RMV math…

[quietlife4me]

Do I have this theoretically correct? I’m trying to make sure I understand the basic concept.

If your RMV is 1.1cuft/min and you want to spend 60 minutes at 33 feet (1ata), you can expect to use 1.1cuft/min * 60 min * 1 ata = 66cuft of gas

…or…

If your RMV is 1.1cuft/min and you want to spend 30 minutes at 66 feet (2ata), you can expect to use 1.1cuft/min * 30 min * 2 ata = 66cuft of gas

…or…

If you have an AL80 cylinder with 51.6cuft of usable air (77.4 cuft total = 51.6cuft “rule of thirds”) and want to dive to 33 feet (1ata) with an RMV of 1.1cuft/min the total dive time before the turn is 51.6cuft / 1.1cuft/min / 1 ata = 56min

..or…

If you have an AL80 cylinder with 51.6cuft of usable air (77.4 cuft total = 51.6cuft “rule of thirds”) and want to dive to 66 feet (2ata) with an RMV of 1.1cuft/min the total dive time before the turn is 51.6cuft / 1.1cuft/min / 2 ata = 23min

 

[johndiver999]

No lol too many questions

 

[attack33]

Do I have this theoretically correct? I’m trying to make sure I understand the basic concept.

If your RMV is 1.1cuft/min and you want to spend 60 minutes at 33 feet (1ata), you can expect to use 1.1cuft/min * 60 min * 1 ata = 66cuft of gas

…or…

If your RMV is 1.1cuft/min and you want to spend 30 minutes at 66 feet (2ata), you can expect to use 1.1cuft/min * 30 min * 2 ata = 66cuft of gas

…or…

If you have an AL80 cylinder with 51.6cuft of usable air (77.4 cuft total = 51.6cuft “rule of thirds”) and want to dive to 33 feet (1ata) with an RMV of 1.1cuft/min the total dive time before the turn is 51.6cuft / 1.1cuft/min / 1 ata = 56min

..or…

If you have an AL80 cylinder with 51.6cuft of usable air (77.4 cuft total = 51.6cuft “rule of thirds”) and want to dive to 66 feet (2ata) with an RMV of 1.1cuft/min the total dive time before the turn is 51.6cuft / 1.1cuft/min / 2 ata = 23min

You’ve forgotten the miserable 1 ATA on the surface…33 feet is 2 ATA. ATA = (depth in feet/33 feet)+ 1

 

[quietlife4me]

You’ve forgotten the miserable 1 ATA on the surface…33 feet is 2 ATA. ATA = (depth in feet/33 feet)+ 1

Damn it!

 

[lowwall]

RMV is equal to number of liters (or cubic feet) per minute drawn from the tank multiplied by the number of atmospheres absolute (ata). Sea level is one atmosphere absolute (1ata). 10m of water below sea level is 2ata.

Thus...

Minutes of gas remaining = tank volume / (RMV x ata)

BTW, I find it easier to do the depths at least in metric: ata = (depth/10) +1 instead of ata = (depth/33) +1

 

[Doc]

Things to do for me tomorrow:
1) buy a calcumalator
2) watch my SPG

 

[inquis]

If you have an AL80 cylinder with 51.6cuft of usable air (77.4 cuft total = 51.6cuft “rule of thirds

You're on the right track with the RMV stuff (with the +1 of course).

I wanted to suggest that you research "Minimum Gas" or "Rock Bottom". This is a reserve strategy that is more appropriate for open water diving than the Rule of Thirds. Min Gas is the amount you need to get you and a stressed out buddy off the bottom and to the surface with a safety stop. Converted into a pressure, it's used as your ascent signal during the dive.

The interesting part is that it varies with depth, which leaves you more gas to enjoy yourself on a shallow dive than Thirds allows. It also shows you a depth where Thirds would be insufficient. (Hint, under some common assumptions, that's anything deeper than 75 ft on an AL80 tank.)

 

[lowwall]

Minutes of gas = tank volume / (RMV x ata)

BTW, I find it easier to do the depths at least in metric: ata = (depth/10) +1 instead of ata = (depth/33) +1

Expanding on this, if you want to find minutes of gas remaining for less than a full cylinder then you really begin to see how much easier it is to do in metric:

Metric:
Minutes of gas remaining = tank internal volume in liters x bar shown on SPG / (RMV x (depth/10 +1))

Imperial:
Minutes of gas remaining = (tank capacity in cubic feet x (psi shown on SPG / psi at which tank capacity is determined)) / (RMV x (depth/33 +1))

or

Minutes of gas remaining = (tank internal volume in cubic feet x (psi shown on SPG/14.5) / (RMV x (depth/33 +1))

(Someone please check this. I'm doing this off the top of my head while walking my dog.)

 

[quietlife4me]

I wanted to suggest that you research "Minimum Gas" or "Rock Bottom". This is a reserve strategy that is more appropriate for open water diving than the Rule of Thirds. Min Gas is the amount you need to get you and a stressed out buddy off the bottom and to the surface with a safety stop. Converted into a pressure, it's used as your ascent signal during the dive.

For anyone who comes to this thread later....see this website for the above...

FROG KICK DIVING | FKD

TRAINING | EXPLORERS

frogkickdiving.com

 

[inquis]

if you want to find minutes of gas remaining for less than a full cylinder

Imperial: you memorize that your AL80 tank factor is about 2.5 cuft/hundred psi. (Similar to memorizing it is 11 liters for the metric route.) From there:

Minutes at depth = Psi hundreds * TF / RMV / PressureAbs​

For instance, 1000 psi at 66 ft with RMV of 0.5 cuft/min: 10*2.5/0.5/3 = about 17 min.

[ETA: that 1000 psi could be after subtracting off the Min Gas reserve.]

In practice, things can be simplified for under water, "on the fly" calcs -- both for imperial or metric. With the combination of that tank and RMV, you get an adjusted factor of 2.5 cuft/HPsi divided by 0.5 cuft/min = 5 min / HPsi. Memorize/use this instead to ease computations.

So my thinking under water would be, "10 times five is 50, divided by 3, that's not quite 20." I might stop right there as "close enough". Or I could go further with, "Not quite 20, so first digit is a one, leaving a remainder of two to make 20. And 7 times 3 is 21, so just shy of 17 minutes remaining. Call it 16 for good measure."