Kevrumbo
Banned
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I prefer the more definitive, consistent & simple terminiology of Surface Consumption Rate (SCR) and Depth Consumption Rate (DCR), both in pressure units of bar/min with respect to the total cylinder rating of the tank(s) in use. And it's easier to work with pressure units for gas planning & management in the Metric System, especially if your pressure Surface Consumption Rate (SCR) turns out to be a convenient integer.
To illustrate --I have a cold water reference Surface Consumption Rate (SCR) of 22 litres/min (roughly 0.75 CubicFt/min in US Imperial units, an average nominal rate that most novice divers can achieve).
The common AL80 Tank holds 11 litres volume at the surface standard of 1 ATA(or 1.01 bar), for a metric rating of 11 litres/bar. 22 litres/min divided-by 11 litres/bar equals 2 bar/min SCR (Surface Consumption Rate) in pressure units --a more useful quantity to utilize during the dive since your SPG reads in bar pressure units. [And 2 bar/min is much easier & quicker to arithmetically manipulate in your head than 29 psi/min equivalent in US Imperial Units.]
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.
Example: 30 meters depth is 4 ATA (divide 30 by 10 and add 1 equals 4); your 2bar/min SCR at depth -or Depth Consumption Rate (DCR)- now becomes 8bar/min. [4 times 2bar/min equals 8bar/min]. So 10 minutes at depth 30m on an AL80 (11L/bar) tank in nominal conditions, you would expect to consume 80bar of gas and your SPG reading to be down or show a delta of 80bar. . .
What if your SCR, or Depth Consumption Rate (DCR) is lower or better than the example above? --Just scale it as a percentage result:
My SCR/RMV in tropical warm waters is typically 30% better than it is in temperate cold SoCal homewaters. However, after a week drift diving in Palau 30deg C water temp, I've lowered that to around 50% of my nominal cold water SCR (from 22 litres/min to 11 litres/min).
This is how I used this value with a 11 litres/bar tank (i.e. an AL80) in Palau:
11 litres/min divided-by- 11 litres/bar equals 1 bar/min pressure SCR.[Compare: how much easier & intuitive is it to work with "1 bar/min" vs "14.5 psi/min equivalent" in US Imperial Units???] All my dives are averaging 20 meters depth 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 1bar/min multiplied by 3 ATA equals a depth consumption rate of 3 bar/min at 20 meters. Checking my bottom timer every 10 minutes, I expect to consume 30 bar (3 bar/min multiplied by 10min equals 30 bar), and accordingly my SPG should read 30 bar less in that 10 minute time frame.
So by 30 minutes elapsed dive time at 20 meters, I expect to be down 90 bar or at half tank (AL80 full tank is 200 bar). At 40 minutes elapsed time, I'm ascending off the wall into the shallow coral plateau around 9 meters (down 120 bar from 200 bar total, 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???
---------- Post added April 18th, 2014 at 11:58 PM ----------
Here's a general Emergency gas planning example & exercise for the OP, for a 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 --let's calculate the "Rock Bottom Reserve" volume to get to the surface starting from 18meters (same as 2.8 ATA):
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 to get you to the surface --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 with your SPG reading 70bar actual, 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, thick surface kelp forest, 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!)
---------- Post added April 19th, 2014 at 02:45 PM ----------
With a normalized reference of 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 :
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 dependent on the metric cylinder rating in use, because your SPG is is in bar pressure units --NOT volumetric liter units.
Now go ahead and apply the concepts to your particular cylinder in use.
---------- Post added April 19th, 2014 at 03:26 PM ----------
Gas Planning Dissimilar Tank Sizes and Gas Matching For Modified Rule of Thirds. . .
For a buddy team with dissimilar tank sizes the point is to derive a usable turn pressure for your own tank, AFTER calculating a normalized volume of Third's Turn Pressure for the teammate's tank with the smallest gas volume. The arithmetic is fairly easy and straight forward (in metric of course):
Example of two HP tanks with short fills:
12 litre/bar tank (High Pressure Steel 100) multiplied by 180 bar fill pressure equals 2160 litres.
15 litre/bar tank (High Pressure Steel 119) multiplied by 150 bar fill pressure equals 2250 litres.
Therefore the 12 litre tank has the smaller gas volume at 2160 litres.
Calculate thirds of smaller volume tank's fill pressure of 180 bar, and you get 60 bar delta turn pressure (one-third of 180 is 60 bar). 12 litres/bar multiplied by 60 bar equals 720 litres volume (the normalized value with respect to the tank with the smaller gas volume).
Hence , the calculated turn pressure for the other tank is:
720 litres divided-by 15 litre/bar equals 48 bar delta turn pressure.
______
Here's how to figure a Rock Bottom for the teammate with the highest Surface Consumption Rate (SCR), and then a Modified Third's value with gas matching in dissimilar tanks:
Example:
28 litres/min SCR, with 18 meters (2.8 ATA) depth NDL 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.
Suppose you have two divers with different cylinder ratings:
In a 12 litre/bar tank, 372.4 litres is 31 bar (divide 372.4 by 12); in a 15 litre/bar tank, 372.4 is 24 bar pressure.
For these two divers sharing gas in an out-of-gas contingency, multiply the respective pressure readings by 2 for actual Rock Bottom:
12 litre/bar tank: 31 times 2 equals 62 bar; 15 litre/bar tank: 24 times 2 equals 48 bar.
The 12 litre/bar tank has a starting pressure of 180 bar; subtract out Rock Bottom of 62 bar yields 118 bar usable.
The 15 litre/bar tank has a starting pressure of 150 bar; subtract out Rock Bottom of 48 bar yields 102 bar usable.
Which of the two tanks above has the smaller remaining volume of usable gas left?
12 litre/bar x 118 bar = 1416 litres
15 litre/bar x 102 bar = 1530 litres
So the 12 litre/bar tank is the smaller, hence we apply the Rule-of-Thirds to the usable pressure of 118 bar:
118 divided-by 3 = 39 bar; so 39 bar delta down from 118 equals an actual turn pressure SPG reading of 79 bar.
39 bar delta in the 12 litre/bar tank equals 468 litres (39 x12 = 468);
Therefore 468 litres divided-by the other cylinder's 15 litre/bar tank rating equals 31 bar; So 31 bar delta from 102 bar equals an actual turn pressure SPG reading of 71 bar with regards to the 15 litre/bar tank.
---------- Post added April 19th, 2014 at 03:36 PM ----------
Modified Thirds,Turn-Around Pressure and Lost Buddy Search Gas Availability Calculations are much easier in metric with a bar SPG. . .
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 some amount of Rock Bottom as well) --in other words, if you do find your lost buddy and worst of all worst case 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.]
To illustrate --I have a cold water reference Surface Consumption Rate (SCR) of 22 litres/min (roughly 0.75 CubicFt/min in US Imperial units, an average nominal rate that most novice divers can achieve).
The common AL80 Tank holds 11 litres volume at the surface standard of 1 ATA(or 1.01 bar), for a metric rating of 11 litres/bar. 22 litres/min divided-by 11 litres/bar equals 2 bar/min SCR (Surface Consumption Rate) in pressure units --a more useful quantity to utilize during the dive since your SPG reads in bar pressure units. [And 2 bar/min is much easier & quicker to arithmetically manipulate in your head than 29 psi/min equivalent in US Imperial Units.]
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.
Example: 30 meters depth is 4 ATA (divide 30 by 10 and add 1 equals 4); your 2bar/min SCR at depth -or Depth Consumption Rate (DCR)- now becomes 8bar/min. [4 times 2bar/min equals 8bar/min]. So 10 minutes at depth 30m on an AL80 (11L/bar) tank in nominal conditions, you would expect to consume 80bar of gas and your SPG reading to be down or show a delta of 80bar. . .
What if your SCR, or Depth Consumption Rate (DCR) is lower or better than the example above? --Just scale it as a percentage result:
My SCR/RMV in tropical warm waters is typically 30% better than it is in temperate cold SoCal homewaters. However, after a week drift diving in Palau 30deg C water temp, I've lowered that to around 50% of my nominal cold water SCR (from 22 litres/min to 11 litres/min).
This is how I used this value with a 11 litres/bar tank (i.e. an AL80) in Palau:
11 litres/min divided-by- 11 litres/bar equals 1 bar/min pressure SCR.[Compare: how much easier & intuitive is it to work with "1 bar/min" vs "14.5 psi/min equivalent" in US Imperial Units???] All my dives are averaging 20 meters depth 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 1bar/min multiplied by 3 ATA equals a depth consumption rate of 3 bar/min at 20 meters. Checking my bottom timer every 10 minutes, I expect to consume 30 bar (3 bar/min multiplied by 10min equals 30 bar), and accordingly my SPG should read 30 bar less in that 10 minute time frame.
So by 30 minutes elapsed dive time at 20 meters, I expect to be down 90 bar or at half tank (AL80 full tank is 200 bar). At 40 minutes elapsed time, I'm ascending off the wall into the shallow coral plateau around 9 meters (down 120 bar from 200 bar total, 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???
---------- Post added April 18th, 2014 at 11:58 PM ----------
Here's a general Emergency gas planning example & exercise for the OP, for a 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 --let's calculate the "Rock Bottom Reserve" volume to get to the surface starting from 18meters (same as 2.8 ATA):
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 to get you to the surface --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 with your SPG reading 70bar actual, 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, thick surface kelp forest, 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!)
---------- Post added April 19th, 2014 at 02:45 PM ----------
With a normalized reference of 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 :
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 dependent on the metric cylinder rating in use, because your SPG is is in bar pressure units --NOT volumetric liter units.
Now go ahead and apply the concepts to your particular cylinder in use.
---------- Post added April 19th, 2014 at 03:26 PM ----------
Gas Planning Dissimilar Tank Sizes and Gas Matching For Modified Rule of Thirds. . .
For a buddy team with dissimilar tank sizes the point is to derive a usable turn pressure for your own tank, AFTER calculating a normalized volume of Third's Turn Pressure for the teammate's tank with the smallest gas volume. The arithmetic is fairly easy and straight forward (in metric of course):
Example of two HP tanks with short fills:
12 litre/bar tank (High Pressure Steel 100) multiplied by 180 bar fill pressure equals 2160 litres.
15 litre/bar tank (High Pressure Steel 119) multiplied by 150 bar fill pressure equals 2250 litres.
Therefore the 12 litre tank has the smaller gas volume at 2160 litres.
Calculate thirds of smaller volume tank's fill pressure of 180 bar, and you get 60 bar delta turn pressure (one-third of 180 is 60 bar). 12 litres/bar multiplied by 60 bar equals 720 litres volume (the normalized value with respect to the tank with the smaller gas volume).
Hence , the calculated turn pressure for the other tank is:
720 litres divided-by 15 litre/bar equals 48 bar delta turn pressure.
______
Here's how to figure a Rock Bottom for the teammate with the highest Surface Consumption Rate (SCR), and then a Modified Third's value with gas matching in dissimilar tanks:
Example:
28 litres/min SCR, with 18 meters (2.8 ATA) depth NDL 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.
Suppose you have two divers with different cylinder ratings:
In a 12 litre/bar tank, 372.4 litres is 31 bar (divide 372.4 by 12); in a 15 litre/bar tank, 372.4 is 24 bar pressure.
For these two divers sharing gas in an out-of-gas contingency, multiply the respective pressure readings by 2 for actual Rock Bottom:
12 litre/bar tank: 31 times 2 equals 62 bar; 15 litre/bar tank: 24 times 2 equals 48 bar.
The 12 litre/bar tank has a starting pressure of 180 bar; subtract out Rock Bottom of 62 bar yields 118 bar usable.
The 15 litre/bar tank has a starting pressure of 150 bar; subtract out Rock Bottom of 48 bar yields 102 bar usable.
Which of the two tanks above has the smaller remaining volume of usable gas left?
12 litre/bar x 118 bar = 1416 litres
15 litre/bar x 102 bar = 1530 litres
So the 12 litre/bar tank is the smaller, hence we apply the Rule-of-Thirds to the usable pressure of 118 bar:
118 divided-by 3 = 39 bar; so 39 bar delta down from 118 equals an actual turn pressure SPG reading of 79 bar.
39 bar delta in the 12 litre/bar tank equals 468 litres (39 x12 = 468);
Therefore 468 litres divided-by the other cylinder's 15 litre/bar tank rating equals 31 bar; So 31 bar delta from 102 bar equals an actual turn pressure SPG reading of 71 bar with regards to the 15 litre/bar tank.
---------- Post added April 19th, 2014 at 03:36 PM ----------
Modified Thirds,Turn-Around Pressure and Lost Buddy Search Gas Availability Calculations are much easier in metric with a bar SPG. . .
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 some amount of Rock Bottom as well) --in other words, if you do find your lost buddy and worst of all worst case 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.]
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