Modified Thirds, Turn-Around Pressure and Lost Buddy Search SPG Calculations.

[Kevrumbo]

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.]

 

[chrisch]

Diving is much easier in metric. Come on you yankee chaps, do keep up with the rest of us

 

[Kevrumbo]

Diving is much easier in metric. Come on you yankee chaps, do keep up with the rest of us

Exactly! Here's the same example using cumbersome US Imperial PSI Pressure Units:

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. . .

Not at all intuitive or easy to do the arithmetic in your head with PSI units compared with bar metrics (that extra order of magnitude in PSI is confounding & superfluous). . .

 

[Superlyte27]

It's easy when you do it everyday in imperial or metric. I never spend more than a second on the math.

But that's just me. I still have students who can't quickly calculate thirds when their tanks are pumped to 4000psi.

 

[Kevrumbo]

Actually . . . It was purported in another Board (Dive Matrix) that the metric example in Post #1 above was "contrived" --that the numbers were "massaged" to make it seem like it was easier to perform the arithmetic. Ed quod est: "It is what it is" -again the above was an actual pre-dive gas plan for several wrecks and revised & re-calculated on-the-fly as described.

Another example, from another thread here in T2T -a gas plan that was originally done in US Imperial units, but with an easier methodology to understand, formulate & implement in re-calculating the plan 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).

Originally Posted by kevrumbo
My point is again, that if you [captndale] understand your pressure per time rate is premised on your tank's rating, then there is no ambiguity. [Translating your figures & using the metric system for better clarity]:

For your example of LP95's at 230 feet:
Your SCR -"Surface Consumption Rate"- of 0.4 SCFM translates to 11 litres/min*ATA in metric;
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 feet or 69 meters (same as 7.9 ATA, simply divide metric depth by ten and add one: 69/10 + 1 = 7.9):
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 feet or 69m.

Thus in 10 minutes of nominal swimming & finning around at 230'/69m, I expect the SPG to be down 32 bar from the previous reading (3.2 bar multiplied by 10min is 32 bar); 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.

Math & Logic 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 bar pressure readings over the time interval periods 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 captndale and is confirmed with metric system, 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 "do-able all in your head, no more complicated than freshman high school algebra", with the cumbersome US Imperial System.

 

[Superlyte27]

I'm reminded of the phone commercial. It's not complicated.

There's thousands of letters typed here for something that's really a non-issue. If I start with 3600psi (regardless of cylinder size) I turn at 2400psi. If I start with 4000psi, I turn at 2700psi. I can do the math in my head, it's a no brainer. And it's only slightly more complicated if I'm matching tank sizes with buddies, but easily done in my head too.

 

[Kevrumbo]

I'm reminded of the phone commercial. It's not complicated.

There's thousands of letters typed here for something that's really a non-issue. If I start with 3600psi (regardless of cylinder size) I turn at 2400psi. If I start with 4000psi, I turn at 2700psi. I can do the math in my head, it's a no brainer. And it's only slightly more complicated if I'm matching tank sizes with buddies, but easily done in my head too.

If it's not that complicated, then why hasn't anyone else (besides UTD) taken the time to explain and apply the concept & methodology above in concise detail before???

"What do you want -a cookie?" -Chris Rock

 

[PfcAJ]

Yawn, we get it Kev. You like metric. We all know this already.

 

[Kevrumbo]

Yawn, we get it Kev. You like metric. We all know this already.

Yep . . .and so does the rest of the world.

You should be "liking" & using it often too AJ, being there in Shenzhen China. . . (?)

There is no way to modify a gas plan in US Imperial units on-the-fly during the dive that's as easy to do using the Metric System -and it's still better than extraneous concepts like "tank factors" per 100psi like GUE & UTD uses for US Imperial.

 

[Superlyte27]

If you say so...
I'm going to go with the old line.... Kev, you don't know what you don't know.