Do you Plan your dive or Dive your plan within NDLs?

[Kevrumbo]

Since we're getting all technical... What you said about RMV is, technically, incorrect.

RMV does not depend on depth.

If I inhale 0.7 cu-ft of gas per min on the surface, I will still inhale 0.7 cu-ft of gas per min at any depth. If I'm at a depth of 33 feet, the MASS of the gas that I inhale will be doubled, but the volume will remain the same. The "surface volume" of the gas will be doubled, but the actual volume is unchanged and RMV is "Volume". It's not RMSV - i.e. it's not Respiratory Minute Surface Volume. The correction that has to be applied when using RMV to calculate remaining time at a certain depth is actually a correction to the gas in the cylinder, not the RMV. If the cylinder has "100" cu-ft of air in it, THAT number is dependent on the depth, is only correct at the surface, and it is that number that has to be corrected for depth. If you're at 33 feet, the cylinder now only has 50 cu-ft of gas in it. RMV is still whatever it is.

Your lungs don't change size at depth.

In practice of course, it doesn't matter.

Technically speaking....

he meant his tidal volume. You technically still consume 0.7cf per minute, the difference is the density of that gas is increased by whatever pressure you are at, so we have to think of tidal volume as normalized for the surface. His statement wasn't incorrect, just not quite as clear as it could have been

Right. I did note that, at 33 feet, the volume is the same, but the mass of the gas used is double. By definition, that means that the density has doubled.

As I said before, the volume of your lungs does not change at depth.

Correct. Briefly and without being too pedantic, you can also increase your volume breathing rate due to physical exertion. But for a given tank fill at the surface and a diver's lung tidal volume, the greater depletion of gas supply at depth is fundamentally due to Dalton's Law, and the gas density at that depth as delivered by your regulator.

 

[dberry]

I stand corrected; Stuart is correct: RMV doesn't change at depth (just the amount of gas you're taking out of the tank per minute, not the volume per minute.) As far as the metric folks making fun of the system over here - I agree completely; it's stupid. Both the way we describe cylinders AND the units we have to wrestle with.

To those who say "SAC or RMV, who cares, *I* know what I'm talking about and so do my buddies." That's fine, but every new diver has to (or should) learn air management at some point, and not understanding the difference between psi per min and cuft per min could be a problem. Teaching them the terms bassackwards at the start doesn't help.

And if you always dive the same cylinder and know your dives very well, that's fine too. I only wish I could squeeze 90min out of an AL80 on a triangular dive to 90ft; Whether it's my SAC or RMV, I just use too much gas and need to become a better diver, for sure.

 

[Basking Ridge Diver]

I only wish I could squeeze 90min out of an AL80 on a triangular dive to 90ft

You may be over thinking this one...
I used to read these forums and think wow these folks don't breath and I am such an air hog - how do I get to be such a great diver?

My SAC/RMV last year hit .481 on a 90 foot dive, avg depth 79 feet for 41 minutes - the trick - it was a drift dive in FL - all I was doing was watching for critters and moving up and down the water column over the canyons and walls. I was not kicking and I was just going for the ride.
In NJ I can not come close to that - unless I am motionless and just floating around taking my time and not doing anything - but it has to be for photos of seahorses or something special. If I am lobster hunting or kicking around some wrecks or shore diving out and around a wreck I am exerting myself - it is not the same comparison...

Everyone on the board dives different conditions and with different objectives - if your goal is only to lower your gas usage - stop moving and enjoy the ride...

 

[oldschoolto]

I think trying to use less gas is a BIG no-no... Should a diver learn not to waste gas... Yes.. But , In affective breathing (both to fast or too slow) leads to CO2 build up and is far more dangers than Narcosis... I use what I use for the dive I'm doing.. Cold water working a wreck is always going to be a high gas consumption then playing with the fish on a warm water reef with little to no current... What to get the most out of a AL80, Sit on the bottom of a pool in 10 feet of water..

Jim....

Oh, And to answer the thread... Most dives are run through my head and done on the fly using a bottom timer and depth gage and the 60-60-120 rule...


oh.. For christmas I Bought 2 Veo 3 computers... So
i'm gonna try and move into the new way of diving..

 

[dberry]

No worries, I'm not trying to lower my air consumption for the sake of bragging rights, but I'm new enough that I can enjoy seeing improvement. I got about 0.55 - 0.65 consistently on Cozumel drift dives last May, which seemed about right. And my rate was a heck of a lot higher dealing with some current on a dive last month in the BVI, which also seemed right. All's good.

 

[lowviz]

How you breathe is very important too. A sharp inhalation will always trigger an autonomic response. Always.

"You saw a threat and inhaled sharply." It is against survival to tolerate out this response. So try not to do that because you are telling yourself that something is wrong. http://www.ncbi.nlm.nih.gov/pubmed/6496020

Convince your bod that all is well by breathing slowly. That is all, nothing more. Just calm down and breathe and deal with the SAC that defines you.

 

[Kevrumbo]

How do you pre-plan a multilevel dive or do you? Do you plan the dive using a manual approach (table or other method) or do you use a paid subscription software application?

Does your pre-plan include your expected RMV, gas volume and time requirements to stay within the NDLs? Or do you jump in and monitor on the fly your dive based on your SPG (AI) and PDC?

It can be a boat or shore dive - I am interested in your approach to a pre-plan or lack of pre-plan.
Thanks in advance.

Here's another manual gas planning example & exercise for a novice OW Diver Buddy Team to the Basic Open Water NDL Limit of 18m/60':

Pre-Dive Plan: Given an Emergency "Stressed" 30 liters/min per ATA Surface Consumption Rate (SCR), with 18 meters depth NDL, and with an arbitrary conservative controlled and slow 0.5 minute stops (30sec) every 3 meters ascent rate to a Safety Stop --let's calculate the "Rock Bottom Reserve" value to get to the surface starting from 18meters (same as 2.8 ATA):

Depth x SCR x Minutes = Liters
2.8 x 30 x 1 = 84
2.5 x 30 x 0.5 = 37.4
2.2 x 30 x 0.5 = 33
1.9 x 30 x 0.5 = 28.5
1.6 x 30 x 2 = 96 [2 min Safety Stop]
1.3 x 30 x 0.5 = 19.5
1.0 x 30 x 0.5 = 15

Sum Total: 313.4 liters 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 liters/bar. So 313 divided-by 11 = round up to approx 30 bar.

That's 30 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: 30 x 2 = 60 bar. Therefore your Rock Bottom Reserve pressure is 60 bar showing on your SPG --if there is no emergency air sharing contingency at that instant with your SPG reading 60 bar 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 60 bar equals 140 bar usable. Let's use 20 bar of this 140 to get squared away in good trim & buoyancy on the descent to 18m which leaves 120 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 120 bar or 60 bar consumed.

So for a dive with a nominal volume SCR of 22 liters/min per ATA on a 11L per bar (AL80) tank, your resulting pressure SCR will be 2 bar/min per ATA [22 divided-by 11 equals 2bar/min per ATA]. 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 per ATA pressure SCR.

18 meters depth is 2.8 ATA (divide 18 by 10 and add 1 equals 2.8 ATA); your 2bar/min per ATA pressure 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 of 56bar less for that 10 minute elapsed time interval. . .

Okay, you splash in to start the dive with 200bar and a 60 bar rock bottom. You use up 20bar on descent --initial exertion, inflating your BCD/wing etc. -so you now have 180bar with 60 bar rock bottom at present depth 18m, a net usable of 120 bar (180 minus 60 is 120 bar).

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 and confirmed with a SPG reading that you've used up 56bar, close to half of a net usable of 120 bar, so you decide to turn the dive coming back around on a reciprocal course. You should now know you have roughly 60 bar of usable gas left before encroaching on your Rock Bottom Reserve, and realize that you must start a nominal ascent from 18m at the end of the next 10 minute interval. (Note: 20 minutes elapsed bottom time at 18 meters depth is well within NDL).

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; because of this unplanned turtle excursion & physical exertion, you immediately check your SPG and it reads 60 bar --bingo! Rock bottom has arrived so start ascending to the shallower depths & eventual Safety Stop . . .or if your Buddy just happens to blow his tank neck O-ring at that instant with a catastrophic loss of his remaining gas supply --you know you've got enough 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!)

 

[Kevrumbo]

This is - IMO - a pretty darned good argument for losing your surface cuFts and tank factors and converting to the Euro way of doing it: Water volume and service pressure.

12L 232 bar = 2800 surface liters.
15L 200 bar = 3000 surface liters.
15L 232 bar = 3500 surface liters.
If your SAC - or RMV, I'm not going to argue that semantic point - is 15 SLM, the 12x232 will give you an hour at 20m/3 ATA, the 15x200 will give you some 5 minutes more, and the 15x232 will give you about an hour and a quarter. Easy peasy, no need to memorize - or look up - those tank factors. All you need is simple, middle school level arithmetic. Which at least I can do even if I'm narked out of my gourd.

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. [11 SLM; 11L 200 bar; 22L 200 bar]

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 metric tank factors- 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.

 

[Rusty Roo]

on shallow dives the planning is non existent just monitor computer.

if i know i'm going to be going 30m+ for a specific reason. ie plateau etc and i know the shallow reef is also good. then the plan will be max ndl maybe stray into it 1-2 minutes then shallow up.

 

[Basking Ridge Diver]

So for shallow dives - less than 33 meters / 100 feet - is it acceptable to jump in without a plan? As long as you monitor your PDC and SPG/AI? (I will assume for this scenario you should have an idea on turn pressure or rock bottom.)
Is this considered a Plan your Dive?