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

[DiveNav]

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

Answer: It depends.
For shallow dives in Laguna when using a wetsuit ..... no planning .... I just turn around when I get cold (very soon)

For deeper dives at Catalina when using a drysuit ..... I really plan them by spending several hours in eDiving so I get quite a good idea of the area / possible dive profiles .... then I bring along few dive computers and an extra pony. And, often, a buddy too

BTW ... this thread is getting better than WAI vs SPG ....

Alberto (aka eDiver)

 

[stuartv]

RMV is the volume of air you use in a minute (cuft / min), again corrected to the surface (1 ATA). This number does NOT depend on the size of your cylinder. It is basically the SAC corrected for the "cylinder constant" for whatever cylinder used when you originally measured your SAC. Now you have a number you can use with any cylinder, but you'll still need to know the volume of the new cylinder (or the cylinder constant) and the planned depth before you can figure out how many minutes you'll have before the tank runs dry.

So, both SAC and RMV are values that are true on the surface ONLY. Both need to be corrected for pressure at depth.

Now MY head hurts.

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....

 

[EFX]

=

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....

Yikes! stuartv, the idea behind the above bolded quote is wrong. I think you've confused volume with a volumetric rate. If what you say is true then a tank of gas at the surface will last just as long at 99 ft. We know that isn't true. Because the pressure is higher at depth a lung full of gas needs to exert a higher pressure (as compared to the surface) to equalize the outside (ambient) pressure. In order to get this higher pressure (as compared to the surface) we must inhale more gas. This means increasing the flow rate. The flow rate is the volumetric rate in ft3/min (or l/min). So, we can say this: in order to fill the same lung volume at depth we must increase the gas flow in the same amount of time as compared to the surface. Therefore: lung volume at depth == lung volume at the surface but, RMV (flow rate into the lungs) at depth != RMV at the surface.

 

[Charles2]

The pedanticism evident in this thread is almost beyond belief. If you will excuse me for a moment or maybe even the rest of my life, I will continue to use the term SAC - meaning Surface Air Consumption - and measured in Cubic Feet per Minute (CFM) as my guide to planning my dives. It has always seemed that every diver I come in contact with understands what I mean.

 

[Storker]

the idea behind the above bolded quote is wrong. I think you've confused volume with a volumetric rate. If what you say is true then a tank of gas at the surface will last just as long at 99 ft.

No, he's right. Formally at least, if not accouring to your slightly twisted terminology. Problem is, you 'murricans measure your gas in surface cu.ft., but you never remember to specify that. For us Euros, it can get somewhat confusing, because the real volume of your Al80s is not 80 cubic feet. The volume is less than 0.4 cu.ft., but since they hold the amount of air that would have measured nearly 80 cu.ft. at atmospheric pressure you think they hold that volume. One inhalation is the same volume of air at 30m/100ft as on the surface, but the amount of air in that volume would've taken up four times as much space on the surface.

Which is another reason why I personally prefer our way of counting, with tanks characterized by water volume and service pressure, and gas consumption given in SLM (surface liters per minute).

 

[tbone1004]

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

 

[KeithG]

youse guys dive too deep! Or suck too much gas? Or just do not have enough to do...Get a life!

Getting back to the original ask...

We plan every dive before we hit the water. Our dives are simple so our plan is simple. Over the years we have learned what style of dive profile works for us. BUT: we confirm the plan with each other and verify it is still feasible via our dive computers before EVERY dive.

On a liveaboard we are limited by the schedule of the boat. No worries, this is either 60 or 70 minute dives. On Bonaire we are limited by cold or hunger.

Our standard default profile is a conservative multilevel dive, 60 feet for 60 minutes. ish. We do not do square profiles. We do triangles. We head for our desired max depth (or there abouts) and then slowly start to ascend for the rest of the dive. Halfway through most dives my computer shows 99 minutes of bottom time left.

My favorite Bonaire profile is 90 feet for 90 minutes. We end up with 1500 PSI in our AL80 after this dive.

What's SAC? figjam....

 

[EFX]

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

Yep. Your right. Looks like I confused density with volume. Should have thought it through more carefully.

 

[Freewillow]

I love all these Nobel price candidates working their way around in Imperial......I never thought that diving was such a difficult endeaveour in your part of the world.

 

[stuartv]

the difference is the density of that gas is increased by whatever pressure you are at

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.