual pressure reading, and place a line-arrow pointing out at this point on your mainline. For example, if you're egressing and you lose your buddy with 120 bar actual reading on your SPG: Subtract the Modified Thirds value of 40 bar from 120 bar -which equals 80 bar- and it is this actual reading that you must have on your SPG when you get back to your line arrow to successfully exit the wreck with all your Rock Bottom still available to reach the surface. Another way of looking at this, at your nominal turn-around point & afterward on egress, the amount of gas tactically available for a lost buddy search is always just your Modified Thirds value --in this case 40 bar.

Remember that on a lost buddy search, you will deliberately encroach and use up the Modified Thirds Reserve Value needed for an emergency gas-sharing egress contingency (and possibly use up Rock Bottom as well) --in other words, if you do find your lost buddy and worst of all worst scenarios he happens to be out-of-gas in a silt-out . . .well dea ex machina. I hope you're in a 3-person Team, somehow make it out and run into other divers on the outside who can donate gas & assist. .

[Note: the above gas plan is taken from wreck penetration dives on the Yukon (San Diego); USS New York (Subic Bay Philippines); HMAS Perth/USS Houston (Sunda Strait Indonesia); and various wrecks in Truk Lagoon. Depth 30m using twin 11L/bar tanks (double AL80's) and Oxygen deco.

 

[victorzamora]

Here's another more comprehensive example:

Modified Thirds Planning and Calculating On-the-Fly Gas Needed to Do a Lost Buddy Search in a Wreck Penetration:

Using difficult & cumbersome US Imperial Units in PSI (the easier Metric calculation in comparison follows later down below in bold

The reason it was easier in Metric is because the numbers you used were easier, due to luck. 200bar-50bar=Number easily divisible by three. If you said that you started with 3000psi, had a MinGas of 900psi, leaving you with 2100psi of usable gas. Thirds means 900psi of penetration gas. It took you 300psi to get down there, though, so you're really starting at 2700psi. Minus 900 for MinGas is 1800. Thirds is 600psi. Easy-peasy.

Also, PROPERLY calculated thirds in your first example wouldn't have rendered 2250psi. Proper "thirds" calculations involve rounding down for safety. You using 450psi to get there (readable only as 500psi) means you're starting with 2500psi. Minus your 800psi min gas gets you 1700psi. Thirds means 500psi of penetration gas, all rounded for safety. If in your example you started with 230 bar, needed 45bar for mingas, used 37 bar to get to the wreck, and were now dealing with dividing 148bar (230-45-37) into thirds and then recalculating, you're running into the same problem. Metric is easier in your example only because it was artificially created to be.

 

[Kevrumbo]

The reason it was easier in Metric is because the numbers you used were easier, due to luck. 200bar-50bar=Number easily divisible by three. If you said that you started with 3000psi, had a MinGas of 900psi, leaving you with 2100psi of usable gas. Thirds means 900psi of penetration gas. It took you 300psi to get down there, though, so you're really starting at 2700psi. Minus 900 for MinGas is 1800. Thirds is 600psi. Easy-peasy.

Also, PROPERLY calculated thirds in your first example wouldn't have rendered 2250psi. Proper "thirds" calculations involve rounding down for safety. You using 450psi to get there (readable only as 500psi) means you're starting with 2500psi. Minus your 800psi min gas gets you 1700psi. Thirds means 500psi of penetration gas, all rounded for safety. If in your example you started with 230 bar, needed 45bar for mingas, used 37 bar to get to the wreck, and were now dealing with dividing 148bar (230-45-37) into thirds and then recalculating, you're running into the same problem. Metric is easier in your example only because it was artificially created to be.

Vic, this was an actual proper dive gas plan for real dives done originally in metric, and not "for tailoring the numbers so it divides better in Metric vs US/imperial." (I actually had a harder time re-writing the conversion example for psi than the original in metric). And your rebuttal above just proves my point how inconveniently confounding working with the superfluous extra significant figures of PSI units. (Tell me Vic, what can you not comprehend about working with multiples or percentages of 1 bar/min metric pressure rate??? Is that really more difficult than working with 14.5 psi/min in US Imperial??? I think not. . .)

Another example where the numbers aren't so "contrived" as you claim Vic (this original quick plan in US Imperial was by "captndale", and confirmed easily & much more smartly & intuitively using Metric):

Originally Posted by captndale
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).

Reply by kevrumbo:
My point is again, that if you understand your pressure per time rate is premised 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':
Your SCR (Surface Consumption Rate) of 0.4 SCFM translates to 11 litres/min*ATA;

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

Therefore your pressure SCR rated for your double LP95's, is 11 divided-by 30 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.

Check: Your fill of 3500 psi is equivalent to 241 bar. 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 down. (As you stated above and is confirmed, your planned third's turn pressure in this example would be at the 25min mark).

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 congruent upon your total tank rating of the cylinder(s) in use. . .and that using the metric system is far easier, more intuitive, and makes greater objective sense than you alluding to the arithmetic "doable all in your head, no more complicated than freshman high school algebra", with the cumbersome US Imperial System. . . . . .

 

[mrdre]

So, in the metric system you DON'T need to know the displacement of the object in order to do the calculation? Hey, that really IS easier!

:d

No you don't, and yes it IS easier. He mentioned "weighing 100 pounds". Weight is NOT the same as mass. Weight is the force on the object. 100 pounds force is 100 pounds force regardless of displacement of the water.. you need to apply 100 "pounds force" in the opposite direction to make it "weightless". If he mentioned "mass" of the object, then we would need to know the displacement of the water to figure out the weight.